Saturn's moon Rhea, as imaged by the Cassini-Huygens spaceprobe. Credit: NASA/JPL-Caltech
The Cronian system (i.e. Saturn and its system of rings and moons) is breathtaking to behold and intriguing to study. Besides its vast and beautiful ring system, it also has the second-most satellites of any planet in the Solar System. In fact, Saturn has an estimated 150 moons and moonlets – and only 53 of them have been officially named – which makes it second only to Jupiter.
For the most part, these moons are small, icy bodies that are believed to house interior oceans. And in all cases, particularly Rhea, their interesting appearances and compositions make them a prime target for scientific research. In addition to being able to tell us much about the Cronian system and its formation, moons like Rhea can also tell us much about the history of our Solar System.
Discovery and Naming:
Rhea was discovered by Italian astronomer Giovanni Domenico Cassini on December 23rd, 1672. Together with the moons of Iapetus, Tethys and Dione, which he discovered between 1671 and 1672, he named them all Sidera Lodoicea (“the stars of Louis”) in honor of his patron, King Louis XIV of France. However, these names were not widely recognized outside of France.
In 1847, John Herschel (the son of famed astronomer William Herschel, who discovered Uranus, Enceladus and Mimas) suggested the name Rhea – which first appeared in his treatise Results of Astronomical Observations made at the Cape of Good Hope. Like all the other Cronian satellites, Rhea was named after a Titan from Greek mythology, the “mother of the gods” and one the sisters of Cronos (Saturn, in Roman mythology).
The moons of Saturn, from left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan (background), Iapetus (top), and Hyperion (bottom). Credit: NASA/JPL/Space Science Institute
Size, Mass and Orbit:
With a mean radius of 763.8±1.0 km and a mass of 2.3065 ×1021 kg, Rhea is equivalent in size to 0.1199 Earths (and 0.44 Moons), and about 0.00039 times as massive (or 0.03139 Moons). It orbits Saturn at an average distance (semi-major axis) of 527,108 km, which places it outside the orbits of Dione and Tethys, and has a nearly circular orbit with a very minor eccentricity (0.001).
With an orbital velocity of about 30,541 km/h, Rhea takes approximately 4.518 days to complete a single orbit of its parent planet. Like many of Saturn’s moons, its rotational period is synchronous with its orbit, meaning that the same face is always pointed towards it.
Composition and Surface Features:
With a mean density of about 1.236 g/cm³, Rhea is estimated to be composed of 75% water ice (with a density of roughly 0.93 g/cm³) and 25% of silicate rock (with a density of around 3.25 g/cm³). This low density means that although Rhea is the ninth-largest moon in the Solar System, it is also the tenth-most massive.
In terms of its interior, Rhea was originally suspected of being differentiated between a rocky core and an icy mantle. However, more recent measurements would seem to indicate that Rhea is either only partly differentiated, or has a homogeneous interior – likely consisting of both silicate rock and ice together (similar to Jupiter’s moon Callisto).
Views of Saturn’s moon Rhea, with false-color image showing elevation data at the right. Credit: NASA/JPL/Space Science Institute
Models of Rhea’s interior also suggest that it may have an internal liquid-water ocean, similar to Enceladus and Titan. This liquid-water ocean, should it exist, would likely be located at the core-mantle boundary, and would be sustained by the heating caused by from decay of radioactive elements in its core.
Rhea’s surface features resemble those of Dione, with dissimilar appearances existing between their leading and trailing hemispheres – which suggests that the two moons have similar compositions and histories. Images taken of the surface have led astronomers to divide it into two regions – the heavily cratered and bright terrain, where craters are larger than 40 km (25 miles) in diameter; and the polar and equatorial regions where craters are noticeably smaller.
Another difference between Rhea’s leading and trailing hemisphere is their coloration. The leading hemisphere is heavily cratered and uniformly bright while the trailing hemisphere has networks of bright swaths on a dark background and few visible craters. It had been thought that these bright areas (aka. wispy terrain) might be material ejected from ice volcanoes early in Rhea’s history when its interior was still liquid.
However, observations of Dione, which has an even darker trailing hemisphere and similar but more prominent bright streaks, has cast this into doubt. It is now believed that the wispy terrain are tectonically-formed ice cliffs (chasmata) which resulted from extensive fracturing of the moon’s surface. Rhea also has a very faint “line” of material at its equator which was thought to be deposited by material deorbiting from its rings (see below).
Hemispheric color differences on Saturn’s moon Rhea are apparent in this false-color view of the anti-Cronian side, from NASA’s Cassini spacecraft. Image Credit: NASA/JPL/SSI
Rhea has two particularly large impact basins, both of which are situated on Rhea’s anti-Cronian side (aka. the side facing away from Saturn). These are known as Tirawa and Mamaldi basins, which measure roughly 360 and 500 km (223.69 and 310.68 mi) across. The more northerly and less degraded basin of Tirawa overlaps Mamaldi – which lies to its southwest – and is roughly comparable to the Odysseus crater on Tethys (which gives it its “Death-Star” appearance).
Atmosphere:
Rhea has a tenuous atmosphere (exosphere) which consists of oxygen and carbon dioxide, which exists in a 5:2 ratio. The surface density of the exosphere is from 105 to 106 molecules per cubic centimeter, depending on local temperature. Surface temperatures on Rhea average 99 K (-174 °C/-281.2 °F) in direct sunlight, and between 73 K (-200 °C/-328 °F) and 53 K (-220 °C/-364 °F) when sunlight is absent.
The oxygen in the atmosphere is created by the interaction of surface water ice and ions supplied from Saturn’s magnetosphere (aka. radiolysis). These ions cause the water ice to break down into oxygen gas (O²) and elemental hydrogen (H), the former of which is retained while the latter escapes into space. The source of the carbon dioxide is less clear, and could be either the result of organics in the surface ice being oxidized, or from outgassing from the moon’s interior.
Saturn’s second-largest moon Rhea, pictured by the Cassini probe on March 29, 2012. Credit: NASA/JPL
Rhea may also have a tenuous ring system, which was inferred based on observed changes in the flow of electrons trapped by Saturn’s magnetic field. The existence of a ring system was temporarily bolstered by the discovered presence of a set of small ultraviolet-bright spots distributed along Rhea’s equator (which were interpreted as the impact points of deorbiting ring material).
However, more recent observations made by the Cassini probe have cast doubt on this. After taking images of the planet from multiple angles, no evidence of ring material was found, suggesting that there must be another cause for the observed electron flow and UV bright spots on Rhea’s equator. If such a ring system were to exist, it would be the first instance where a ring system was found orbiting a moon.
Exploration:
The first images of Rhea were obtained by the Voyager 1 and 2 spacecraft while they studied the Cronian system, in 1980 and 1981, respectively. No subsequent missions were made until the arrival of the Cassini orbiter in 2005. After it’s arrival in the Cronian system, the orbiter made five close targeted fly-bys and took many images of Saturn from long to moderate distances.
The Cronian system is definitely a fascinating place, and we’ve really only begun to scratch its surface in recent years. In time, more orbiters and perhaps landers will be traveling to the system, seeking to learn more about Saturn’s moons and what exists beneath their icy surfaces. One can only hope that any such mission includes a closer look at Rhea, and the other “Death Star Moon”, Dione.
"Tiger stripes" -- sources of ice spewing -- in this image of Saturn's Enceladus taken by the Cassini spacecraft in 2009. Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA
In the ongoing drive to unlock the secrets of Saturn and its system of moons, some truly fascinating and awe-inspiring things have been discovered. In addition to things like methane lakes and propane-rich atmospheres (Titan) to moon’s that resemble the Death Star (Mimas), it is also becoming abundantly clear that planet’s beyond Earth may harbor interior oceans and even the extra-terrestrial organisms.
Nowhere is this more apparent than on Enceladus, Saturn’s sixth largest moon, which also possesses some of the most interesting characteristics in the outer Solar System. These include long veins of blue ice that resemble stripes, not to mention amazing plumes of water ice that have been spotted periodically blasting out of the moon’s southern pole. These, in turn, raise the possibility of liquid water beneath the surface, and possibly even life!
Discovery and Naming:
Discovered in 1789 by William Herschel, Enceladus is named after one of the giants in Greek mythology. In fact, all of the large moons of Saturn are named after the Titans, as suggested by William Herschel’s son, John Herschel. He chose these names because Saturn (known in Greek mythology as Kronos) was the father of the Titans.
In contrast, in accordance with the IAU naming conventions for Enceladus, features are named after characters and places from the classic story One Thousand and One Nights (aka. Arabian Nights). Impact craters are named after characters, whereas other feature types – such as fossae (long, narrow depressions), dorsa (ridges), planitia (plains), and sulci (long parallel grooves), are named after places.
Size comparison between the Cronian moon Enceladus, the Moon, and Earth. Credit: NASA/JPL-Caltech/Tom Reding
Size, Mass and Orbit:
With a mean radius of 252 km, Enceladus is equivalent in size to 0.0395 Earths (or 0.1451 Moons). But with a mass of 1.08 ×1020 kg, it is only 0.000018 as massive. The planet has a very minor eccentricity (0.0047) and orbits Saturn at an average distance (semi-major axis) of 237,948 km, between the orbits of Mimas and Tethys.
Enceladus takes 32.9 hours (1.37 days) to complete a single orbit around Saturn, and is currently in a 2:1 mean-motion orbital resonance with Dione; meaning that it completes two orbits of Saturn for every orbit completed by Dione. This forced resonance is what maintains Enceladus’s orbital eccentricity and results in tidal deformation, and the resulting heat dissipation is the main heating source for Enceladus’s geologic activity.
Like most of the larger natural satellites of Saturn, Enceladus rotates synchronously with its orbital period, keeping one face pointed toward Saturn. The planet also experiences forced libration, where it appears to oscillate relative to Saturn’s other moons – which may also provide Enceladus with an internal heat source.
Composition and Surface Features:
Enceladus has a density of 1.61 g/cm³, which is higher than Saturn’s other mid-sized, icy satellites, suggesting a composition that includes a greater percentage of silicates and iron. It is also believed to be largely differentiated between a geologically active core and an icy mantle, with a liquid water ocean nestled between.
Gravity measurements by NASA’s Cassini spacecraft and Deep Space Network suggest that Saturn’s moon Enceladus harbors a large interior ocean beneath it’s south pole. Credit: NASA/JPL-Caltech
The existence of this liquid water ocean has been the subject of scientific debate since 2005, when scientists first observed plumes containing water vapor spewing from Enceladus’s south polar surface. These jets are capable of dispensing 250 kg of water vapor every second at speeds of up to 2,189 km/h (1,360 mph), and reaching 500 km into space.
In 2006, it was determined that Enceladus’s plumes are the source of Saturn’s E Ring and actively replenish it. According to measurements made by the Cassini-Huygens probe, these emissions are composed mostly of water vapor, as well as minor components like molecular nitrogen, methane, and carbon dioxide. Further observations noted the presence of simple hydrocarbons such as methane, propane, acetylene and formaldehyde.
The combined analysis of imaging, mass spectrometry, and magnetospheric data suggests that the observed south polar plume emanates from pressurized subsurface chambers. The intensity of the eruptions varies significantly due to changes in Enceladus’s orbit. Basically, the plumes are about four times brighter when Enceladus is at apoapsis (farthest from Saturn), which is consistent with geophysical calculations that predict that the south polar fissures will be under less compression, thus opening them wider.
The existence of subsurface water was confirmed thanks to evidence provided by the Cassini mission in 2014. This included gravity measurements obtained during the flybys of 2010-2012, which confirmed the existence of a south polar subsurface ocean of liquid water within Enceladus with a thickness of around 10 km.
Artist’s rendering of possible hydrothermal activity that may be taking place on and under the seafloor of Enceladus. Credit: NASA/JPL
In addition, during the July 14, 2005 flyby, the Cassini probe also detected the presence of escaping internal heat in the southern polar region. These temperatures were too high to be attributed to solar heating, and combined with the geyser activity, seemed to indicate that the interior of the planet is still geologically active.
Further studies from measurements of Enceladus’s libration as it orbits Saturn strongly suggest that the entire icy crust is detached from the rocky core, which would mean that the ocean beneath its surface is planet-wide. The amount of libration implies that this global ocean is about 26 to 31 kilometers in depth (compared to Earth’s average ocean depth of 3.7 kilometers).
Observations of Enceladus’ surface has revealed five types of terrain – cratered terrain, smooth (young) terrain, ridged terrain (often bordering on smooth areas), linear cracks, scarps, troughs, and grooves. Surveys of the cratered terrain, smooth plains, and other features indicate a level of resurfacing that suggests that tectonics are an important factor in the geological history of Enceladus.
Recent observations by Cassini have provided a closer look at the crater distribution and size. These features have been named by the IAU after characters and places from Burton’s translation of The Book of One Thousand and One Nights – i.e. the Shahrazad crater, the Diyar plains, the Anbar depression.
Artist impression of the view of Saturn from Enceladus, with geysers erupting at the right in the foreground. Credit: Michael Carroll
The smooth plains are dominated by fresh clean ice, which gives Enceladus what is possibly the most reflective surface in the Solar System (with a visual geometric albedo of 1.38). These areas have few craters, which indicate that they are likely younger than a few hundred million years old. In addition, the relative youthfulness of these regions are an indication that cryovolcanism and other processes actively renew the surface.
The older terrain is not only cratered, but numerous fractures have also been observed – suggesting that the surface has been subject to extensive deformation since the craters formed. Some areas show regions with no craters, indicating major resurfacing events in the geologically recent past. The fissures, plains, corrugated terrain and other crustal deformations also indicate that Enceladus is geologically active.
One of the more dramatic types of tectonic features found on Enceladus are its rift canyons. These canyons can be up to 200 km long, 5–10 km wide, and 1 km deep. Such features are geologically young, because they cut across other tectonic features and have sharp topographic relief with prominent outcrops along the cliff faces.
Evidence of tectonics on Enceladus is also derived from grooved terrain, consisting of lanes of curved formations and ridges that often separate smooth plains from cratered regions. Deep fractures are another, which are often found in bands cutting across cratered terrain, and which were probably influenced by the formation of weakened regolith produced by impact craters.
Enceladus, showing the famous “Tiger Stripes” feature – a series of fractures bound on either side by colorful ice. Credit: NASA/JPL/Space Science Institute
Linear grooves can also be seen cutting across other terrain types, like the groove and ridge belts. Like the deep rifts, they are among the youngest features on Enceladus. However, some linear grooves have been softened like the craters nearby, suggesting that they are older. Ridges have also been observed on Enceladus, though they are relatively limited in extent and are up to one kilometer tall.
Other interesting features include the “Tiger stripes“: a series of fractures bounded on either side by ridges in the southern polar region that are are surrounded by mint-green-colored, coarse-grained water ice. These fractures appear to be the youngest features in this region, and combined with a lack of impact craters in this area, are further evidence of geological activity.
Atmosphere:
Saturn’s moon Enceladus has an atmosphere greater than that of all others in the Solar System, with the exception of Titan. The source of the atmosphere is attributed to the periodic cryovolcanism, which leads to gases and vapor escaping from the surface or the interior. Evidence of a tenuous atmosphere came from magnetometer readings provided by the Cassini‘s probe in 2005.
