Matt Williams is a space journalist and science communicator for Universe Today and Interesting Engineering. He's also a science fiction author, podcaster (Stories from Space), and Taekwon-Do instructor who lives on Vancouver Island with his wife and family.
NASA Selects Investigations for Future Key Planetary Mission
Artist's concept of the Psyche spacecraft, a proposed mission for NASA's Discovery program that would conduct a direct exploration of an object thought to be a stripped planetary core. Credit: NASA/JPL-Caltech
In their drive to set exploration goals for the future, NASA’s Discovery Program put out the call for proposals for their thirteenth Discovery mission in February 2014. After reviewing the 27 initial proposals, a panel of NASA and other scientists and engineers recently selected five semifinalists for additional research and development, one or two of which will be launching by the 2020s.
With an eye to Venus, near-Earth objects and asteroids, these missions are looking beyond Mars to address other questions about the history and formation of our Solar System. Among them is the proposed Psyche mission, a robotic spacecraft that will explore the metallic asteroid of the same name – 16 Psyche – in the hopes of shedding some light on the mysteries of planet formation.
Discovered by Italian astronomer Annibale de Gasparis on March 17th, 1852 – and named after a Greek mythological figure – Psyche is one the ten most-massive asteroids in the Asteroid Belt. It is also the most massive M-type asteroid, a special class pertaining to asteroids composed primarily of nickel and iron.
For some time, scientists have speculated that this metallic asteroid is in fact the survivor of a protoplanet. In this scenario, a violent collision with a planetesimal stripped off Psyche’s outer, rocky layers, leaving behind only the dense, metallic interior. This theory is supported by estimates of Psyche’s bulk density, spectra, and radar surface properties; all of which show it to be an object unlike any others in the Belt.
Promotional artwork for the proposed Psyche mission. Credit: Peter Rubin/JPL-CALTECH.
In addition, this composition of 16 Psyche is strikingly similar to that of Earth’s metal core. Given that astronomers think that larger planets like Venus, Earth and Mars formed from the collision and merger of smaller worlds, Psyche could be the remains of a protoplanet that did not get to create a larger body.
Had such a planetesimal been struck by a large enough object, it would have been able to lose its lower-mass exterior while keeping its core intact. Thus, studying this 250 km (155 mile) wide body, offers a unique opportunity to learn more about the interiors of planets and large moons, whose cores are hidden beneath many miles of rock.
Dr. Linda Elkins-Tanton of Arizona State University’s School of Earth and Space Exploration is the Principle Investigator of this mission. As she and her team stated in their mission proposal paper, which was originally submitted as part of the 45th Lunar and Planetary Science Conference (2014):
“This mission would be a journey back in time to one of the earliest periods of planetary accretion, when the first bodies were not only differentiating, but were being pulverized, shredded, and accreted by collisions. It is also an exploration, by proxy, of the interiors of terrestrial planets and satellites today: we cannot visit a metallic core any other way.
“For all of these reasons, coupled with the relative accessibility to low- cost rendezvous and orbit, Psyche is a superb target for a Discovery-class mission that would characterize its geology, shape, elemental composition, magnetic field , and mass distribution.”
The huge metal asteroid Psyche may have a strong remnant magnetic field. Credit: Damir Gamulin/Ben Weiss
A robotic mission to Pysche would also help astronomers learn more about metal worlds, a type of solar system object that scientists know very little about. But perhaps the greatest reason to study 16 Psyche is the fact that it is unique. So far, this body is the only metallic core-like body that has been discovered in the Solar System.
The proposed spacecraft would orbit Psyche for six months, studying its topography, surface features, gravity, magnetism, and other characteristics. The mission would also be cost-effective and quick to launch, since it is largely based on technology that went into the making of NASA’s Dawn probe. Currently in orbit around Ceres, the Dawn mission has demonstrated the effectiveness of many new technologies, not the least of which was the xenon ion thruster.
The Psyche orbiter mission was selected as one of the Discovery Program’s five semifinalists on September 30th, 2015. Each proposal has received $3 million for year-long studies to lay out detailed mission plans and reduce risks. One or two finalist will be selected to receive the program’s budget of $450 million (minus the cost of a launch vehicle and mission operations) and will launch in 2020 at the earliest.