This consisted of an increased detection in the power of ion cyclotron waves, which are produced by the interaction of ionized particles and magnetic fields. During the next two encounters, the magnetometer team determined that gases in Enceladus’s atmosphere are concentrated over the south polar region, with atmospheric density away from the pole being much lower.
Water vapour geysers erupting from Enceladus’ south pole. Credit: NASA/JPL
Much like the content of the jet plumes, this atmosphere is composed primarily of water vapor (91%), but also shows signs of minor components like molecular nitrogen (4%) and carbon dioxide (3.2%). There has also been evidence of simple hydrocarbons, which take the form of methane (1.7%) as well as trace amounts of propane, acetylene and formaldehyde.
Habitability:
Ever since the discovery of Enceladus’s geysers and evidence that suggested an interior ocean, scientists have speculated about the possibility of there being life on Enceladus. Because it reflects so much sunlight, the mean surface temperature at noon only reaches -198 °C, making it somewhat colder than other Cronian satellites. However, within the core, multiple indications of life exist.
It’s resonance with Dione excites its orbital eccentricity, which tidal forces damp, resulting in tidal heating of its interior. This offers a possible explanation for its geological activity, and also suggests that its interior oceans are warmer closer to the core. In addition, geological models have indicated that the large rocky core is porous, allowing water to flow through it to pick up heat.
A model of Enceladus’s ocean created by Christopher R. Glein et al. (2015) suggests that it has an alkaline pH of 11 to 12. This high pH (alkaline) is interpreted to be a consequence of serpentinization of chondritic rock, which leads to the generation of molecular hydrogen (H²). This geochemical source of energy can be metabolized by methanogen microbes to provide energy for life.
The presence of an internal salty ocean with an energy source and simple organic compounds are all strong indications that microbes may exist closer to the core, where the water is warm and the basic building blocks of life exist.
Exploration:
Although it was first discovered in the late 18th century, astronomers didn’t know much about this moon for many centuries. It was not until it was first visited in a series of flybys by NASA’s two Voyager spacecraft in the 1980’s that certain things began to become apparent about Enceladus.
Artist’s impression of Voyager 1 reaching Saturn and its system of moons. Image Credit: NASA/JPL – Caltech
For starters, the Voyager missions showed that the planet has a diameter of only 500 km (310 miles), which makes it less than one-tenth the diameter of Saturn’s largest moon of Titan. They also noted that most of the surface is covered in fresh, clean ice; giving it a pure, snow-white appearance that also attracts close to 100% of the sunlight that strikes its surface.
The Voyager 1 mission also confirmed that Enceladus was embedded in the densest part of Saturn’s diffuse E-ring. Combined with the apparent youthful appearance of the surface, Voyager scientists suggested that the E-ring consisted of particles vented from Enceladus’s surface. The Voyager 2 mission provided better photographs than its predecessor, confirming the presence of a youthful surface, but also other features.
By 2005, the Cassini spacecraft began performing multiple close flybys of Enceladus, revealing its surface and environment in greater detail. In particular, Cassini discovered the water-rich plumes venting from the south polar region of Enceladus, which became the subject of much research and speculation.
Cassini has provided strong evidence that Enceladus has an ocean with an energy source, nutrients and organic molecules, making Enceladus one of the best places for the study of potentially habitable environments for extraterrestrial life. By contrast, the water thought to be on Jupiter’s moon Europa is located under a much thicker layer of ice.
An artist illustration of the Cassini spacecraft. Image Credit: NASA/JPL
Cassini’s latest flyby took place on October 14th, 2015, passing the moon at an altitude of 1,839 kilometers (1,142 miles) above the northern polar region. This was the first time that Cassini had been able to observe the northern polar region, due to the fact that on all previous occasions, the northern region was experiencing its winter cycle and was concealed by darkness.
Cassini’s instruments took pictures of multiple surface features, including craters (many of which look like they are melting), fractures and wrinkles. The latter features are believed to be an indication that the moon’s spin rate has changed, which may be another indication that the surface has undergone multiple episodes of geologic activity over the course of much of its lifetime.
The discoveries Cassini has made at Enceladus have prompted studies into follow-up mission concepts. In 2013, NASA proposed a possible sample-return mission to Enceladus that would involve a low-cost orbiter. This mission would launch during the 2020s and last 15 years.
Another proposal for a probe flyby, known as Journey to Enceladus and Titan (JET) would analyze plume contents in-situ. Proposed in response to NASA’s 2010 Discovery Announcement of Opportunity, the mission would involve an orbiter conducting high-resolution mass spectroscopy surveys of Enceladus and Titan, assessing them for biological potential.
The German Aerospace Center has also proposed studying the habitability of Enceladus’s subsurface ocean using an Enceladus Explorer, and two astrobiology-oriented mission concepts (the Enceladus Life Finder and Life Investigation For Enceladus). In 2007, the European Space Agency (ESA) proposed sending a probe to Enceladus in a mission to be combined with studies of Titan – known as TandEM (Titan and Enceladus Mission).
Additionally, there’s the Titan Saturn System Mission (TSSM), a joint NASA/ESA flagship-class proposal to explore Saturn’s moons (with a focus on Enceladus). TSSM was competing against the Europa Jupiter System Mission (EJSM) proposal for funding. In February 2009, it was announced that NASA/ESA had given the EJSM mission priority ahead of TSSM, although TSSM will continue to be studied and evaluated.
Enceladus is a tempting target for future research and exploration, and for good reason. For starters, it is one of the few Solar System bodies (alongside with Earth, Io, and Triton) to have confirmed contemporary volcanic activity. Second is the distinct possibility that life exists beneath its icy surface, much like Europa. But with Enceladus, getting to a place where we could study that life would be much easier.
As such, it is almost certain that any missions to Saturn and/or the outer Solar System in the coming years will likely involve a close flyby of Enceladus. Maybe we’ll even pop in a lander and an aquatic explorer to examine the surface and peak underneath it!
Craters near Enceladus' north pole region appear to be 'melting' into each other. Image taken by Cassini spacecraft on October 14, 2015. Credit: NASA/JPL-Caltech/Space Science Institute
If you thought Saturn’s moon Enceladus couldn’t get any more bizzare — with its magnificent plumes, crazy tiger-stripe-like fissures and global subsurface salty ocean — think again. New images of this moon’s northern region just in from the Cassini spacecraft show surprising and perplexing features: a tortured surface where craters look like they are melting, and fractures that cut straight across the landscape.
“We’ve been puzzling over Enceladus’ south pole for so long, time to be puzzled by the north pole!” tweeted NASA engineer Sarah Milkovich, who formerly worked on the Cassini mission.
While the Cassini mission has been at the Saturn system since 2004 and flown by this moon several times, this is the spacecraft’s first close-up look at the north polar region of Enceladus. On October 14, 2015 the spacecraft passed at an altitude of just 1,839 kilometers (1,142 miles) above the moon’s surface.
See more imagery below:
Craters and a possible straight fracture line mar the surface of Enceladus in this raw image from the Cassini spacecraft taken on October 14, 2015. Credit: NASA/JPL-Caltech/Space Science Institute.
The reason Cassini hasn’t been able to see the northern terrain of Enceladus previously is that it was concealed by the darkness of winter. It’s now summer in the high northern latitudes, and scientists have been anxious to take a look at this previously unseen region. Gauging by the posts of “Wow!” and “Enceladus what are you doing??” by scientists on social media, the Cassini team is as excited and perplexed by these images as the rest of us.
“We’ve been following a trail of clues on Enceladus for 10 years now,” said Bonnie Buratti, a Cassini science team member and icy moons expert at NASA’s Jet Propulsion Laboratory. “The amount of activity on and beneath this moon’s surface has been a huge surprise to us. We’re still trying to figure out what its history has been, and how it came to be this way.”
Craters and fractures dot the landscape of the northern region of Enceladus in this raw image from the Cassini spacecraft taken on October 14, 2015. Credit: NASA/JPL-Caltech/Space Science Institute.
While these raw images just arrived this morning, already image editing enthusiasts have dived into the data to create composite and color images. Here are two from UT writer Jason Major and image contributor Kevin Gill:
A beautiful view of the night side of a crescent Enceladus, lovingly lit by Saturnshine. This was captured by the Cassini spacecraft during a close pass on Oct. 14, 2015. The 6.5-mile-wide Bahman cater is visible near the center. Credit: NASA/JPL-Caltech/Space Science Institute, image editing by Jason Major.Saturn’s icy moon Enceladus on October 14th, 2015 during Cassini’s latest encounter. Assembled from uncalibrated images using infrared, green, and ultraviolet light. Image Credit: NASA/JPL-CalTech/ISS/Kevin M. Gill
In an email, Cassini imaging team leader Carolyn Porco explained the flyby: “Our cameras were active during most of this encounter, allowing the imaging team and other remote-sensing instrument teams to observe the Saturn-opposing side of Enceladus on the inbound leg of the encounter, and a narrow, sunlit crescent outbound.”
From previous imagery and study of this moon, it has been suggested that the fractured and wrinkled terrain on Enceladus could be the scars of a shift in the moon’s spin rate. The moon has likely undergone multiple episodes of geologic activity spanning a considerable portion of its lifetime.
A complex region of craters and fractures near the north polar region on Saturn’s moon Enceladus. Image from Cassini spacecraft taken on October 14, 2015. Credit: NASA/JPL-Caltech/Space Science Institute
While these images are incredible, get ready for even more. An even closer flyby of Enceladus is scheduled for Wednesday, Oct. 28, during which Cassini will come dizzyingly close to the icy moon, passing just 49 kilometers (30 miles) above the moon’s south polar region. NASA says that during this encounter, Cassini will make its deepest-ever dive through the moon’s plume of icy spray, collecting images and valuable data about what’s going on beneath the frozen surface. Cassini scientists are hopeful data from that flyby will provide evidence of how much hydrothermal activity is occurring in the moon’s ocean, and how the amount of activity impacts the habitability of Enceladus’ ocean.
Then another flyby — Cassini’s final scheduled close flyby of Enceladus — on Dec. 19 will examine how much heat is coming from the moon’s interior from an altitude of 4,999 kilometers (3,106 miles).
Enceladus hovers over Saturn’s rings in this raw image from the Cassini spacecraft taken on October 14, 2015. Credit: NASA/JPL-Caltech/Space Science Institute.
An interesting side note is that the Cassini mission launched 18 years ago today (October 15, 1997).
Again stay tuned for more, and you can see all of Cassini’s raw image here, and find out more details of the upcoming flybys at this CICLOPS page.
Lucy, an SwRI mission proposal to study primitive asteroids orbiting near Jupiter, is one of five science investigations under the NASA Discovery Program up for possible funding. Credit: swri.org
In February of 2014, NASA’s Discovery Program put out the call for mission proposals, one or two of which will have the honor of taking part in Discovery Mission Thirteen. Hoping to focus the next round of exploration efforts to places other than Mars, the five semifinalists (which were announced this past September) include proposed missions to Venus, Near-Earth Objects, and asteroids.
When it comes to asteroid exploration, one of the possible contenders is Lucy – a proposed reconnaissance orbiter that would study Jupiter‘s Trojan Asteroids. In addition to being the first mission of its kind, examining the Trojans Asteroids could also lead to several scientific finds that will help us to better understand the history of the Solar System.
By definition, Trojan are populations of asteroids that share their orbit with other planets or moons, but do not collide with it because they orbit in one of the two Lagrangian points of stability. The most significant population of Trojans in the Solar System are Jupiter’s, with a total of 6,178 having been found as of January 2015. In accordance with astronomical conventions, objects found in this population are named after mythical figures from the Trojan War.
There are two main theories as to where Jupiter’s Trojans came from. The first suggests that they formed in the same part of the Solar System as Jupiter and were caught by the gas giant’s gravity as it accumulated hydrogen and helium from the protoplanetary disk. Since they would have shared the same approximate orbit as the forming gas giant, they would have been caught in its gravity and orbited it ever since.
The asteroids of the Inner Solar System and Jupiter. Credit: Wikipedia Commons
The second theory, part of the Nice model, proposes that the Jupiter Trojans were captured about 500-600 million years after the Solar System’s formation. During this period Uranus, Neptune – and to a lesser extent, Saturn – moved outward, whereas Jupiter moved slightly inward. This migration could have destabilized the primordial Kuiper Belt, throwing millions of objects into the inner Solar System, some of which Jupiter then captured.
In either case, the presence of Trojan asteroids around Jupiter can be traced back to the early Solar System. Studying them therefore presents an opportunity to learn more about its history and formation. And if in fact the Trojans are migrant from the Kuiper Belt, it would also be a chance for scientists to learn more about the most distant reaches of the solar system without having to send a mission all the way out there.
The mission would be led by Harold Levison of the Southwest Research Institute (SwRI) in Boulder, Colorado, with the Goddard Space Center managing the project. Its targets would most likely include asteroid (3548) Eurybates, (21900) 1999 VQ10, (11351) 1997 TS25, and the binary (617) Patroclus/Menoetius. It would also visit a main-belt asteroid (1981 EQ5) on the way.
The spacecraft would perform scans of the asteroids and determine their geology, surface features, compositions, masses and densities using a sophisticated suite of remote-sensing and radio instruments. In addition, during it’s proposed 11-year mission, Lucy would also gather information on the asteroids thermal and other physical properties from close range.
Artist’s concept of Jupiter’s Trojan asteroids hovering in the foreground in Jupiter’s path, with the “Greeks” at left in the background. Credit: NASA.
The project is named Lucy in honor of one of the most influential human fossils found on Earth. Discovered in the Awash Valley of Ethiopia in 1974, Lucy’s remains – several hundred bone fragments that belonged to a member the hominid species of Australopithecus afarensis – proved to be an extraordinary find that advanced our knowledge of hominid species evolution.
Levison and his team are hoping that a similar find can be made using the probe of the same name. As he and his colleagues describe it, the Lucy mission is aimed at “Surveying the diversity of Trojan asteroids: The fossils of planet formation.”
“This is a once-in-a-lifetime opportunity,” said Levinson. “Because the Trojan asteroids are remnants of that primordial material, they hold vital clues to deciphering the history of the solar system. These asteroids are in an area that really is the last population of objects in the solar system to be visited.”
The payload is expected to include three complementary imaging and mapping instruments, including a color imaging and infrared mapping spectrometer, a high-resolution visible imager, and a thermal infrared spectrometer. NASA has also offered an additional $5 to $30 million in funding if mission planners choose to incorporate a laser communications system, a 3D woven heat shield, a Deep Space atomic clock, and/or ion engines.
As one of the semifinalists, the Lucy mission has received $3 million dollars to conduct concept design studies and analyses over the course of the next year. After a detailed review and evaluation of the concept studies, NASA will make the final selections by September 2016. In the end, one or two missions will receive the mission’s budget of $450 million (not including launch vehicle funding or post-launch operations) and will be launched by 2020 at the earliest.
Titan's dense, hydrocarbon rich atmosphere remains a focal point of scientific research. Credit: NASA
In ancient Greek lore, the Titans were giant deities of incredible strength who ruled during the legendary Golden Age and gave birth to the Olympian gods we all know and love. Saturn‘s largest moon, known as Titan, is therefore appropriately named. In addition to being Saturn’s largest moon – and the second-largest moon in the Solar System (after Jupiter’s moon Ganymede) – it is larger by volume than even the smallest planet, Mercury.
Beyond its size, Titan is also fascinating because it is the only natural satellite known to have a dense atmosphere, a fact which has made it very difficult to study until recently. On top of all that, it is the only object other than Earth where clear evidence of stable bodies of surface liquid has been found. All of this makes Titan the focal point of a great deal of curiosity, and a prime location for future scientific missions.
Discovery and Naming:
Titan was discovered on March 25th, 1655, by the Dutch astronomer Christiaan Huygens. Huygens had been inspired by Galileo’s improvements in telescopes and his discovery of moons circling Jupiter in 1610. By 1650, he went about developing a telescope of his own with the help of his brother (Constantijn Huygens, Jr.) and observed the first moon of Saturn.
In 1655, Huygens named it Saturni Luna (Latin for “Saturn’s moon”) in a tract De Saturni Luna Observatio Nova (“A New Observation of Saturn’s Moon”). As Giovanni Domenico Cassini discoveries four more moons around Saturn between 1673 and 1686, astronomers began to refer to them as Saturn I through V (with Titan being in the fourth position as Saturn IV).
A replica of the telescope which William Herschel used to observe Uranus. Credit: Alun Salt/Wikimedia Commons
After William Herschel’s discovery of Mimas and Enceladus in 1789, which are closer to Saturn than any of the larger moons, Saturn’s moons once again had to be re-designated. Thenceforth, Titan status became fixed as Saturn VI, despite the discovery of several smaller moons that were closer to Saturn since then.
The name Titan, along with the names for all the seven major satellites of Saturn, were suggested by William Herschel’s son, John. In 1847, John Herschel published Results of Astronomical Observations Made at the Cape of Good Hope, in which he suggested that the moons be named after the mythological Titans – the brothers and sisters of Cronus, who is the Greek equivalent to Saturn.
In 1907, Spanish astronomer Josep Comas i Solà observed limb darkening of Titan. This effect, where the center part of a planet or star appears brighter than the edge (or limb), was the first indication that Titan had an atmosphere. In 1944, Gerard P. Kuiper used a spectroscopic technique to determine that Titan had an atmosphere composed of methane.
Size. Mass and Orbit:
With a mean radius of 2576 ± 2 km and a mass of 1.345 × 1023 kg, Titan is 0.404 the size of Earth (or 1.480 Moons) and 0.0225 times as massive (1.829 Moons). Its orbit has a minor eccentricity of 0.0288, and its orbital plane is inclined 0.348 degrees relative to Saturn’s equator. It’s average distance from Saturn (semi-major axis) is 1,221,870 km – ranging from 1,186,680 km at periapsis (closest) to 1,257,060 km at apoapsis (farthest).
Diameter comparison of Titan, the Moon, and Earth. Credit: NASA/JPL/Space Science Institute/Gregory H. Revera
Titan takes 15 days and 22 hours to complete a single orbit of Saturn. Like the Moon and many satellites that orbit the other gas giants, its rotational period is identical to its orbital period. Thus, Titan is tidally-locked and in a synchronous rotation with Saturn, which means one face is permanently pointed towards the planet.
Composition and Surface Features:
Though similar in composition to Dione and Enceladus, Titan is denser due to gravitational compression. In terms of diameter and mass (and hence density) Titan is more similar to the Jovian moons of Ganymede and Callisto. Based on its bulk density of 1.88 g/cm3, Titan’s composition is believed to consist half of water ice and half of rocky material.
It’s internal makeup is likely differentiated into several layers, with a 3,400-kilometre (2,100 mi) rocky center surrounded by layers composed of different forms of crystalized ice. Based on evidence provided by the Cassini-Huygens mission in 2005, it is believed that Titan may also have a subsurface ocean which exists between the crust and several deeper layers of high-pressure ice.
This subsurface ocean is believed to be made up of water and ammonia, which allows the water to remain in a liquid state even at temperatures as low as 176 K (-97 °C). Evidence of a systematic shift of the moon’s surface features (which took place between October 2005 and May 2007) suggests that the crust is decoupled from the interior – possibly by a liquid layer in between – as well as the way the gravity field varies as Titan orbits Saturn.
Diagram of the internal structure of Titan according to the fully differentiated dense-ocean model. Credit: Wikipedia Commons/Kelvinsong
The surface of Titan is relatively young – between 100 million and 1 billion years old – despite having been formed during the early Solar System. In addition, it appears to be relatively smooth, with impact craters having been filled in. Height variation is also low, ranging by little more than 150 meters, but with the occasional mountain reaching between 500 meters and 1 km in height.
This is believed to due to geological processes which have reshaped Titan’s surface over time. For example, a range measuring 150 km (93 mi) long, 30 km (19 mi) wide, and 1.5 km (0.93 mi) tall has been potted in the southern hemisphere, composed of icy material and covered in methane snow. The movement of tectonic plates, perhaps influenced by a nearby impact basin, could have opened a gap through which the mountain’s material upwelled.
Then there is Sotra Patera, a chain of mountains that is 1000 to 1500 m (0.62 and 0.93 mi) in height, has some peaks topped by craters, and what appears to be frozen lava flows at its base. If volcanism on Titan really exists, the hypothesis is that it is driven by energy released from the decay of radioactive elements within the mantle, tidal flexing caused by Saturn’s influence, or possibly the interaction of Titan’s subsurface ice layers.
An alternate theory is that Titan is a geologically dead world and that the surface is shaped by a combination of impact cratering, flowing-liquid and wind-driven erosion, mass wasting and other externally-motivated processes. According to this hypothesis, methane is not emitted by volcanoes but slowly diffuses out of Titan’s cold and stiff interior.
Updated maps of Titan, based on the Cassini imaging science subsystem. Credit: NASA/JPL/Space Science Institute
The few impact craters discovered on Titan’s surface include a 440 km (270 mi) wide two-ring impact basin named Menrva, which is identifiable from its bright-dark concentric pattern. A smaller, 60 km (37 mi) wide, flat-floored crater named Sinlap and a 30 km (19 mi) crater with a central peak and dark floor named Ksa have also been observed.
Radar and orbital imaging has also revealed a number of “crateriforms” on the surface, circular features that may be impact related. These include a 90 km (56 mi) wide ring of bright, rough material known as Guabonito, which is thought to be an impact crater filled in by dark, windblown sediment. Several other similar features have been observed in the dark Shangri-la and Aaru regions.
The presence of cryovolcanism has also been theorized based on the fact that there is apparently not enough liquid methane on Titan’s surface (see below) to account for the atmospheric methane. However, to date, the only indications of cryovolcanism are particularly bright and dark features on the surface and 200 m (660 ft) structures resembling lava flows that were spotted in the region called Hotei Arcus.
Titan’s surface is also permeated by streaky features (aka. “sand dunes“), some of which are hundreds of kilometers in length and several meters high. These appear to be caused by powerful, alternating winds that are caused by the interaction of the Sun and Titan’s dense atmosphere. Titan’s surface is also marked by broad regions of bright and dark terrain.
Radar image of rows of dunes on Titan. Credit: NASA/JPL-Caltech
These include Xanadu, a large, reflective equatorial area that was first identified by the Hubble Space Telescope in 1994 and later by the Cassini spacecraft. This region (which is about the same size as Australia) is very diverse, being filled with hills, valleys, chasms and crisscrossed in places by dark lineaments – sinuous topographical features resembling ridges or crevices.
These could be an indication of tectonic activity, which would mean that Xanadu is geologically young. Alternatively, the lineaments may be liquid-formed channels, suggesting old terrain that has been cut through by stream systems. There are dark areas of similar size elsewhere on Titan, which have been revealed to be the patches of water ice and organic compounds that darkened due to exposure to UV radiation.
Methane Lakes:
Titan is also home to its famous “hydrocarbon seas”, lakes of liquid methane and other hydrocarbon compounds. Many of these have been spotted near the polar regions, such as Ontario Lacus. This confirmed methane lake near the south pole has a surface area of 15,000 km² (making it 20% smaller than its namesake, Lake Ontario) and a maximum depth of 7 meters (23 feet).
But the largest body of liquid is Kraken Mare, a methane lake near the north pole. With a surface area of about 400,000 km², it is larger than the Caspian Sea and is estimated to be 160 meters deep. Shallow capillary waves (aka. ripple waves) that are 1.5 centimeters high and moving at speeds of 0.7 meters per second have also been detected.
Mosaic of images taken in near infrared light showing Titan’s polar seas (left) and a radar image of Kraken Mare (right), both taken by the Cassini spacecraft. Credit: NASA/JPL
Then there is Ligeia Mare, the second largest known body of liquid on Titan, which is connected to Kraken Mare and also located near the north pole. With a surface area of about 126,000 km² and a shoreline that is over 2000 km (1240 mi) in length, it is larger than Lake Superior. Much like Kraken Mare, it takes its name from Greek mythology; in this case, after one of the sirens.
It was here that NASA first noticed a bright object measuring 260 km² (100 square miles), which they named “Magic Island”. This object was first spotted in July 2013, then disappeared later, only to reappear again (slightly changed) in August 2014 . It is believed to be inked to Titan’s changing seasons, and suggestions as to what it might be range from surface waves and rising bubbles to floating solids suspended beneath the surface.
Although most of the lakes are concentrated near the poles (where low levels of sunlight prevent evaporation), a number of hydrocarbon lakes have also been discovered in the equatorial desert regions. This includes one near the Huygens landing site in the Shangri-la region, which is about half the size of Utah’s Great Salt Lake. Like desert oases on Earth, it is speculated that these equatorial lakes are fed by underground aquifers.
Overall, the Cassini radar observations have shown that lakes cover only a few percent of the surface, making Titan much drier than Earth. However, the probe also provided strong indications that considerable liquid water exists 100 km below the surface. Further analysis of the data suggests that this ocean may be as salty as the Dead Sea.
During previous flybys, ‘Magic Island’ was not visible near Ligeia Mare’s coastline (left). Then, during Cassini’s July 20, 2013, flyby the feature appeared (right). Credit: NASA/JPL-Caltech/ASI/Cornell
Other studies suggest methane rainfall (see below) on Titan may interact with icy materials underground to produce ethane and propane that may eventually feed into rivers and lakes.
Atmosphere:
Titan is the only moon in the Solar System with a significant atmosphere, and the only body other than Earth who’s atmosphere is nitrogen-rich. Recent observations have shown that Titan’s atmosphere is denser than Earth’s, with a surface pressure of about 1.469 KPa – 1.45 times that of Earths. It is also about 1.19 times as massive as Earth’s atmosphere overall, or about 7.3 times more massive on a per-surface-area basis.
The atmosphere is made up of opaque haze layers and other sources that block most visible light from the Sun and obscure its surface features (similar to Venus). Titan’s lower gravity also means that its atmosphere is far more extended than Earth’s. In the stratosphere, the atmospheric composition is 98.4% nitrogen with the remaining 1.6% composed mostly of methane (1.4%) and hydrogen (0.1–0.2%).
There are trace amounts of other hydrocarbons, such as ethane, diacetylene, methylacetylene, acetylene and propane; as well as other gases such as cyanoacetylene, hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen, argon and helium. The hydrocarbons are thought to form in Titan’s upper atmosphere in reactions resulting from the breakup of methane by the Sun’s ultraviolet light, producing a thick orange smog.
Energy from the Sun should have converted all traces of methane in Titan’s atmosphere into more complex hydrocarbons within 50 million years—a short time compared to the age of the Solar System. This suggests that methane must be replenished by a reservoir on or within Titan itself. The ultimate origin of the methane in its atmosphere may be its interior, released via eruptions from cryovolcanoes.
False color image of Titan’s atmosphere. Credit: NASA/JPL/Space Science Institute/ESA
Titan’s surface temperature is about 94 K (-179.2 °C), which is due to the fact that Titan receives about 1% as much sunlight as Earth. At this temperature, water ice has an extremely low vapor pressure, so the little water vapor present appears limited to the stratosphere. The moon would be much colder, were it not for the fact that the atmospheric methane creates a greenhouse effect on Titan’s surface.
Conversely, haze in Titan’s atmosphere contributes to an anti-greenhouse effect by reflecting sunlight back into space, cancelling a portion of the greenhouse effect and making its surface significantly colder than its upper atmosphere. In addition, Titan’s atmosphere periodically rains liquid methane and other organic compounds onto its surface.
Based on studies simulating the atmosphere of Titan, NASA scientists have speculated that complex organic molecules could arise on Titan (see below). In addition, propene – aka. propylene, a class of hydrocarbon – has also been detected in Titan’s atmosphere. This is the first time propene has been found on any moon or planet other than Earth, and is thought to be formed from recombined radicals created by the UV photolysis of methane.
Habitability:
Titan is thought to be a prebiotic environment rich in complex organic chemistry with a possible subsurface liquid ocean serving as a biotic environment. Ongoing research of Titan’s atmosphere has led many scientists to theorize that conditions there are similar to what existed on a primordial Earth, with the important exception of a lack of water vapor.
Numerous experiments have shown that an atmosphere similar to that of Titan, with the addition of UV radiation, could give rise to complex molecules and polymer substances like tholins. In addition, independent research conducted by the University of Arizona reported that when energy was applied to a combination of gases like those found in Titan’s atmosphere, many organic compounds were produced. These includes the five nucleotide bases – the building blocks of DNA and RNA – as well as amino acids, which are the building blocks of protein.
Multiple laboratory simulations have been conducted that have led to the suggestion that enough organic material exists on Titan to start a chemical evolution process analogous to what is thought to have started life here on Earth. While this theory assumes the presence of water that would remain in a liquid state for longer periods that have been observed, organic life could theoretically survive in Titan’s hypothetical subsurface ocean.
Much like on Europa and other moons, this life would likely take the form of extremophiles – organisms that thrive in extreme environments. Heat transfer between the interior and upper layers would be critical in sustaining any subsurface oceanic life, most likely through hydrothermal vents located at the ocean-core boundary. That the atmospheric methane and nitrogen might be of biological origin has also been examined.
It has also been suggested that life could exist in Titan’s lakes of liquid methane, just as organisms on Earth live in water. Such organisms would inhale dihydrogen (H²) in place of oxygen gas (O²), metabolize it with acetylene instead of glucose, and then exhale methane instead of carbon dioxide. Although all living things on Earth use liquid water as a solvent, it is speculated that life on Titan could actually live in liquid hydrocarbons.
Several experiments and models have been constructed to test this hypothesis. For instance, atmospheric models have shown that molecular hydrogen is in greater abundance in the upper atmosphere and disappears near the surface – which is consistent with the possibility of methanogenic life-forms. Another study has shown that there are low levels of acetylene on Titan’s surface, which is also in line with the hypothesis of organisms consuming hydrocarbons.
In 2015, a team of chemical engineers at Cornell University went as far as to construct a hypothetical cell membrane that was capable of functioning in liquid methane under conditions similar to that on Titan. Composed of small molecules containing carbon, hydrogen, and nitrogen, this cell was said to have the same stability and flexibility as cell membranes on Earth. This hypothetical cell membrane was termed an “azotosome” (a combination of “azote”, French for nitrogen, and “liposome”).
However, NASA has gone on record as stating that these theories remain entirely hypothetical. Furthermore, it has been emphasized that other theories as to why hydrogen and acetylene levels are lower nearer to the surface are more plausible. These include a as-of-yet unidentified physical or chemical processes – such as a surface catalyst accepting hydrocarbons or hydrogen – or the existence of flaws in the current models of material flow.