Artist's concept of the VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy) spacecraft, a proposed mission for NASA's Discovery Program that would launch by the end of 2021. Credit: NASA/JPL-Caltech
In February of 2014, NASA’s Discovery Program asked for proposals for the their 13th mission. Last week, five semifinalist were selected from the original 27 submissions for further investigation and refinement. Of the possible missions that could be going up, two involve sending a robotic spacecraft to a planet that NASA has not been to in decades: Venus!
The first is the DAVINCI spacecraft, which would study the chemical composition of Venus’ atmosphere. Meanwhile, the proposed VERITAS mission – or The Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy spacecraft – would investigate the planet’s surface to determine just how much it has in common with Earth, and whether or not it was ever habitable.
In many respects, this mission would pick up where Magellan left off in the early 1990s. Having reached Venus in 1990, the Magellan spacecraft (otherwise known as the Venus Radar Mapper) mapped nearly the entire surface with an S-band Synthetic Aperture Radar (SAR) and microwave radiometer. From the data obtained, NASA scientists were able to make radar altimeter measurements of the planet’s topography.
Deployment of the Magellan spacecraft with the Inertial Upper Stage (IUS) booster during the STS 30 Atlantis flight. Credit: NASA
These measurements revolutionized our understanding of Venus’ geology and the geophysical processes that have shaped the planet’s surface. In addition to revealing a young surface with few impact craters, Magellan also showed evidence of volcanic activity and signs of plate tectonics.
However, the lack of finer resolution imagery and topography of the surface hampered efforts to answer definitively what role these forces have played in the formation and evolution of the surface. As a result, scientists have remained unclear as to what extent certain forces have shaped (and continue to shape) the surface of Venus.
With a suite of modern instruments, the VERITAS spacecraft would produce global, high-resolution topography and imaging of Venus’ surface and produce the first maps of deformation and global surface composition. These include an X-band radar configured as a single pass radar interferometer (known as VISAR) which would be coupled with a multispectral NIR emissivity mapping capability.
Three-dimensional simulation of Gula Mons captured by the Magellan Synthetic Aperture Radar (SAR) combined with radar altimetry. Credit: NASA/JPL
Using these, the VERITAS probe will be able to see through Venus’ thick clouds, map the surface at higher resolution than Magellan, and attempt to accomplish three major scientific goals: get a better understanding of Venus’ geologic evolution; determine what geologic processes are currently operating on Venus (including whether or not active volcanoes still exist); and find evidence for past or present water.
Suzanne Smrekar of NASA’s Jet Propulsion Laboratory (JPL) is the mission’s principal investigator, while the JPL would be responsible for managing the project. As she explained to Universe Today via email:
“VERITAS’ objectives are to reveal Venus’ geologic history, determine how active it is, and search for the fingerprints of past and present water. The overarching question is ‘How Earthlike is Venus?’ As more and more exoplanets are discovered, this information is essential to predicting whether Earth-sized planets are more likely to resemble Earth or Venus.”
Venus, as imaged by the Magellan spacecraft using Synthetic Aperture Radar (SAR). Credit: NASA/JPL
In many ways, VERITAS and DAVINCI represent a vindication for Venus scientists in the United States, who have not sent a probe to the planet since the Magellan orbiter mission ended in 1994. Since that time, efforts have been largely focused on Mars, where orbiters and landers have been looking for evidence of past and present water, and trying to piece together what Mars’ atmosphere used to look like.
But with Discovery Mission 13 and its five semi-finalists, the focus has now shifted onto Venus, near-Earth objects, and a variety of asteroids. As John Grunsfeld, astronaut and associate administrator for NASA’s Science Mission Directorate in Washington, explained:
“The selected investigations have the potential to reveal much about the formation of our solar system and its dynamic processes. Dynamic and exciting missions like these hold promise to unravel the mysteries of our solar system and inspire future generations of explorers. It’s an incredible time for science, and NASA is leading the way.”