Also, life on Titan would face tremendous obstacles compared to life on Earth – thus making any analogy to Earth problematic. For one, Titan is too far from the Sun, and its atmosphere lacks carbon monoxide (CO), which results in it not retaining enough heat or energy to trigger biological processes. Also, water only exists on Titan’s surface in solid form.
So while the prebiotic conditions that are associated with organic chemistry exist on Titan, life itself may not. However, the existence of these conditions remains a subject of fascination among scientists. And since its atmosphere is thought to be analogous to Earth’s in the distant past, researching Titan could help advance our understanding of the early history of the terrestrial biosphere.
Exploration:
Titan cannot be spotted without the help of instrumentation, and is often difficult for amateur astronomers because of interference from Saturn’s brilliant globe and ring system. And even after the development of high-powered telescopes, Titan’s dense, hazy, atmosphere made observations of the surface very difficult. Hence, observations of both Titan and its surface features prior to the space age were limited.
The first probe to visit the Saturnian system was Pioneer 11 in 1979, which took images of Titan and Saturn together and revealed that Titan was probably too cold to support life. Titan was examined in 1980 and 1981 by both the Voyager 1 and 2 space probes, respectively. While Voyager 2 managed to take snapshots of Titan on its way to Uranus and Neptune, only Voyager 1 managed to conduct a flyby and take pictures and readings.
This included readings on Titan’s density, composition, and temperature of the atmosphere, and obtain a precise measurement of Titan’s mass. Atmospheric haze prevented direct imaging of the surface; though in 2004, intensive digital processing of images taken through Voyager 1‘s orange filter did reveal hints of the light and dark features now known as Xanadu and Shangri-la.
Voyager 2 photograph of Titan, taken on Aug. 23rd, 1981, which shows some detail in the cloud systems on this Saturnian moon. Credit: NASA/JPL
Even so, much of the mystery surrounding Titan would not begin to be dispelled until the Cassini-Huygens mission – a joint project between NASA and the European Space Agency (ESA) named in honor of the astronomers who made the greatest discoveries involving Saturn’s moons. The spacecraft reached Saturn on July 1st, 2004, and began the process of mapping Titan’s surface by radar.
The Cassini probe flew by Titan on October 26th, 2004, and took the highest-resolution images ever of Titan’s surface, discerning patches of light and dark that were otherwise invisible to the human eye. Over the course of many close flybys of Titan, Cassini managed to detect abundant sources of liquid on the surface in the north polar region, in the form of many lakes and seas.
The Huygens probe landed on Titan on January 14th, 2005, making Titan the most distant body from Earth to have a space probe land on its surface. During the course of its investigations, it would discover that many of the surface features appear to have been formed by fluids at some point in the past.
After landing just off the easternmost tip of the bright region now called Adiri, the probe photographed pale hills with dark “rivers” running down to a dark plain. The current theory is that these hills (aka. “highlands”) are composed mainly of water ice, and that dark organic compounds – created in the upper atmosphere – may come down from Titan’s atmosphere with methane rain and become deposited on the plains over time.
Artist depiction of Huygens landing on Titan. Credit: ESA
Huygens also obtained photographs of a dark plain covered in small rocks and pebbles (composed of water ice) that showed evidence of erosion and/or fluvial activity. The surface is darker than originally expected, consisting of a mixture of water and hydrocarbon ice. The “soil” visible in the images is interpreted to be precipitation from the hydrocarbon haze above.
Several proposals for returning a robotic space probe to Titan have been made in recent years. These include the Titan Saturn System Mission (TSSM) – a joint NASA/ESA proposal for the exploration of Saturn’s moons – that envisions a hot-air balloon floating in Titan’s atmosphere and conducting research for a period of six months.
In 2009, it was announced that the TSSM lost out to a competing concept known the Europa Jupiter System Mission (EJSM) – a joint NASA/ESA mission that will consist of sending two probes to Europa and Ganymede to study their potential habitability.
There was also a proposal known as Titan Mare Explorer (TiME), a concept under consideration by NASA in conjunction with Lockheed Martin. This mission would involve a low-cost lander splashing down in a lake in Titan’s northern hemisphere and floating on the surface of the lake for 3 to 6 months. However, NASA announced in 2012 that it favored the lower-cost InSight Mars lander instead, which is scheduled to be sent to Mars in 2016.
Another mission to Titan was proposed in early 2012 by Jason Barnes, a scientist at the University of Idaho. Known as the Aerial Vehicle for In-situ and Airborne Titan Reconnaissance (AVIATR), this unmanned plane (or drone) would fly through Titan’s atmosphere and take high-definition images of the surface. NASA did not approve the requested $715 million at the time and the future of the project is uncertain.
Another lake lander project known as the Titan Lake In-situ Sampling Propelled Explorer (TALISE) was proposed in late 2012 by the Spanish-based private engineering firm SENER and the Centro de Astrobiología in Madrid. The major difference between this and the TiME probe is that the TALISE concept includes its own propulsion system, and would therefore not be limited to simply drifting on the lake when it splashes down.
In response to NASA’s 2010 Discovery Announcement, the concept known as Journey to Enceladus and Titan (JET) was proposed. Developed by Caltech and JPL, this mission would consist of a low-cost astrobiology orbiter that would be sent to the Saturnian system to asses the habitability potential of Enceladus and Titan.
In 2015, NASA’s Innovative Advanced Concepts (NIAC) awarded a Phase II grant to a proposed robotic submarine in order to further investigate and develop the concept. This submarine explorer, should it be deployed to Titan, will explore the depths of Kraken Mare to investigate its makeup and potential for supporting life.
Colonization:
The colonization of the Saturn system presents numerous advantages compared to other gas giants in the Solar System. According to Dr. Robert Zubrin – an American aerospace engineer, author, and an advocate for the exploration Mars – these include its relative proximity to Earth, its low radiation, and its excellent system of moons. Zubrin has also stated that Titan is the most important of these moons when it comes to building a base to develop the system’s resources.
Artist’s conception of possible Titan “floater” designed by NASA and the ESA. Credit: bisbos.com
For starters, Titan possess an abundance of all the elements necessary to support life, such as atmospheric nitrogen and methane, liquid methane, and liquid water and ammonia. Water could easily be used to generate breathable oxygen, and nitrogen is ideal as a buffer gas to create a pressurized, breathable atmosphere. In addition, nitrogen, methane and ammonia could all be used to produce fertilizer for growing food.
Additionally, Titan has an atmospheric pressure one and a half times that of Earth, which means that the interior air pressure of landing craft and habitats could be set equal or close to the exterior pressure. This would significantly reduce the difficulty and complexity of structural engineering for landing craft and habitats compared with low or zero pressure environments such as on the Moon, Mars, or the Asteroid Belt.
The thick atmosphere also makes radiation a non-issue, unlike with other planets or Jupiter’s moons. And while Titan’s atmosphere does contain flammable compounds, these only present a danger if they are mixed with sufficient enough oxygen – otherwise, combustion cannot be achieved or sustained. Finally, the very high ratio of atmospheric density to surface gravity also greatly reduces the wingspan needed for aircraft to maintain lift.
Beyond this, Titan presents many challenges for human colonization. For starters, the moon has a surface gravity of 0.138 g, which is slightly less than that of the Moon. Managing the long-term effects of this presents a challenge, and what those effects would be (especially for children born on Titan) are not currently known. However, they would likely include loss of bone density, muscle deterioration, and a weakened immune system.
Artist’s impression of future colonists flying over Ligeia Mare on Titan. Credit: Erik Wernquist/erikwernquist.com
The temperature on Titan is also considerably lower than on Earth, with an average temperature of 94 K (-179 °C, or -290.2 °F). Combined with the increased atmospheric pressure, temperatures vary very little over time and from one local to the next. Unlike in a vacuum, the high atmospheric density makes thermoinsulation a significant engineering problem. Nevetherless, compared to other cases for colonization, the problems associated with creating a human presence on Titan are relatively surmountable.
Titan is a moon that is shrouded in mystery, both literally and metaphorically. Until very recently, we were unable to discern what secrets it held because its atmosphere was simply too thick to see beneath. However, in recent years, we have managed to pull back that shroud and get a better look at the moon’s surface. But in many ways, doing this has only confounded the sense of mystery surrounding this world.
Perhaps someday we will send astronauts to Titan and find life forms there that completely alter our conception of what life is and where it can thrive. Perhaps we will find only extremophiles, life forms that live in the deepest parts of its interior ocean huddled around hydorthermal vents, since these spots are the only place on Titan where lifeforms can exist.
Perhaps we will even colonize Titan someday, and use it as a base for further exploration of the Solar System and resource extraction. Then, we may come to know the pleasures of looking up at a ringed planet in the sky while sailing on a methane lake, the hazy light of the Sun trickling down onto the cold, hydrocarbon seas. One can only hope… and dream!
We have many interesting articles about Titan here at Universe Today. Here are some on Titan’s atmosphere, it’s mysterious sand dunes, and how we might explore it with a robotic sailboat.
For more information on Titan’s methane lakes, check out this article on Titan’s north pole, and this one about Kraken Mare.
Special Guests:
Dr. Sara Seager, whose research focuses on computer models of exoplanet atmospheres, interiors, and biosignatures. Her favorite projects involve the search for planets like Earth with signs of life
on them.
Neptune photographed by Voyager 2. Image credit: NASA/JPL
Neptune is the eight planet from our Sun, one of the four gas giants, and one of the four outer planets in our Solar System. Since the “demotion” of Pluto by the IAU to the status of a dwarf planet – and/or Plutoid and Kuiper Belt Object (KBO) – Neptune is now considered to be the farthest planet in our Solar System.
As one of the planets that cannot be seen with the naked eye, Neptune was not discovered until relatively recently. And given its distance, it has only been observed up close on one occasion – in 1989 by the Voyager 2 spaceprobe. Nevertheless, what we’ve come to know about this gas (and ice) giant in that time has taught us much about the outer Solar System and the history of its formation.
Discovery and Naming:
Neptune’s discovery did not take place until the 19th century, though there are indications that it was observed before long that. For instance, Galileo’s drawings from December 28th, 1612, and January 27th, 1613, contained plotted points which are now known to match up with the positions of Neptune on those dates. However, in both cases, Galileo appeared to have mistaken it for a star.
1821, French astronomer Alexis Bouvard published astronomical tables for the orbit of Uranus. Subsequent observations revealed substantial deviations from the tables, which led Bouvard to hypothesize that an unknown body was perturbing Uranus’ orbit through gravitational interaction.
New Berlin Observatory at Linden Street, where Neptune was discovered observationally. Credit: Leibniz-Institute for Astrophysics Potsdam
In 1843, English astronomer John Couch Adams began work on the orbit of Uranus using the data he had and produced several different estimates in the following years of the planet’s orbit. In 1845–46, Urbain Le Verrier, independently of Adams, developed his own calculations, which he shared with Johann Gottfried Galle of the Berlin Observatory. Galle confirmed the presence of a planet at the coordinates specified by Le Verrier on September 23rd, 1846.
The announcement of the discovery was met with controversy, as both Le Verrier and Adams claimed responsibility. Eventually, an international consensus emerged that both Le Verrier and Adams jointly deserved credit. However, a re-evaluation by historians in 1998 of the relevant historical documents led to the conclusion that Le Verrier was more directly responsible for the discovery and deserves the greater share of the credit.
Claiming the right of discovery, Le Verrier suggested the planet be named after himself, but this met with stiff resistance outside of France. He also suggested the name Neptune, which was gradually accepted by the international community. This was largely because it was consistent with the nomenclature of the other planets, all of which were named after deities from Greco-Roman mythology.
Neptune’s Size, Mass and Orbit:
With a mean radius of 24,622 ± 19 km, Neptune is the fourth largest planet in the Solar System and four times as large as Earth. But with a mass of 1.0243×1026 kg – which is roughly 17 times that of Earth – it is the third most massive, outranking Uranus. The planet has a very minor eccentricity of 0.0086, and orbits the Sun at a distance of 29.81 AU (4.459 x 109 km) at perihelion and 30.33 AU (4.537 x 109 km) at aphelion.
A size comparison of Neptune and Earth. Credit: NASA
Neptune takes 16 h 6 min 36 s (0.6713 days) to complete a single sidereal rotation, and 164.8 Earth years to complete a single orbit around the Sun. This means that a single day lasts 67% as long on Neptune, whereas a year is the equivalent of approximately 60,190 Earth days (or 89,666 Neptunian days).
Because Neptune’s axial tilt (28.32°) is similar to that of Earth (~23°) and Mars (~25°), the planet experiences similar seasonal changes. Combined with its long orbital period, this means that the seasons last for forty Earth years. Also owing to its axial tilt being comparable to Earth’s is the fact that the variation in the length of its day over the course of the year is not any more extreme than it on Earth.
Neptune’s orbit also has a profound impact on the region directly beyond it, known as the Kuiper Belt (otherwise known as the “Trans-Neptunian Region”). Much in the same way that Jupiter’s gravity dominates the Asteroid Belt, shaping its structure, so Neptune’s gravity dominates the Kuiper Belt. Over the age of the Solar System, certain regions of the Kuiper belt became destabilised by Neptune’s gravity, creating gaps in the Kuiper belt’s structure.
There also exists orbits within these empty regions where objects can survive for the age of the Solar System. These resonances occur when Neptune’s orbital period is a precise fraction of that of the object – meaning they complete a fraction of an orbit for every orbit made by Neptune. The most heavily populated resonance in the Kuiper belt, with over 200 known objects, is the 2:3 resonance.
Objects in this resonance complete 2 orbits for every 3 of Neptune, and are known as plutinos because the largest of the known Kuiper belt objects, Pluto, is among them. Although Pluto crosses Neptune’s orbit regularly, the 2:3 resonance ensures they can never collide.
Neptune has a number of known trojan objects occupying both the Sun–Neptune L4 and L5Lagrangian Points – regions of gravitational stability leading and trailing Neptune in its orbit. Some Neptune trojans are remarkably stable in their orbits, and are likely to have formed alongside Neptune rather than being captured.
Neptune’s Composition:
Due to its smaller size and higher concentrations of volatiles relative to Jupiter and Saturn, Neptune (much like Uranus) is often referred to as an “ice giant” – a subclass of a giant planet. Also like Uranus, Neptune’s internal structure is differentiated between a rocky core consisting of silicates and metals; a mantle consisting of water, ammonia and methane ices; and an atmosphere consisting of hydrogen, helium and methane gas.
The core of Neptune is composed of iron, nickel and silicates, with an interior model giving it a mass about 1.2 times that of Earth. The pressure at the center is estimated to be 7 Mbar (700 GPa), about twice as high as that at the center of Earth, and with temperatures as high as 5,400 K. At a depth of 7000 km, the conditions may be such that methane decomposes into diamond crystals that rain downwards like hailstones.
The mantle is equivalent to 10 – 15 Earth masses and is rich in water, ammonia and methane. This mixture is referred to as icy even though it is a hot, dense fluid, and is sometimes called a “water-ammonia ocean”. Meanwhile, the atmosphere forms about 5% to 10% of its mass and extends perhaps 10% to 20% of the way towards the core, where it reaches pressures of about 10 GPa – or about 100,000 times that of Earth’s atmosphere.
Composition of Neptune. Image credit: NASA
Increasing concentrations of methane, ammonia and water are found in the lower regions of the atmosphere. Unlike Uranus, Neptune’s composition has a higher volume of ocean, whereas Uranus has a smaller mantle.
Neptune’s Atmosphere:
At high altitudes, Neptune’s atmosphere is 80% hydrogen and 19% helium, with a trace amount of methane. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue, although Neptune’s is darker and more vivid. Because Neptune’s atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune’s more intense coloring.
Neptune’s atmosphere is subdivided into two main regions: the lower troposphere (where temperature decreases with altitude), and the stratosphere (where temperature increases with altitude). The boundary between the two, the tropopause, lies at a pressure of 0.1 bars (10 kPa). The stratosphere then gives way to the thermosphere at a pressure lower than 10-5 to 10-4 microbars (1 to 10 Pa), which gradually transitions to the exosphere.
Neptune’s spectra suggest that its lower stratosphere is hazy due to condensation of products caused by the interaction of ultraviolet radiation and methane (i.e. photolysis), which produces compounds such as ethane and ethyne. The stratosphere is also home to trace amounts of carbon monoxide and hydrogen cyanide, which are responsible for Neptune’s stratosphere being warmer than that of Uranus.
A modified color/contrast image emphasizing Neptune’s atmospheric features, including wind speed. Credit Erich Karkoschka)
For reasons that remain obscure, the planet’s thermosphere experiences unusually high temperatures of about 750 K (476.85 °C/890 °F). The planet is too far from the Sun for this heat to be generated by ultraviolet radiation, which means another heating mechanism is involved – which could be the atmosphere’s interaction with ion’s in the planet’s magnetic field, or gravity waves from the planet’s interior that dissipate in the atmosphere.
Because Neptune is not a solid body, its atmosphere undergoes differential rotation. The wide equatorial zone rotates with a period of about 18 hours, which is slower than the 16.1-hour rotation of the planet’s magnetic field. By contrast, the reverse is true for the polar regions where the rotation period is 12 hours.
This differential rotation is the most pronounced of any planet in the Solar System, and results in strong latitudinal wind shear and violent storms. The three most impressive were all spotted in 1989 by the Voyager 2 space probe, and then named based on their appearances.
The first to be spotted was a massive anticyclonic storm measuring 13,000 x 6,600 km and resembling the Great Red Spot of Jupiter. Known as the Great Dark Spot, this storm was not spotted five later (Nov. 2nd, 1994) when the Hubble Space Telescope looked for it. Instead, a new storm that was very similar in appearance was found in the planet’s northern hemisphere, suggesting that these storms have a shorter life span than Jupiter’s.
Reconstruction of Voyager 2 images showing the Great Black spot (top left), Scooter (middle), and the Small Black Spot (lower right). Credit: NASA/JPL
The Scooter is another storm, a white cloud group located farther south than the Great Dark Spot. This nickname first arose during the months leading up to the Voyager 2 encounter in 1989, when the cloud group was observed moving at speeds faster than the Great Dark Spot.
The Small Dark Spot, a southern cyclonic storm, was the second-most-intense storm observed during the 1989 encounter. It was initially completely dark; but as Voyager 2 approached the planet, a bright core developed and could be seen in most of the highest-resolution images.
Neptune’s Moons:
Neptune has 14 known satellites, all but one of which are named after Greek and Roman deities of the sea (S/2004 N 1 is currently unnamed). These moons are divided into two groups – the regular and irregular moons – based on their orbit and proximity to Neptune. Neptune’s Regular Moons – Naiad, Thalassa, Despina, Galatea, Larissa, S/2004 N 1, and Proteus – are those that are closest to the planet and which follow circular, prograde orbits that lie in the planet’s equatorial plane.
They range in distance from 48,227 km (Naiad) to 117,646 km (Proteus) from Neptune, and all but the outermost two (S/2004 N 1, and Proteus) orbit Neptune slower than its orbital period of 0.6713 days. Based on observational data and assumed densities, these moons range in size and mass from 96 x 60 x 52 km and 1.9 x 1017 kg (Naiad) to 436 x 416 x 402 km and 50.35 x 1017 kg (Proteus).
This composite Hubble Space Telescope picture shows the location of a newly discovered moon, designated S/2004 N 1, orbiting the giant planet Neptune, nearly 4.8 billion km (3 billion miles) from Earth. Credit: NASA, ESA, and M. Showalter (SETI Institute).
With the exception of Larissa and Proteus (which are largely rounded) all of Neptune’s inner moons are believed to be elongated in shape. Their spectra also indicates that they are made from water ice contaminated by some very dark material, probably organic compounds. In this respect, the inner Neptunian moons are similar to the inner moons of Uranus.
Neptune’s irregular moons consist of the planet’s remaining satellites (including Triton). They generally follow inclined eccentric and often retrograde orbits far from Neptune. The only exception is Triton, which orbits close to the planet, following a circular orbit, though retrograde and inclined.
In order of their distance from the planet, the irregular moons are Triton, Nereid, Halimede, Sao, Laomedeia, Neso and Psamathe – a group that includes both prograde and retrograde objects. With the exception of Triton and Nereid, Neptune’s irregular moons are similar to those of other giant planets and are believed to have been gravitationally captured by Neptune.
In terms of size and mass, the irregular moons are relatively consistent, ranging from approximately 40 km in diameter and 4 x 1016 kg in mass (Psamathe) to 62 km and 16 x 1016 kg for Halimede. Triton and Nereid are unusual irregular satellites and are thus treated separately from the other five irregular Neptunian moons. Between these two and the other irregular moons, four major differences have been noted.
First of all, they are the largest two known irregular moons in the Solar System. Triton itself is almost an order of magnitude larger than all other known irregular moons and comprises more than 99.5% of all the mass known to orbit Neptune (including the planet’s rings and thirteen other known moons).
Global Color Mosaic of Triton, taken by Voyager 2 in 1989. Credit: NASA/JPL/USGS
Secondly, they both have atypically small semi-major axes, with Triton’s being over an order of magnitude smaller than those of all other known irregular moons. Thirdly, they both have unusual orbital eccentricities: Nereid has one of the most eccentric orbits of any known irregular satellite, and Triton’s orbit is a nearly perfect circle. Finally, Nereid also has the lowest inclination of any known irregular satellite
With a mean diameter of around 2700 km and a mass of 214080 ± 520 x 1017 kg, Triton is the largest of Neptune’s moons, and the only one large enough to achieve hydrostatic equilibrium (i.e. is spherical in shape). At a distance of 354,759 km from Neptune, it also sits between the planet’s inner and outer moons.
Triton follows a retrograde and quasi-circular orbit, and is composed largely of nitrogen, methane, carbon dioxide and water ices. With a geometric albedo of more than 70% and a Bond albedo as high as 90%, it is also one of the brightest objects in the Solar System. The surface has a reddish tint, owning to the interaction of ultraviolet radiation and methane, causing tholins.
Triton is also one of the coldest moons in the Solar System, with surface temperature of about 38 K (-235.2 °C). However, owing to the moon being geologically active (which results in cryovolcanism) and surface temperature variations that cause sublimation, Triton is one of only two moons in the Solar System that has a substantial atmosphere. Much like it’s surface, this atmosphere is composed primarily of nitrogen with small amounts of methane and carbon monoxide, and with an estimated pressure of about 14 nanobar.
Triton has a relatively high density of about 2 g/cm3 indicating that rocks constitute about two thirds of its mass, and ices (mainly water ice) the remaining one third. There also may be a layer of liquid water deep inside Triton, forming a subterranean ocean. Surface features include the large southern polar cap, older cratered planes cross-cut by graben and scarps, as well as youthful features caused by endogenic resurfacing.
Because of its retrograde orbit and relative proximity to Neptune (closer than the Moon is to Earth), Triton is grouped with the planet’s irregular moons (see below). In addition, it is believed to be a captured object, possibly a dwarf planet that was once part of the Kuiper Belt. At the same time, these orbital characteristics are the reason why Triton experiences tidal deceleration. and will eventually spiral inward and collide with the planet in about 3.6 billion years.
Nereid is the third-largest moon of Neptune. It has a prograde but very eccentric orbit and is believed to be a former regular satellite that was scattered to its current orbit through gravitational interactions during Triton’s capture. Water ice has been spectroscopically detected on its surface. Nereid shows large, irregular variations in its visible magnitude, which are probably caused by forced precession or chaotic rotation combined with an elongated shape and bright or dark spots on the surface.
Neptune’s Ring System:
Neptune has five rings, all of which are named after astronomers who made important discoveries about the planet – Galle, Le Verrier, Lassell, Arago, and Adams. The rings are composed of at least 20% dust (with some containing as much as 70%) while the rest of the material consists of small rocks. The planet’s rings are difficult to see because they are dark and vary in density and size.
The Galle ring was named after Johann Gottfried Galle, the first person to see the planet using a telescope; and at 41,000–43,000 km, it is the nearest of Neptune’s rings. The La Verrier ring – which is very narrow at 113 km in width – is named after French astronomer Urbain Le Verrier, the planet’s co-founder.
At a distance of between 53,200 and 57,200 km from Neptune (giving it a width of 4,000 km) the Lassell ring is the widest of Neptune’s rings. This ring is named after William Lassell, the English astronomer who discovered Triton just seventeen days after Neptune was discovered. The Arago ring is 57,200 kilometers from the planet and less than 100 kilometers wide. This ring section is named after Francois Arago, Le Verrier’s mentor and the astronomer who played an active role in the dispute over who deserved credit for discovering Neptune.
The outer Adams ring was named after John Couch Adams, who is credited with the co-discovery of Neptune. Although the ring is narrow at only 35 kilometers wide, it is the most famous of the five due to its arcs. These arcs accord with areas in the ring system where the material of the rings is grouped together in a clump, and are the brightest and most easily observed parts of the ring system.
Although the Adams ring has five arcs, the three most famous are the “Liberty”, “Equality”, and “Fraternity” arcs. Scientists have been traditionally unable to explain the existence of these arcs because, according to the laws of motion, they should distribute the material uniformly throughout the rings. However, stronomers now estimate that the arcs are corralled into their current form by the gravitational effects of Galatea, which sits just inward from the ring.
The rings of Neptune as seen from Voyager 2 during the 1989 flyby. Credit: NASA/JPL
The rings of Neptune are very dark, and probably made of organic compounds that have been altered due to exposition to cosmic radiation. This is similar to the rings of Uranus, but very different to the icy rings around Saturn. They seem to contain a large quantity of micrometer-sized dust, similar in size to the particles in the rings of Jupiter.
It’s believed that the rings of Neptune are relatively young – much younger than the age of the Solar System, and much younger than the age of Uranus’ rings. Consistent with the theory that Triton was a KBO that was seized, by Neptune’s gravity, they are believed to be the result of a collision between some of the planet’s original moons.
Exploration:
The Voyager 2 probe is the only spacecraft to have ever visited Neptune. The spacecraft’s closest approach to the planet occurred on August 25th, 1989, which took place at a distance of 4,800 km (3,000 miles) above Neptune’s north pole. Because this was the last major planet the spacecraft could visit, it was decided to make a close flyby of the moon Triton – similar to what had been done for Voyager 1‘s encounter with Saturn and its moon Titan.
The spacecraft performed a near-encounter with the moon Nereid before it came to within 4,400 km of Neptune’s atmosphere on August 25th, then passed close to the planet’s largest moon Triton later the same day. The spacecraft verified the existence of a magnetic field surrounding the planet and discovered that the field was offset from the center and tilted in a manner similar to the field around Uranus.
Neptune’s rotation period was determined using measurements of radio emissions and Voyager 2 also showed that Neptune had a surprisingly active weather system. Six new moons were discovered during the flyby, and the planet was shown to have more than one ring.
While no missions to Neptune are currently being planned, some hypothetical missions have been suggested. For instance, a possible Flagship Mission has been envisioned by NASA to take place sometime during the late 2020s or early 2030s. Other proposals include a possible Cassini-Huygens-style “Neptune Orbiter with Probes”, which was suggested back in 2003.
Another, more recent proposal by NASA was for Argo – a flyby spacecraft that would be launched in 2019, which would visit Jupiter, Saturn, Neptune, and a Kuiper belt object. The focus would be on Neptune and its largest moon Triton, which would be investigated around 2029.
With its icy-blue color, liquid surface, and wavy weather patterns, Neptune was appropriately named after the Roman god of the sea. And given its distance from our planet, there is still a great deal that remains to be learned about it. In the coming decades, one can only hope that a mission to the outer Solar System and/or Kuiper Belt includes a flyby of Neptune.
We have many interesting articles about Neptune here at Universe Today. Below is a comprehensive list for your viewing (and possibly researching) pleasure!
The Universe is a very big place, and we occupy a very small corner of it. Known as the Solar System, our stomping grounds are not only a tiny fraction of the Universe as we know it, but is also a very small part of our galactic neighborhood (aka. the Milky Way Galaxy). When it comes right down to it, our world is just a drop of water in an endless cosmic sea.
Nevertheless, the Solar System is still a very big place, and one which is filled with its fair share of mysteries. And in truth, it was only within the relatively recent past that we began to understand its true extent. And when it comes to exploring it, we’ve really only begun to scratch the surface.
Discovery:
With very few exceptions, few people or civilizations before the era of modern astronomy recognized the Solar System for what it was. In fact, the vast majority of astronomical systems posited that the Earth was a stationary object and that all known celestial objects revolved around it. In addition, they viewed it as being fundamentally different from other stellar objects, which they held to be ethereal or divine in nature.
Although there were some Greek, Arab and Asian astronomers during Antiquity and the Medieval period who believed that the universe was heliocentric in nature (i.e. that the Earth and other bodies revolved around the Sun) it was not until Nicolaus Copernicus developed his mathematically predictive model of a heliocentric system in the 16th century that it began to become widespread.
Galileo (1564 – 1642) would often show people how to use his telescope to view the sky in Saint Mark’s square in Venice. Note the lack of adaptive optics. Credit: Public Domain
During the 17th-century, scientists like Galileo Galilei, Johannes Kepler, and Isaac Newton developed an understanding of physics which led to the gradual acceptance that the Earth revolves round the Sun. The development of theories like gravity also led to the realization that the other planets are governed by the same physical laws as Earth.
The widespread use of the telescope also led to a revolution in astronomy. After Galileo discovered the moons of Jupiter in 1610, Christian Huygens would go on to discover that Saturn also had moons in 1655. In time, new planets would also be discovered (such as Uranus and Neptune), as well as comets (such as Halley’s Comet) and the Asteroids Belt.
By the 19th century, three observations made by three separate astronomers determined the true nature of the Solar System and its place the universe. The first was made in 1839 by German astronomer Friedrich Bessel, who successfully measured an apparent shift in the position of a star created by the Earth’s motion around the Sun (aka. stellar parallax). This not only confirmed the heliocentric model beyond a doubt, but revealed the vast distance between the Sun and the stars.
In 1859, Robert Bunsen and Gustav Kirchhoff (a German chemist and physicist) used the newly invented spectroscope to examined the spectral signature of the Sun. They discovered that it was composed of the same elements as existed on Earth, thus proving that Earth and the heavens were composed of the same elements.
With parallax technique, astronomers observe object at opposite ends of Earth’s orbit around the Sun to precisely measure its distance. Credit: Alexandra Angelich, NRAO/AUI/NSF.
Then, Father Angelo Secchi – an Italian astronomer and director at the Pontifical Gregorian University – compared the spectral signature of the Sun with those of other stars, and found them to be virtually identical. This demonstrated conclusively that our Sun was composed of the same materials as every other star in the universe.