Each investigation team will receive $3 million to conduct concept design studies and analyses. After a detailed review and evaluation of the concept studies, NASA will make the final selections by September 2016 for continued development. This final mission (or missions) that are selected will launcd by 2020 at the earliest.
NASA's latest round of Discovery Program missions. Credit: NASA
It’s no secret that there has been a resurgence in interest in space exploration in recent years. Much of the credit for this goes to NASA’s ongoing exploration efforts on Mars, which in the past few years have revealed things like organic molecules on the surface, evidence of flowing water, and that the planet once had a denser atmosphere – all of which indicate that the planet may have once been hospitable to life.
But when it comes to the future, NASA is looking beyond Mars to consider missions that will send missions to Venus, near-Earth objects, and a variety of asteroids. With an eye to Venus, they are busy investigating the possibility of sending the Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging (DAVINCI) spacecraft to the planet by the 2020s.
Led by Lori Glaze of the Goddard Spaceflight Center, the DAVINCI descent craft would essentially pick up where the American and Soviet space programs left off with the Pioneer and Venera Programs in the 1970s and 80s. The last time either country sent a probe into Venus’ atmosphere was in 1985, when the Soviet probes Vega 1 and 2 both orbited the planet and released a balloon-supported aerobot into the upper atmosphere.
Model of the Vega 1 probe and landing apparatus at the Udvar-Hazy Center, Dulles International Airport, Chantilly, Virginia. Credit: historicspacecraft.com
These probes both remained operational for 46 hours and discovered just how turbulent and powerful Venus’ atmosphere was. In contrast, the DAVINCI probe’s mission will be to study both the atmosphere and surface of Venus, and hopefully shed some light on some of the planet’s newfound mysteries. According to the NASA release:
“DAVINCI would study the chemical composition of Venus’ atmosphere during a 63-minute descent. It would answer scientific questions that have been considered high priorities for many years, such as whether there are volcanoes active today on the surface of Venus and how the surface interacts with the atmosphere of the planet.”
These studies will attempt to build upon the data obtained by the Venus Express spacecraft, which in 2008/2009 noted the presence of several infrared hot spots in the Ganis Chasma region near the the shield volcano of Maat Mons (shown below). Believed to be due to volcanic eruptions, this activity was thought to be responsible for significant changes that were noted in the sulfur dioxide (SO²) content in the atmosphere at the time.
What’s more, the Pioneer Venus spacecraft – which studied the planet’s atmosphere from 1978 until its orbit decayed in 1992 – noted a tenfold decreased in the density of SO² at the cloud tops, which was interpreted as a decline following an episode of volcanogenic upwelling from the lower atmosphere.
3-D perspective of the Venusian volcano, Maat Mons, generated from radar data from NASA’s Magellan mission. Credit: NASA/JPL
Commonly associated with volcanic activity here on Earth, SO² is a million times more abundant in Venus’ atmosphere, where it helps to power the runaway greenhouse effect that makes the planet so inhospitable. However, any SO² released into Venus’ atmosphere is also short-lived, being broken down by sunlight within a matter of days.
Hence, any significant changes in SO² levels in the upper atmosphere must have been a recent addition, and some scientists believe that the spike observed in 2008/2009 was due to a large volcano (or several) erupting. Determining whether or not this is the case, and whether or not volcanic activity plays an active role in the composition of Venus’s thick atmosphere, will be central to DAVINCI’s mission.
Along with four other mission concepts, DAVINCI was selected as a semifinalist for the NASA Discovery Program‘s latest calls for proposed missions. Every few years, the Discovery Program – a low-cost planetary missions program that is managed by the JPL’s Planetary Science Division – puts out a call for missions with an established budget of around $500 million (not counting the cost of launch or operation).
The latest call for submissions took place in February 2014, as part of the Discovery Mission 13. At the time, a total of 27 teams threw their hats into the ring to become part of the next round of space exploration missions. Last Wednesday, September 30th, 2015, five semifinalists were announced, one (or possibly two) of which will be chosen as the winner(s) by September 2016.
Artist rendition of NASA’s Mars InSight (Interior exploration using Seismic Investigations, Geodesy and Heat Transport) Lander, which was selected as part of the Discovery Programs 2010 call for submissions and will be launched by 2016. Credit: JPL/NASA
These finalists will receive $3 million in federal grants for detailed concept studies, and the mission (or missions) that are ultimately chosen will be launched by December 31st, 2021. The Discovery Program began back in 1992, and launched its first mission- the Mars Pathfinder – in 1996. Other Discovery missions include the NEAR Shoemaker probe that first orbited an asteroid, and the Stardust-NExT project, which returned samples of comet and interstellar dust to Earth.
NASA’s MESSENGER spacecraft, the planet-hunting Kepler telescope, and the Dawn spacecraft were also developed and launched under the Discovery program. The winning proposal of the Discovery Program’s 12th mission, which was issued back in 2010, was the InSight Mars lander. Set to launch in March of 2016, the lander will touch down on the red planet, deploy instruments to the planet’s interior, and measure its seismic activity.
NASA hopes to infuse the next mission with new technologies, offering up government-furnished equipment with incentives to sweeten the deal for each proposal. These include a supply of deep space optical communications system that are intended to test new high-speed data links with Earth. Science teams that choose to incorporate the laser telecom unit will be entitled to an extra $30 million above their $450 million cost cap.
If science teams wish to send entry probes into the atmospheres of Venus or Saturn, they will need a new type of heat shield. Hence, NASA’s solicitation includes a provision to furnish a newly-developed 3D-woven heat shield with a $10 million incentive. A deep space atomic clock is also available with a $5 million bonus, and NASA has offered to provide xenon ion thrusters and radioisotope heater units without incentives.
As with previous Discovery missions, NASA has stipulated that the mission must use solar power, limiting mission possibilities beyond Jupiter and Saturn. Other technologies may include the NEXT ion thruster and/or re-entry technology.
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.
Jupiter’s icy moon Europa. Credits: NASA/Jet Propulsion Laboratory, SETI Institute
Jupiter‘s four largest moons – aka. the Galilean Moons, consisting of Io, Europa, Ganymede, and Callisto – are nothing if not fascinating. Ever since their discovery over four centuries ago, these moons have been a source of many great discoveries. These include the possibility of internal oceans, the presence of atmospheres, volcanic activity, a magnetosphere (Ganymede), and the possibility of having more water than Earth.
But arguably, the most fascinating of the Galilean Moons is Europa: the sixth closest moon to Jupiter, the smallest of the four, and the sixth-largest moon in the Solar System. In addition to having an icy surface and a possible warm-water interior, this moon is considered to be one of the most likely places for finding life beyond Earth.
The Sun. Credit: NASA & European Space Agency (ESA)
The Sun is the center of the Solar System and the source of all life and energy here on Earth. It accounts for more than 99.86% of the mass of the Solar System and it’s gravity dominates all the planets and objects that orbit it. Since the beginning of history, human beings have understood the Sun’s importance to our world, it’s seasons, the diurnal cycle, and the life-cycle of plants.
Because of this, the Sun has been at the center of many ancient culture’s mythologies and systems of worship. From the Aztecs, Mayans and Incas to the ancient Sumerians, Egyptians, Greeks, Romans and Druids, the Sun was a central deity because it was seen as the bringer of all light and life. In time, our understanding of the Sun has changed and become increasingly empirical. But that has done nothing to diminish it’s significance.
This photo of the Moon was taken on October 2, 2011 in Angera, Lombardy, IT. Credit: Milo.
Look up in the night sky. On a clear night, if you’re lucky, you’ll catch a glimpse of the Moon shining in all it’s glory. As Earth‘s only satellite, the Moon has orbited our planet for over three and a half billion years. There has never been a time when human beings haven’t been able to look up at the sky and see the Moon looking back at them.
As a result, it has played a vital role in the mythological and astrological traditions of every human culture. A number of cultures saw it as a deity while others believed that its movements could help them to predict omens. But it is only in modern times that the true nature and origins of the Moon, not to mention the influence it has on planet Earth, have come to be understood.