Further apparent discrepancies in the orbits of the outer planets led American astronomer Percival Lowell to conclude that yet another planet, which he referred to as “Planet X“, must lie beyond Neptune. After his death, his Lowell Observatory conducted a search that ultimately led to Clyde Tombaugh’s discovery of Pluto in 1930.
Also in 1992, astronomers David C. Jewitt of the University of Hawaii and Jane Luu of the MIT discovered the Trans-Neptunian Object (TNO) known as (15760) 1992 QB1. This would prove to be the first of a new population, known as the Kuiper Belt, which had already been predicted by astronomers to exist at the edge of the Solar System.
Further investigation of the Kuiper Belt by the turn of the century would lead to additional discoveries. The discovery of Eris and other “plutoids” by Mike Brown, Chad Trujillo, David Rabinowitz and other astronomers would lead to the Great Planet Debate – where IAU policy and the convention for designating planets would be contested.
The Sun contains 99.86% of the system’s known mass, and its gravity dominates the entire system. Most large objects in orbit around the Sun lie near the plane of Earth’s orbit (the ecliptic) and most planets and bodies rotate around it in the same direction (counter-clockwise when viewed from above Earth’s north pole). The planets are very close to the ecliptic, whereas comets and Kuiper belt objects are frequently at greater angles to it.
It’s four largest orbiting bodies (the gas giants) account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four terrestrial planets, the dwarf planets, moons, asteroids, and comets) together comprise less than 0.002% of the Solar System’s total mass.
The Sun and planets to scale. Credit: Illustration by Judy Schmidt, texture maps by Björn Jónsson
Astronomers sometimes informally divide this structure into separate regions. First, there is the Inner Solar System, which includes the four terrestrial planets and the Asteroid Belt. Beyond this, there’s the outer Solar System that includes the four gas giant planets. Meanwhile, there’s the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune (i.e. Trans-Neptunian Objects).
Most of the planets in the Solar System possess secondary systems of their own, being orbited by planetary objects called natural satellites (or moons). In the case of the four giant planets, there are also planetary rings – thin bands of tiny particles that orbit them in unison. Most of the largest natural satellites are in synchronous rotation, with one face permanently turned toward their parent.
The Sun, which comprises nearly all the matter in the Solar System, is composed of roughly 98% hydrogen and helium. The terrestrial planets of the Inner Solar System are composed primarily of silicate rock, iron and nickel. Beyond the Asteroid Belt, planets are composed mainly of gases (such as hydrogen, helium) and ices – like water, methane, ammonia, hydrogen sulfide and carbon dioxide.
Objects farther from the Sun are composed largely of materials with lower melting points. Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (hence why they are sometimes referred to as “ice giants”) and the numerous small objects that lie beyond Neptune’s orbit.
Together, gases and ices are referred to as volatiles. The boundary in the Solar System beyond which those volatile substances could condense is known as the frost line, which lies roughly 5 AU from the Sun. Within the Kuiper Belt, objects and planetesimals are composed mainly of these materials and rock.
Formation and Evolution:
The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud composed of hydrogen, helium, and small amounts of heavier elements fused by previous generations of stars. As the region that would become the Solar System (known as the pre-solar nebula) collapsed, conservation of angular momentum caused it to rotate faster.
The center, where most of the mass collected, became increasingly hotter than the surrounding disc. As the contracting nebula rotated faster, it began to flatten into a protoplanetary disc with a hot, dense protostar at the center. The planets formed by accretion from this disc, in which dust and gas gravitated together and coalesced to form ever larger bodies.
Due to their higher boiling points, only metals and silicates could exist in solid form closer to the Sun, and these would eventually form the terrestrial planets of Mercury, Venus, Earth, and Mars. Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large.
In contrast, the giant planets (Jupiter, Saturn, Uranus, and Neptune) formed beyond the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid (i.e. the frost line).
The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium. Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud.
Within 50 million years, the pressure and density of hydrogen in the center of the protostar became great enough for it to begin thermonuclear fusion. The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved.
At this point, the Sun became a main-sequence star. Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space, ending the planetary formation process.
The terrestrial planets of our Solar System at approximately relative sizes. From left, Mercury, Venus, Earth and Mars. Credit: Lunar and Planetary Institute
The Solar System will remain roughly as we know it today until the hydrogen in the core of the Sun has been entirely converted to helium. This will occur roughly 5 billion years from now and mark the end of the Sun’s main-sequence life. At this time, the core of the Sun will collapse, and the energy output will be much greater than at present.
The outer layers of the Sun will expand to roughly 260 times its current diameter, and the Sun will become a red giant. The expanding Sun is expected to vaporize Mercury and Venus and render Earth uninhabitable as the habitable zone moves out to the orbit of Mars. Eventually, the core will be hot enough for helium fusion and the Sun will burn helium for a time, after which nuclear reactions in the core will start to dwindle.
At this point, the Sun’s outer layers will move away into space, leaving a white dwarf – an extraordinarily dense object that will have half the original mass of the Sun, but will be the size of Earth. The ejected outer layers will form what is known as a planetary nebula, returning some of the material that formed the Sun to the interstellar medium.
Inner Solar System:
In the inner Solar System, we find the “Inner Planets” – Mercury, Venus, Earth, and Mars – which are so named because they orbit closest to the Sun. In addition to their proximity, these planets have a number of key differences that set them apart from planets elsewhere in the Solar System.
For starters, the inner planets are rocky and terrestrial, composed mostly of silicates and metals, whereas the outer planets are gas giants. The inner planets are also much more closely spaced than their outer Solar System counterparts. In fact, the radius of the entire region is less than the distance between the orbits of Jupiter and Saturn.
Generally, inner planets are smaller and denser than their counterparts, and have few to no moons or rings circling them. The outer planets, meanwhile, often have dozens of satellites and rings composed of particles of ice and rock.
The terrestrial inner planets are composed largely of refractory minerals such as the silicates, which form their crusts and mantles, and metals such as iron and nickel which form their cores. Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather. All of them have impact craters and tectonic surface features as well, such as rift valleys and volcanoes.
Of the inner planets, Mercury is the closest to our Sun and the smallest of the terrestrial planets. Its magnetic field is only about 1% that of Earth’s, and it’s very thin atmosphere means that it is hot during the day (up to 430°C) and freezing at night (as low as -187 °C) because the atmosphere can neither keep heat in or out. It has no moons of its own and is comprised mostly of iron and nickel. Mercury is one of the densest planets in the Solar System.
Venus, which is about the same size as Earth, has a thick toxic atmosphere that traps heat, making it the hottest planet in the Solar System. This atmosphere is composed of 96% carbon dioxide, along with nitrogen and a few other gases. Dense clouds within Venus’ atmosphere are composed of sulphuric acid and other corrosive compounds, with very little water. Much of Venus’ surface is marked with volcanoes and deep canyons – the biggest of which is over 6400 km (4,000 mi) long.
Earth is the third inner planet and the one we know best. Of the four terrestrial planets, Earth is the largest, and the only one that currently has liquid water, which is necessary for life as we know it. Earth’s atmosphere protects the planet from dangerous radiation and helps keep valuable sunlight and warmth in, which is also essential for life to survive.
Like the other terrestrial planets, Earth has a rocky surface with mountains and canyons, and a heavy metal core. Earth’s atmosphere contains water vapor, which helps to moderate daily temperatures. Like Mercury, the Earth has an internal magnetic field. And our Moon, the only one we have, is comprised of a mixture of various rocks and minerals.
Mars, as it appears today, Credit: NASA
Mars is the fourth and final inner planet, and is also known as the “Red Planet” due to the oxidization of iron-rich materials that form the planet’s surface. Mars also has some of the most interesting terrain features of any of the terrestrial planets. These include the largest mountain in the Solar System (Olympus Mons) which rises some 21,229 m (69,649 ft) above the surface, and a giant canyon called Valles Marineris – which is 4000 km (2500 mi) long and reaches depths of up to 7 km (4 mi).
Much of Mars’ surface is very old and filled with craters, but there are geologically newer areas of the planet as well. At the Martian poles are polar ice caps that shrink in size during the Martian spring and summer. Mars is less dense than Earth and has a smaller magnetic field, which is indicative of a solid core, rather than a liquid one.
Mars’ thin atmosphere has led some astronomers to believe that the surface water that once existed there might have actually taken liquid form, but has since evaporated into space. The planet has two small moons called Phobos and Deimos.
Outer Solar System:
The outer planets (sometimes called Jovian planets or gas giants) are huge planets swaddled in gas that have rings and plenty of moons. Despite their size, only two of them are visible without telescopes: Jupiter and Saturn. Uranus and Neptune were the first planets discovered since antiquity, and showed astronomers that the solar system was bigger than previously thought.
The outer planets of our Solar System at approximately relative sizes. From left, Jupiter, Saturn, Uranus and Neptune. Credit: Lunar and Planetary Institute
Jupiter is the largest planet in our Solar System and spins very rapidly (10 Earth hours) relative to its orbit of the sun (12 Earth years). Its thick atmosphere is mostly made up of hydrogen and helium, perhaps surrounding a terrestrial core that is about Earth’s size. The planet has dozens of moons, some faint rings and a Great Red Spot – a raging storm that has happening for the past 400 years at least.
Saturn is best known for its prominent ring system – seven known rings with well-defined divisions and gaps between them. How the rings got there is one subject under investigation. It also has dozens of moons. Its atmosphere is mostly hydrogen and helium, and it also rotates quickly (10.7 Earth hours) relative to its time to circle the Sun (29 Earth years).
Uranus was first discovered by William Herschel in 1781. The planet’s day takes about 17 Earth hours and one orbit around the Sun takes 84 Earth years. Its mass contains water, methane, ammonia, hydrogen and helium surrounding a rocky core. It has dozens of moons and a faint ring system. The only spacecraft to visit this planet was the Voyager 2 spacecraft in 1986.
Neptune is a distant planet that contains water, ammmonia, methane, hydrogen and helium and a possible Earth-sized core. It has more than a dozen moons and six rings. NASA’s Voyager 2 spacecraft also visited this planet and its system by 1989 during its transit of the outer Solar System.
How many moons are there in the Solar System? Image credit: NASA
Trans-Neptunian Region:
There have been more than a thousand objects discovered in the Kuiper Belt, and it’s theorized that there are as many as 100,000 objects larger than 100 km in diameter. Given to their small size and extreme distance from Earth, the chemical makeup of KBOs is very difficult to determine.
However, spectrographic studies conducted of the region since its discovery have generally indicated that its members are primarily composed of ices: a mixture of light hydrocarbons (such as methane), ammonia, and water ice – a composition they share with comets. Initial studies also confirmed a broad range of colors among KBOs, ranging from neutral grey to deep red.
This suggests that their surfaces are composed of a wide range of compounds, from dirty ices to hydrocarbons. In 1996, Robert H. Brown et al. obtained spectroscopic data on the KBO 1993 SC, revealing its surface composition to be markedly similar to that of Pluto (as well as Neptune’s moon Triton) in that it possessed large amounts of methane ice.
Water ice has been detected in several KBOs, including 1996 TO66, 38628 Huya and 20000 Varuna. In 2004, Mike Brown et al. determined the existence of crystalline water ice and ammonia hydrate on one of the largest known KBOs, 50000 Quaoar. Both of these substances would have been destroyed over the age of the Solar System, suggesting that Quaoar had been recently resurfaced, either by internal tectonic activity or by meteorite impacts.
Keeping Pluto company out in the Kuiper belt are many other objects worthy of mention. Quaoar, Makemake, Haumea, Orcus and Eris are all large icy bodies in the Belt and several of them even have moons of their own. These are all tremendously far away, and yet, very much within reach.
Oort Cloud and Farthest Regions:
The Oort Cloud is thought to extend from between 2,000 and 5,000 AU (0.03 and 0.08 ly) to as far as 50,000 AU (0.79 ly) from the Sun, though some estimates place the outer edge as far as 100,000 and 200,000 AU (1.58 and 3.16 ly). The Cloud is thought to be comprised of two regions – a spherical outer Oort Cloud of 20,000 – 50,000 AU (0.32 – 0.79 ly), and disc-shaped inner Oort (or Hills) Cloud of 2,000 – 20,000 AU (0.03 – 0.32 ly).
The outer Oort cloud may have trillions of objects larger than 1 km (0.62 mi), and billions that measure 20 kilometers (12 mi) in diameter. Its total mass is not known, but – assuming that Halley’s Comet is a typical representation of outer Oort Cloud objects – it has the combined mass of roughly 3×1025 kilograms (6.6×1025 pounds), or five Earths.
The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA
Based on the analyses of past comets, the vast majority of Oort Cloud objects are composed of icy volatiles – such as water, methane, ethane, carbon monoxide, hydrogen cyanide, and ammonia. The appearance of asteroids thought to be originating from the Oort Cloud has also prompted theoretical research that suggests that the population consists of 1-2% asteroids.
Earlier estimates placed its mass up to 380 Earth masses, but improved knowledge of the size distribution of long-period comets has led to lower estimates. The mass of the inner Oort Cloud, meanwhile, has yet to be characterized. The contents of both Kuiper Belt and the Oort Cloud are known as Trans-Neptunian Objects (TNOs), because the objects of both regions have orbits that that are further from the Sun than Neptune’s orbit.
Exploration:
Our knowledge of the Solar System also benefited immensely from the advent of robotic spacecraft, satellites, and robotic landers. Beginning in the mid-20th century, in what was known as “The Space Age“, manned and robotic spacecraft began exploring planets, asteroids and comets in the Inner and Outer Solar System.
All planets in the Solar System have now been visited to varying degrees by spacecraft launched from Earth. Through these unmanned missions, humans have been able to get close-up photographs of all the planets. In the case of landers and rovers, tests have been performed on the soils and atmospheres of some.
Photograph of a Russian technician putting the finishing touches on Sputnik 1, humanity’s first artificial satellite. Credit: NASA/Asif A. Siddiqi
The first artificial object sent into space was the Soviet satellite Sputnik 1, which was launched in space in 1957, successfully orbited the Earth for months, and collected information on the density of the upper atmosphere and the ionosphere. The American probe Explorer 6, launched in 1959, was the first satellite to capture images of the Earth from space.
Robotic spacecraft conducting flybys also revealed considerable information about the planet’s atmospheres, geological and surface features. The first successful probe to fly by another planet was the Soviet Luna 1 probe, which sped past the Moon in 1959. The Mariner program resulted in multiple successful planetary flybys, consisting of the Mariner 2 mission past Venus in 1962, the Mariner 4 mission past Mars in 1965, and the Mariner 10 mission past Mercury in 1974.
By the 1970’s, probes were being dispatched to the outer planets as well, beginning with the Pioneer 10 mission which flew past Jupiter in 1973 and the Pioneer 11 visit to Saturn in 1979.The Voyager probes performed a grand tour of the outer planets following their launch in 1977, with both probes passing Jupiter in 1979 and Saturn in 1980-1981. Voyager 2 then went on to make close approaches to Uranus in 1986 and Neptune in 1989.
Launched on January 19th, 2006, the New Horizons probe is the first man-made spacecraft to explore the Kuiper Belt. This unmanned mission flew by Pluto in July 2015. Should it prove feasible, the mission will also be extended to observe a number of other Kuiper Belt Objects (KBOs) in the coming years.