Size, Mass and Orbit:
With a mean radius of 1737 km and a mass of 7.3477 x 10²² kg, the Moon is 0.273 times the size of Earth and 0.0123 as massive. Its size, relative to Earth, makes it quite large for a satellite – second only to Charon‘s size relative to Pluto. With a mean density of 3.3464 g/cm³, it is 0.606 times as dense as Earth, making it the second densest moon in our Solar System (after Io). Last, it has a surface gravity equivalent to 1.622 m/s2, which is 0.1654 times, or 17%, the Earth standard (g).
The Moon’s orbit has a minor eccentricity of 0.0549, and orbits our planet at a distance of between 356,400-370,400 km at perigee and 404,000-406,700 km at apogee. This gives it an average distance (semi-major axis) of 384,399 km, or 0.00257 AU. The Moon has an orbital period of 27.321582 days (27 d 7 h 43.1 min), and is tidally-locked with our planet, which means the same face is always pointed towards Earth.
Structure and Composition:
Much like Earth, the Moon has a differentiated structure that includes an inner core, an outer core, a mantle, and a crust. It’s core is a solid iron-rich sphere that measures 240 km (150 mi) across, and it surrounded by a outer core that is primarily made of liquid iron and which has a radius of roughly 300 km (190 mi).
Around the core is a partially molten boundary layer with a radius of about 500 km (310 mi). This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon’s formation 4.5 billion years ago. Crystallization of this magma ocean would have created a mantle rich in magnesium and iron nearer to the top, with minerals like olivine, clinopyroxene, and orthopyroxene sinking lower.
The mantle is also composed of igneous rock that is rich in magnesium and iron, and geochemical mapping has indicated that the mantle is more iron rich than Earth’s own mantle. The surrounding crust is estimated to be 50 km (31 mi) thick on average, and is also composed of igneous rock.
The Moon is the second densest satellite in the Solar System after Io. However, the inner core of the Moon is small, at around 20% of its total radius. Its composition is not well constrained, but it is probably a metallic iron alloy with a small amount of sulfur and nickel and analyses of the Moon’s time-variable rotation indicate that it is at least partly molten.
Artist concept illustration of the internal structure of the moon. Credit: NOAJ
The presence of water has also been confirmed on the Moon, the majority of which is located at the poles in permanently-shadowed craters, and possibly also in reservoirs located beneath the lunar surface. The widely accepted theory is that most of the water was created through the Moon’s interaction of solar wind – where protons collided with oxygen in the lunar dust to create H²O – while the rest was deposited by cometary impacts.
Surface Features:
The geology of the Moon (aka. selenology) is quite different from that of Earth. Since the Moon lacks a significant atmosphere, it does not experience weather – hence there is no wind erosion. Similarly, since it lacks liquid water, there is also no erosion caused by flowing water on its surface. Because of its small size and lower gravity, the Moon cooled more rapidly after forming, and does not experience tectonic plate activity.
Instead, the complex geomorphology of the lunar surface is caused by a combination of processes, particularly impact cratering and volcanoes. Together, these forces have created a lunar landscape that is characterized by impact craters, their ejecta, volcanoes, lava flows, highlands, depressions, wrinkle ridges and grabens.
The most distinctive aspect of the Moon is the contrast between its bright and dark zones. The lighter surfaces are known as the “lunar highlands” while the darker plains are called maria (derived from the Latin mare, for “sea”). The highlands are made of igneous rock that is predominately composed of feldspar, but also contains trace amounts of magnesium, iron, pyroxene, ilmenite, magnetite, and olivine.
LROC Wide Angle Camera (WAC) mosaic of the lunar South Pole region, width ~600 km. Credit: NASA/GSFC/Arizona State University.
Mare regions, in contrast, are formed from basalt (i.e. volcanic) rock. The maria regions often coincide with the “lowlands,” but it is important to note that the lowlands (such as within the South Pole-Aitken basin) are not always covered by maria. The highlands are older than the visible maria, and hence are more heavily cratered.
Other features include rilles, which are long, narrow depressions that resemble channels. These generally fall into one of three categories: sinuous rilles, which follow meandering paths; arcuate rilles, which have a smooth curve; and linear rilles, which follow straight paths. These features are often the result of the formation of localized lava tubes that have since cooled and collapsed, and can be traced back to their source (old volcanic vents or lunar domes).