Orbiters, rovers, and landers began being deployed to other planets in the Solar System by the 1960’s. The first was the Soviet Luna 10 satellite, which was sent into lunar orbit in 1966. This was followed in 1971 with the deployment of the Mariner 9 space probe, which orbited Mars, and the Soviet Venera 9 which orbited Venus in 1975.
The Galileo probe became the first artificial satellite to orbit an outer planet when it reached Jupiter in 1995, followed by the Cassini–Huygens probe orbiting Saturn in 2004. Mercury and Vesta were explored by 2011 by the MESSENGER and Dawn probes, respectively, with Dawn establishing orbit around the asteroid/dwarf planet Ceres in 2015.
The first probe to land on another Solar System body was the Soviet Luna 2 probe, which impacted the Moon in 1959. Since then, probes have landed on or impacted on the surfaces of Venus in 1966 (Venera 3), Mars in 1971 (Mars 3 and Viking 1 in 1976), the asteroid 433 Eros in 2001 (NEAR Shoemaker), and Saturn’s moon Titan (Huygens) and the comet Tempel 1 (Deep Impact) in 2005.
Curiosity Rover self portrait mosaic, taken with the MAHLI camera while sitting on flat sedimentary rocks at the “John Klein” outcrop in Feb. 2013. Credit: NASA/JPL-Caltech/MSSS/Marco Di Lorenzo/KenKremer
To date, only two worlds in the Solar System, the Moon and Mars, have been visited by mobile rovers. The first robotic rover to land on another planet was the Soviet Lunokhod 1, which landed on the Moon in 1970. The first to visit another planet was Sojourner, which traveled 500 meters across the surface of Mars in 1997, followed by Spirit(2004), Opportunity (2004), and Curiosity (2012).
Manned missions into space began in earnest in the 1950’s, and was a major focal point for both the United States and Soviet Union during the “Space Race“. For the Soviets, this took the form of the Vostok program, which involved sending manned space capsules into orbit.
The first mission – Vostok 1 – took place on April 12th, 1961, and was piloted by Soviet cosmonaut Yuri Gagarin (the first human being to go into space). On June 6th, 1963, the Soviets also sent the first woman – Valentina Tereshvoka – into space as part of the Vostok 6 mission.
In the US, Project Mercury was initiated with the same goal of placing a crewed capsule into orbit. On May 5th, 1961, astronaut Alan Shepard went into space aboard the Freedom 7mission and became the first American (and second human) to go into space.
After the Vostok and Mercury programs were completed, the focus of both nations and space programs shifted towards the development of two and three-person spacecraft, as well as the development of long-duration spaceflights and extra-vehicular activity (EVA).
Bootprint in the moon dust from Apollo 11. Credit: NASA
This took the form of the Voshkod and Gemini programs in the Soviet Union and US, respectively. For the Soviets, this involved developing a two to three-person capsule, whereas the Gemini program focused on developing the support and expertise needed for an eventual manned mission to the Moon.
These latter efforts culminated on July 21st, 1969 with the Apollo 11 mission, when astronauts Neil Armstrong and Buzz Aldrin became the first men to walk on the Moon. As part of the Apollo program, five more Moon landings would take place through 1972, and the program itself resulted in many scientific packages being deployed on the Lunar surface, and samples of moon rocks being returned to Earth.
After the Moon Landing took place, the focus of the US and Soviet space programs then began to shift to the development of space stations and reusable spacecraft. For the Soviets, this resulted in the first crewed orbital space stations dedicated to scientific research and military reconnaissance – known as the Salyut and Almaz space stations.
The first orbital space station to host more than one crew was NASA’s Skylab, which successfully held three crews from 1973 to 1974. The first true human settlement in space was the Soviet space station Mir, which was continuously occupied for close to ten years, from 1989 to 1999. It was decommissioned in 2001, and its successor, the International Space Station, has maintained a continuous human presence in space since then.
Space Shuttle Columbia launching on its maiden voyage on April 12th, 1981. Credit: NASA
The United States’ Space Shuttle, which debuted in 1981, became the only reusable spacecraft to successfully make multiple orbital flights. The five shuttles that were built (Atlantis, Endeavour, Discovery, Challenger, Columbiaand Enterprise) flew a total of 121 missions before being decommissioned in 2011.
During their history of service, two of the craft were destroyed in accidents. These included the Space Shuttle Challenger – which exploded upon take-off on Jan. 28th, 1986 – and the Space Shuttle Columbia which disintegrated during re-entry on Feb. 1st, 2003.
In 2004, then-U.S. President George W. Bush announced the Vision for Space Exploration, which called for a replacement for the aging Shuttle, a return to the Moon and, ultimately, a manned mission to Mars. These goals have since been maintained by the Obama administration, and now include plans for an Asteroid Redirect mission, where a robotic craft will tow an asteroid closer to Earth so a manned mission can be mounted to it.
All the information gained from manned and robotic missions about the geological phenomena of other planets – such as mountains and craters – as well as their seasonal, meteorological phenomena (i.e. clouds, dust storms and ice caps) have led to the realization that other planets experience much the same phenomena as Earth. In addition, it has also helped scientists to learn much about the history of the Solar System and its formation.
As our exploration of the Inner and Outer Solar System has improved and expanded, our conventions for categorizing planets has also changed. Our current model of the Solar System includes eight planets (four terrestrial, four gas giants), four dwarf planets, and a growing number of Trans-Neptunian Objects that have yet to be designated. It also contains and is surrounded by countless asteroids and planetesimals.
Given its sheer size, composition and complexity, researching our Solar System in full detail would take an entire lifetime. Obviously, no one has that kind of time to dedicate to the topic, so we have decided to compile the many articles we have about it here on Universe Today in one simple page of links for your convenience.
There are thousands of facts about the solar system in the links below. Enjoy your research.
This portrait looking down on Saturn and its rings was created from images obtained by NASA's Cassini spacecraft on Oct. 10, 2013. Credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic
The farthest planet from the Sun that can be observed with the naked eye, the existence of Saturn has been known for thousands of years. And much like all celestial bodies that can be observed with the aid of instruments – i.e. Mercury, Venus, Mars, Jupiter and the Moon – it has played an important role in the mythology and astrological systems of many cultures.
Saturn is one of the four gas giants in our Solar System, also known as the Jovian planets, and the sixth planet from the Sun. It’s ring system, which is it famous for, is also the most observable – consisting of nine continuous main rings and three discontinuous arcs.
Saturn’s Size, Mass and Orbit:
With a polar radius of 54364±10 km and an equatorial radius of 60268±4 km, Saturn has a mean radius of 58232±6 km, which is approximately 9.13 Earth radii. At 5.6846×1026 kg, and a surface area, at 4.27×1010 km2, it is roughly 95.15 as massive as Earth and 83.703 times it’s size. However, since it is a gas giant, it has significantly greater volume – 8.2713×1014 km3, which is equivalent to 763.59 Earths.
The sixth most distant planet, Saturn orbits the Sun at an average distance of 9 AU (1.4 billion km; 869.9 million miles). Due to its slight eccentricity, the perihelion and aphelion distances are 9.022 (1,353.6 million km; 841.3 million mi) and 10.053 AU (1,513,325,783 km; 940.13 million mi), on average respectively.
Saturn Compared to Earth. Image credit: NASA/JPL
With an average orbital speed of 9.69 km/s, it takes Saturn 10,759 Earth days to complete a single revolution of the Sun. In other words, a single Cronian year is the equivalent of about 29.5 Earth years. However, as with Jupiter, Saturn’s visible features rotate at different rates depending on latitude, and multiple rotation periods have been assigned to various regions.
The latest estimate of Saturn’s rotation as a whole are based on a compilation of various measurements from the Cassini, Voyager and Pioneer probes. Saturn’s rotation causes it to have the shape of an oblate spheroid; flattened at the poles but bulging at the equator.
Saturn’s Composition:
As a gas giant, Saturn is predominantly composed of hydrogen and helium gas. With a mean density of 0.687 g/cm3, Saturn is the only planet in the Solar System that is less dense than water; which means that it lacks a definite surface, but is believed to have a solid core. This is due to the fact that Saturn’s temperature, pressure, and density all rise steadily toward the core.
Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a small rocky core surrounded by hydrogen and helium with trace amounts of various volatiles. This core is similar in composition to the Earth, but more dense due to the presence of metallic hydrogen, which as a result of the extreme pressure.
Diagram of Saturn’s interior. Credit: Kelvinsong/Wikipedia Commons
Saturn has a hot interior, reaching 11,700 °C at its core, and it radiates 2.5 times more energy into space than it receives from the Sun. This is due in part to the Kelvin-Helmholtz mechanism of slow gravitational compression, but may also be attributable to droplets of helium rising from deep in Saturn’s interior out to the lower-density hydrogen. As these droplets rise, the process releases heat by friction and leaves Saturn’s outer layers depleted of helium. These descending droplets may have accumulated into a helium shell surrounding the core.
In 2004, French astronomers Didier Saumon and Tristan Guillot estimated that the core must 9-22 times the mass of Earth, which corresponds to a diameter of about 25,000 km. This is surrounded by a thicker liquid metallic hydrogen layer, followed by a liquid layer of helium-saturated molecular hydrogen that gradually transitions to a gas with increasing altitude. The outermost layer spans 1,000 km and consists of gas.
Saturn’s Atmosphere:
The outer atmosphere of Saturn contains 96.3% molecular hydrogen and 3.25% helium by volume. The gas giant is also known to contain heavier elements, though the proportions of these relative to hydrogen and helium is not known. It is assumed that they would match the primordial abundance from the formation of the Solar System.
Trace amounts of ammonia, acetylene, ethane, propane, phosphine and methane have been also detected in Saturn’s atmosphere. The upper clouds are composed of ammonia crystals, while the lower level clouds appear to consist of either ammonium hydrosulfide (NH4SH) or water. Ultraviolet radiation from the Sun causes methane photolysis in the upper atmosphere, leading to a series of hydrocarbon chemical reactions with the resulting products being carried downward by eddies and diffusion.
NASA’s Cassini spacecraft captures a composite near-true-color view of the huge storm churning through the atmosphere in Saturn’s northern hemisphere. Image credit: NASA/JPL-Caltech/SSI
Saturn’s atmosphere exhibits a banded pattern similar to Jupiter’s, but Saturn’s bands are much fainter and wider near the equator. As with Jupiter’s cloud layers, they are divided into the upper and lower layers, which vary in composition based on depth and pressure. In the upper cloud layers, with temperatures in range of 100–160 K and pressures between 0.5–2 bar, the clouds consist of ammonia ice.
Water ice clouds begin at a level where the pressure is about 2.5 bar and extend down to 9.5 bar, where temperatures range from 185–270 K. Intermixed in this layer is a band of ammonium hydrosulfide ice, lying in the pressure range 3–6 bar with temperatures of 290–235 K. Finally, the lower layers, where pressures are between 10–20 bar and temperatures are 270–330 K, contains a region of water droplets with ammonia in an aqueous solution.
On occasion, Saturn’s atmosphere exhibits long-lived ovals, similar to what is commonly observed on Jupiter. Whereas Jupiter has the Great Red Spot, Saturn periodically has what’s known as the Great White Spot (aka. Great White Oval). This unique but short-lived phenomenon occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere’s summer solstice.
These spots can be several thousands of kilometers wide, and have been observed in 1876, 1903, 1933, 1960, and 1990. Since 2010, a large band of white clouds called the Northern Electrostatic Disturbance have been observed enveloping Saturn, which was spotted by the Cassini space probe. If the periodic nature of these storms is maintained, another one will occur in about 2020.
The huge storm churning through the atmosphere in Saturn’s northern hemisphere overtakes itself as it encircles the planet in this true-color view from NASA’s Cassini spacecraft. Image credit: NASA/JPL-Caltech/SSI
The winds on Saturn are the second fastest among the Solar System’s planets, after Neptune’s. Voyager data indicate peak easterly winds of 500 m/s (1800 km/h). Saturn’s northern and southern poles have also shown evidence of stormy weather. At the north pole, this takes the form of a hexagonal wave pattern, whereas the south shows evidence of a massive jet stream.
The persisting hexagonal wave pattern around the north pole was first noted in the Voyager images. The sides of the hexagon are each about 13,800 km (8,600 mi) long (which is longer than the diameter of the Earth) and the structure rotates with a period of 10h 39m 24s, which is assumed to be equal to the period of rotation of Saturn’s interior.
The south pole vortex, meanwhile, was first observed using the Hubble Space Telescope. These images indicated the presence of a jet stream, but not a hexagonal standing wave. These storms are estimated to be generating winds of 550 km/h, are comparable in size to Earth, and believed to have been going on for billions of years. In 2006, the Cassini space probe observed a hurricane-like storm that had a clearly defined eye. Such storms had not been observed on any planet other than Earth – even on Jupiter.
Saturn’s Moons:
Saturn has at least 150 moons and moonlets, but only 53 of these moons have been given official names. Of these moons, 34 are less than 10 km in diameter and another 14 are between 10 and 50 km in diameter. However, some of its inner and outer moons are rather large, ranging from 250 to over 5000 km.
Moons of Saturn (from left to right): Mimas, Enceladus, Tethys, Dione, Rhea, Titan in the background; Iapetus (top) and irregularly shaped Hyperion (bottom). Credit: NASA/JPL/Space Science Institute
Traditionally, most of Saturn’s moons have been named after the Titans of Greek mythology, and are grouped based on their size, orbits, and proximity to Saturn. The innermost moons and regular moons all have small orbital inclinations and eccentricities and prograde orbits. Meanwhile, the irregular moons in the outermost regions have orbital radii of millions of kilometers, orbital periods lasting several years, and move in retrograde orbits.
The Inner Large Moons, which orbit within the E Ring (see below), includes the larger satellites Mimas, Enceladus, Tethys, and Dione. These moons are all composed primarily of water ice, and are believed to be differentiated into a rocky core and an icy mantle and crust. With a diameter of 396 km and a mass of 0.4×1020 kg, Mimas is the smallest and least massive of these moons. It is ovoid in shape and orbits Saturn at a distance of 185,539 km with an orbital period of 0.9 days.
Enceladus, meanwhile, has a diameter of 504 km, a mass of 1.1×1020 km and is spherical in shape. It orbits Saturn at a distance of 237,948 km and takes 1.4 days to complete a single orbit. Though it is one of the smaller spherical moons, it is the only Cronian moon that is endogenously active – and one of the smallest known bodies in the Solar System that is geologically active. This results in features like the famous “tiger stripes” – a series of continuous, ridged, slightly curved and roughly parallel faults within the moon’s southern polar latitudes.
Large geysers have also been observed in the southern polar region that periodically release plumes of water ice, gas and dust which replenish Saturn’s E ring. These jets are one of several indications that Enceladus has liquid water beneath it’s icy crust, where geothermal processes release enough heat to maintain a warm water ocean closer to its core. With a geometrical albedo of more than 140%, Enceladus is one of the brightest known objects in the Solar System.
Artist’s rendering of possible hydrothermal activity that may be taking place on and under the seafloor of Enceladus. Image Credit: NASA/JPL
At 1066 km in diameter, Tethys is the second-largest of Saturn’s inner moons and the 16th-largest moon in the Solar System. The majority of its surface is made up of heavily cratered and hilly terrain and a smaller and smoother plains region. Its most prominent features are the large impact crater of Odysseus, which measures 400 km in diameter, and a vast canyon system named Ithaca Chasma – which is concentric with Odysseus and measures 100 km wide, 3 to 5 km deep and 2,000 km long.