Lunar domes are another feature that is related to volcanic activity. When relatively viscous, possibly silica-rich lava erupts from local vents, it forms shield volcanoes that are referred to as lunar domes. These wide, rounded, circular features have gentle slopes, typically measure 8-12 km in diameter and rise to an elevation of a few hundred meters at their midpoint.
Wrinkle ridges are features created by compressive tectonic forces within the maria. These features represent buckling of the surface and form long ridges across parts of the maria. Grabens are tectonic features that form under extension stresses and which are structurally composed of two normal faults, with a down-dropped block between them. Most grabens are found within the lunar maria near the edges of large impact basins.
Rima Ariadaeus as photographed from Apollo 10. The crater to the south of the rille in the left half of the image is Silberschlag. The dark patch at the top right is the floor of the crater Boscovich. Credit: NASA
Impact craters are the Moon’s most common feature, and are created when a solid body (an asteroid or comet) collides with the surface at a high velocity. The kinetic energy of the impact creates a compression shock wave that creates a depression, followed by a rarefaction wave that propels most of the ejecta out of the crater, and then a rebounds to form a central peak.
These craters range in size from tiny pits to the immense South Pole–Aitken Basin, which has a diameter of nearly 2,500 km and a depth of 13 km. In general, the lunar history of impact cratering follows a trend of decreasing crater size with time. In particular, the largest impact basins were formed during the early periods, and these were successively overlaid by smaller craters.
There are estimated to be roughly 300,000 craters wider than 1 km (0.6 mi) on the Moon’s near side alone. Some of these are named for scholars, scientists, artists and explorers. The lack of an atmosphere, weather and recent geological processes mean that many of these craters are well-preserved.
Another feature of the lunar surface is the presence of regolith (aka. Moon dust, lunar soil). Created by billions of years of collisions by asteroids and comets, this fine grain of crystallized dust covers much of the lunar surface. The regolith contains rocks, fragments of minerals from the original bedrock, and glassy particles formed during the impacts.
The historic boot print left behind by the Apollo 11 crew in the lunar regolith. Credit: NASA
The chemical composition of the regolith varies according to its location. Whereas the regolith in the highlands is rich in aluminum and silica, the regolith in the maria is rich in iron and magnesium and is silica-poor, as are the basaltic rocks from which it is formed.
Geological studies of the Moon are based on a combination of Earth-based telescope observations, measurements from orbiting spacecraft, lunar samples, and geophysical data. A few locations were sampled directly during the Apollo missions in the late 1960s and early 1970s, which returned approximately 380 kilograms (838 lb) of lunar rock and soil to Earth, as well as several missions of the Soviet Luna programme.
Atmosphere:
Much like Mercury, the Moon has a tenuous atmosphere (known as an exosphere), which results in severe temperature variations. These range from -153°C to 107°C on average, though temperatures as low as -249°C have been recorded. Measurements from NASA’s LADEE have mission determined the exosphere is mostly made up of helium, neon and argon.
The helium and neon are the result of solar wind while the argon comes from the natural, radioactive decay of potassium in the Moon’s interior. There is also evidence of frozen water existing in permanently shadowed craters, and potentially below the soil itself. The water may have been blown in by the solar wind or deposited by comets.
Formation:
Several theories have been proposed for the formation of the Moon. These include the fission of the Moon from the Earth’s crust through centrifugal force, the Moon being a preformed object that was captured by Earth’s gravity, and the Earth and Moon co-forming together in the primordial accretion disk. The estimated age of the Moon also ranges from it being formed 4.40-4.45 billion years ago to 4.527 ± 0.010 billion years ago,roughly 30–50 million years after the formation of the Solar System.
The prevailing hypothesis today is that the Earth-Moon system formed as a result of an impact between the newly-formed proto-Earth and a Mars-sized object (named Theia) roughly 4.5 billion years ago. This impact would have blasted material from both objects into orbit, where it eventually accreted to form the Moon.
This has become the most accepted hypothesis for several reasons. For one, such impacts were common in the early Solar System, and computer simulations modelling the impact are consistent with the measurements of the Earth-Moon system’s angular momentum, as well as the small size of the lunar core.