With a diameter and mass of 1,123 km and 11×1020 kg, Dione is the largest inner moon of Saturn. The majority of Dione’s surface is heavily cratered old terrain, with craters that measure up to 250 km in diameter. However, the moon is also covered with an extensive network of troughs and lineaments which indicate that in the past it had global tectonic activity.
The Large Outer Moons, which orbit outside of the Saturn’s E Ring, are similar in composition to the Inner Moons – i.e. composed primarily of water ice and rock. Of these, Rhea is the second largest – measuring 1,527 km in diameter and 23 × 1020 kg in mass – and the ninth largest moon of the Solar System. With an orbital radius of 527,108 km, it is the fifth-most distant of the larger moons, and takes 4.5 days to complete an orbit.
Like other Cronian satellites, Rhea has a rather heavily cratered surface, and a few large fractures on its trailing hemisphere. Rhea also has two very large impact basins on its anti-Saturnian hemisphere – the Tirawa crater (similar to Odysseus on Tethys) and an as-yet unnamed crater – that measure 400 and 500 km across, respectively.
A composite image of Titan’s atmosphere, created using blue, green and red spectral filters to create an enhanced-color view. Image Credit: NASA/JPL/Space Science Institute
At 5150 km in diameter, and 1,350×1020 kg in mass, Titan is Saturn’s largest moon and comprises more than 96% of the mass in orbit around the planet. Titan is also the only large moon to have its own atmosphere, which is cold, dense, and composed primarily of nitrogen with a small fraction of methane. Scientists have also noted the presence of polycyclic aromatic hydrocarbons in the upper atmosphere, as well as methane ice crystals.
The surface of Titan, which is difficult to observe due to persistent atmospheric haze, shows only a few impact craters, evidence of cryo-volcanoes, and longitudinal dune fields that were apparently shaped by tidal winds. Titan is also the only body in the Solar System beside Earth with bodies of liquid on its surface, in the form of methane–ethane lakes in Titan’s north and south polar regions.
With an orbital distance of 1,221,870 km, it is the second-farthest large moon from Saturn, and completes a single orbit every 16 days. Like Europa and Ganymede, it is believed that Titan has a subsurface ocean made of water mixed with ammonia, which can erupt to the surface of the moon and lead to cryovolcanism.
Hyperion is Titan’s immediate neighbor. At an average diameter of about 270 km, it is smaller and lighter than Mimas. It is also irregularly shaped and quite odd in composition. Essentially, the moon is an ovoid, tan-colored body with an extremely porous surface (which resembles a sponge). The surface of Hyperion is covered with numerous impact craters, most of which are 2 to 10 km in diameter. It also has a highly unpredictable rotation, with no well-defined poles or equator.
The two sides of Iapetus, which is known as “Saturn’s yin yang moon” because of the contrast in its color composition. Credit: NASA/JPL
At 1,470 km in diameter and 18×1020 kg in mass, Iapetus is the third-largest of Saturn’s large moons. And at a distance of 3,560,820 km from Saturn, it is the most distant of the large moons, and takes 79 days to complete a single orbit. Due to its unusual color and composition – its leading hemisphere is dark and black whereas its trailing hemisphere is much brighter – it is often called the “yin and yang” of Saturn’s moons.
Beyond these larger moons are Saturn’s Irregular Moons. These satellites are small, have large-radii, are inclined, have mostly retrograde orbits, and are believed to have been acquired by Saturn’s gravity. These moons are made up of three basic groups – the Inuit Group, the Gallic Group, and the Norse Group.
The Inuit Group consists of five irregular moons that are all named from Inuit mythology – Ijiraq, Kiviuq, Paaliaq, Siarnaq, and Tarqeq. All have prograde orbits that range from 11.1 to 17.9 million km, and from 7 to 40 km in diameter. They are all similar in appearance (reddish in hue) and have orbital inclinations of between 45 and 50°.
The Gallic group are a group of four prograde outer moons named for characters in Gallic mythology -Albiorix, Bebhionn, Erriapus, and Tarvos. Here too, the moons are similar in appearance and have orbits that range from 16 to 19 million km. Their inclinations are in the 35°-40° range, their eccentricities around 0.53, and they range in size from 6 to 32 km.
Saturns rings and moons, shown to scale. Credit: NASA
Last, there is the Norse group, which consists of 29 retrograde outer moons that take their names from Norse mythology. These satellites range in size from 6 to 18 km, their distances from 12 and 24 million km, their inclinations between 136° and 175°, and their eccentricities between 0.13 and 0.77. This group is also sometimes referred to as the Phoebe group, due to the presence of a single larger moon in the group – which measures 240 km in diameter. The second largest, Ymir, measures 18 km across.
Within the Inner and Outer Large Moons, there are also those belonging to Alkyonide group. These moons – Methone, Anthe, and Pallene – are named after the Alkyonides of Greek mythology, are located between the orbits of Mimas and Enceladus, and are among the smallest moons around Saturn.
Some of the larger moons even have moons of their own, which are known as Trojan moons. For instance, Tethys has two trojans – Telesto and Calypso, while Dione has Helene and Polydeuces.
Saturn’s Ring System:
Saturn’s rings are believed to be very old, perhaps even dating back to the formation of Saturn itself. There are two main theories as to how these rings formed, each of which have variations. One theory is that the rings were once a moon of Saturn whose orbit decayed until it came close enough to be ripped apart by tidal forces.
In version of this theory, the moon was struck by a large comet or asteroid – possible during the Late Heavy Bombardment – that pushed it beneath the Roche Limit. The second theory is that the rings were never part of a moon, but are instead left over from the original nebular material from which Saturn formed billions of years ago.
The structure is subdivided into seven smaller ring sets, each of which has a division (or gap) between it and its neighbor. The A and B Rings are the densest part of the Cronian ring system and are 14,600 and 25,500 km in diameter, respectively. They extend to a distance of 92,000 – 117,580 km (B Ring) and 122,170 – 136,775 km (A Ring) from Saturn’s center, and are separated by the 4,700 km wide Cassini Division.
Saturn’s rings. Credit: NASA/JPL/Space Science Institute.
The C Ring, which is separated from the B Ring by the 64 km Maxwell Gap, is approximately 17,500 km in width and extends 74,658 – 92,000 from Saturn’s center. Together with the A and B Rings, they comprise the main rings, which are denser and contain larger particles than the “dusty rings”.
These tenuous rings are called “dusty” due to the small particles that make them up. They include the D Ring, a 7,500 km ring that extends inward to Saturn’s cloud tops (66,900 – 74,510 km from Saturn’s center) and is separated from the C Ring by the 150 km Colombo Gap. On the other end of the system, the G and E Rings are located, which are also “dusty” in composition.
The G Ring is 9000 km in width and extends 166,000 – 175,000 km from Saturn’s center. The E Ring, meanwhile, is the largest single ring section, measuring 300,000 km in width and extending 166,000 to 480,000 km from Saturn’s center. It is here where the majority of Saturn’s moons are located (see above).
The narrow F Ring, which sits on the outer edge of the A Ring, is more difficult to categorize. While some parts of it are very dense, it also contains a great deal of dust-size particles. For this reason, estimates on its width range from 30 to 500 km, and it extends roughly 140,180 km from Saturn’s center.
History of Observing Saturn:
Because it is visible to the naked eye in the night sky, human beings have been observing Saturn for thousands of years. In ancient times, it was considered the most distant of five known the planets, and thus was accorded special meaning in various mythologies. The earliest recorded observations come from the Babylonians, where astronomers systematically observed and recorded its movements through the zodiac.
Roman astrological calendar, from the stone plate of the 3rd—4th centuries CE, Rome. Credit: Museo della civiltà romana
To the ancient Greeks, this outermost planet was named Cronus (Kronos), after the Greek god of agriculture and youngest of the Titans. The Greek scientist Ptolemy made calculations of Saturn’s orbit based on observations of the planet while it was in opposition.The Romans followed in this tradition, identifying it with their equivalent of Cronos (named Saturnus).
In ancient Hebrew, Saturn is called ‘Shabbathai’, whereas in Ottoman Turkish, Urdu and Malay, its name is ‘Zuhal’, which derived is from the original Arabic. In Hindu astrology, there are nine astrological objects known as Navagrahas. Saturn, which is one of them, is known as “Shani”, who judges everyone based on the good and bad deeds performed in life. In ancient China and Japan, the planet was designated as the “earth star” – based on the Five Elements of earth, air, wind, water and fire.
However, the planet was not directly observed until 1610, when Galileo Galilee first discerned the presence of rings. At the time, he mistook them for two moons that were located on either side. It was not until Christiaan Huygens used a telescope with greater magnification that this was corrected. Huygens also discovered Saturn’s moon Titan, and Giovanni Domenico Cassini later discovered the moons of Iapetus, Rhea, Tethys and Dione.
No further discoveries of significance were made again until the 181th and 19th centuries. The first occurred in 1789 when William Herschel discovered the two distant moons of Mimas and Enceladus, and then in 1848 when a British team discovered the irregularly-shaped moon of Hyperion.
Drawing of Saturn by Robert Hook, taken from Philosophical Transactions (1666). Credit: Wikipedia Commons
In 1899 William Henry Pickering discovered Phoebe, noting that it had a highly irregular orbit that did not rotate synchronously with Saturn as the larger moons do. This was the first time any satellite had been found to move about a planet in retrograde orbit. And by 1944, research conducted throughout the early 20th century confirmed that Titan has a thick atmosphere – a feature unique among the Solar System’s moons.
Exploration of Saturn:
By the late 20th century, unmanned spacecraft began to conduct flybys of Saturn, gathering information on its composition, atmosphere, ring structure, and moons. The first flyby was conducted by NASA using the Pioneer 11 robotic space probe, which passed Saturn at a distance of 20,000 km in September of 1979.
Images were taken of the planet and a few of its moons, although their resolution was too low to discern surface detail. The spacecraft also studied Saturn’s rings, revealing the thin F Ring and the fact that dark gaps in the rings are bright when facing towards the Sun, meaning that they contain fine light-scattering material. In addition, Pioneer 11 measured the temperature of Titan.
The next flyby took place in November of 1980 when the Voyager 1 space probe passed through the Saturn system. It sent back the first high-resolution images of the planet, its rings and satellites – which included features of various moons that had never before been seen.
These six narrow-angle color images were made from the first ever ‘portrait’ of the solar system taken by Voyager 1 in November 1980. Credit: NASA/JPL
In August 1981, Voyager 2 conducted its flyby and gathered more close-up images of Saturn’s moons, as well as evidence of changes in the atmosphere and the rings. The probes discovered and confirmed several new satellites orbiting near or within the planet’s rings, as well as the small Maxwell Gap and Keeler gap (a 42 km wide gap in the A Ring).
In June of 2004, the Cassini–Huygens space probe entered the Saturn system and conducted a close flyby of Phoebe, sending back high-resolution images and data. By July 1st, 2004, the probe entered orbit around Saturn, and by December, it had completed two flybys of Titan before releasing the Huygens probe. This lander reached the surface and began transmitting data on the atmospheric and surface by by Jan. 14th, 2005. Cassini has since conducted multiple flybys of Titan and other icy satellites.
In 2006, NASA reported that Cassini had found evidence of liquid water reservoirs that erupt in geysers on Saturn’s moon Enceladus. Over 100 geysers have since been identified, which are concentrated around the southern polar region. In May 2011, NASA scientists at an Enceladus Focus Group Conference reported that Enceladus’ interior ocean may be the most likely candidate in the search for extra-terrestrial life.
In addition, Cassini photographs have revealed a previously undiscovered planetary ring, eight new satellites, and evidence of hydrocarbon lakes and seas near Titan’s north pole. The probe was also responsible for sending back high-resolution images of the intense storm activity at Saturn’s northern and southern poles.
Cassini’s primary mission ended in 2008, but the probe’s mission has been extended twice since then – first to September 2010 and again to 2017. In the coming years, NASA hopes to use the probe to study a full period of Saturn’s seasons.
Artist Illustration of the Cassini space probe to Saturn and Titan, a joint NASA, ESA mission. Credit: NASA/JPL
From being a very important part of the astrological systems of many cultures to becoming the subject of ongoing scientific fascination, Saturn continues to occupy a special place in our hearts and minds. Whether it’s Saturn’s fantastically large and beautiful ring system, its many many moons, its tempestuous weather, or its curious composition, this gas giant continues to fascinate and inspire.
In the coming years and decades, additional robotic explorer missions will likely to be sent to investigate Saturn, its rings and its system of moons in greater detail. What we find may constitute some of the most groundbreaking discoveries of all time, and will likely teach us more about the history of our Solar System.
Enhanced-color image from Cassini showing red streaks on Tethys (NASA/JPL-Caltech/Space Science Institute)
Resembling what the skin on my arms looks like after giving my cat a bath, the surface of Saturn’s moon Tethys is seen above in an extended-color composite from NASA’s Cassini spacecraft showing strange long red streaks. They stretch for long distances across the moon’s surface following the rugged terrain, continuing unbroken over hills and down into craters… and their cause isn’t yet known.
According to a NASA news release, “The origin of the features and their reddish color is currently a mystery to Cassini scientists. Possibilities being studied include ideas that the reddish material is exposed ice with chemical impurities, or the result of outgassing from inside Tethys. The streaks could also be associated with features like fractures that are below the resolution of the available images.”
The images were taken by Cassini during a flyby of the 660-mile-wide (1,062 km) Tethys on April 11, 2015 at a resolution of about 2,300 feet (700 meters) per pixel. They were acquired in visible green, infrared, and ultraviolet light wavelengths and so the composite image reveals colors our eyes can’t directly perceive. The combination of this and the solar illumination needed to image this particular area as the spacecraft passed by are why these features haven’t been seen so well until now.
“The red arcs really popped out when we saw the new images,” said Cassini participating scientist Paul Schenk of the Lunar and Planetary Institute in Houston. “It’s surprising how extensive these features are.”
Extended color mosaic of Tethys from Cassini images acquired on April 11, 2015. The region where the streaks are is outlined. Click for original hi-res version. (NASA/JPL-Caltech/SSI)
While the nature of Tethys’ streaks isn’t understood, the observations do indicate a relatively young age compared to the surrounding surface.
“The red arcs must be geologically young because they cut across older features like impact craters, but we don’t know their age in years.” said Paul Helfenstein, a Cassini imaging scientist at Cornell University in Ithaca. “If the stain is only a thin, colored veneer on the icy soil, exposure to the space environment at Tethys’ surface might erase them on relatively short time scales.”
Reprocessed Galileo image of Europa’s streaked surface by Ted Stryk (NASA/JPL/Ted Stryk)
Could these arcs be signs of an underground ocean or reservoir of briny liquid, like Enceladus’ tiger stripes (aka sulcae) or the streaks that crisscross Europa’s ice? Or are they the results of infalling material from one of Saturn’s other moons? More observations with Cassini, now in its eleventh year in orbit at Saturn, are being planned to “study the streaks.”
“We are planning an even closer look at one of the Tethys red arcs in November to see if we can tease out the source and composition of these unusual markings,” said Linda Spilker, Cassini project scientist at JPL.