In addition, examinations of various meteorites show that other inner Solar System bodies (such as Mars and Vesta) have very different oxygen and tungsten isotopic compositions to Earth. In contrast, examinations of the lunar rocks brought back by the Apollo missions show that Earth and the Moon have nearly identical isotopic compositions.
This is the most compelling evidence suggesting that the Earth and the Moon have a common origin.
Relationship to Earth:
The Moon makes a complete orbit around Earth with respect to the fixed stars about once every 27.3 days (its sidereal period). However, because Earth is moving in its orbit around the Sun at the same time, it takes slightly longer for the Moon to show the same phase to Earth, which is about 29.5 days (its synodic period). The presence of the Moon in orbit influences conditions here on Earth in a number of ways.
The most immediate and obvious are the ways its gravity pulls on Earth – aka. it’s tidal effects. The result of this is an elevated sea level, which are commonly referred to as ocean tides. Because Earth spins about 27 times faster than the Moon moves around it, the bulges are dragged along with Earth’s surface faster than the Moon moves, rotating around Earth once a day as it spins on its axis.
The ocean tides are magnified by other effects, such as frictional coupling of water to Earth’s rotation through the ocean floors, the inertia of water’s movement, ocean basins that get shallower near land, and oscillations between different ocean basins. The gravitational attraction of the Sun on Earth’s oceans is almost half that of the Moon, and their gravitational interplay is responsible for spring and neap tides.
Gravitational coupling between the Moon and the bulge nearest the Moon acts as a torque on Earth’s rotation, draining angular momentum and rotational kinetic energy from Earth’s spin. In turn, angular momentum is added to the Moon’s orbit, accelerating it, which lifts the Moon into a higher orbit with a longer period.
As a result of this, the distance between Earth and Moon is increasing, and Earth’s spin is slowing down. Measurements from lunar ranging experiments with laser reflectors (which were left behind during the Apollo missions) have found that the Moon’s distance to Earth increases by 38 mm (1.5 in) per year.
This speeding and slowing of Earth and the Moon’s rotation will eventually result in a mutual tidal locking between the Earth and Moon, similar to what Pluto and Charon experience. However, such a scenario is likely to take billions of years, and the Sun is expected to have become a red giant and engulf Earth long before that.
The lunar surface also experiences tides of around 10 cm (4 in) amplitude over 27 days, with two components: a fixed one due to Earth (because they are in synchronous rotation) and a varying component from the Sun. The cumulative stress caused by these tidal forces produces moonquakes. Despite being less common and weaker than earthquakes, moonquakes can last longer (one hour) since there is no water to damp out the vibrations.
Another way the Moon effects life on Earth is through occultation (i.e. eclipses). These only happen when the Sun, the Moon, and Earth are in a straight line, and take one of two forms – a lunar eclipse and a solar eclipse. A lunar eclipse occurs when a full Moon passes behind Earth’s shadow (umbra) relative to the Sun, which causes it to darken and take on a reddish appearance (aka. a “Blood Moon” or “Sanguine Moon”.)
A solar eclipse occurs during a new Moon, when the Moon is between the Sun and Earth. Since they are the same apparent size in the sky, the moon can either partially block the Sun (annular eclipse) or fully block it (total eclipse). In the case of a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye.
The geometry that creates a total lunar eclipse. Credit: NASA
Because the Moon’s orbit around Earth is inclined by about 5° to the orbit of Earth around the Sun, eclipses do not occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes.The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by the “Saros Cycle“, which is a period of approximately 18 years.
History of Observation:
Human beings have been observing the Moon since prehistoric times, and understanding the Moon’s cycles was one of the earliest developments in astronomy. The earliest examples of this comes from the 5th century BCE, when Babylonian astronomers had recorded the 18-year Satros cycle of lunar eclipses, and Indian astronomers had described the Moon’s monthly elongation.
The ancient Greek philosopher Anaxagoras (ca. 510 – 428 BCE) reasoned that the Sun and Moon were both giant spherical rocks, and the latter reflected the light of the former. In Aristotle’s “On the Heavens“, which he wrote in 350 BCE, the Moon was said to mark the boundary between the spheres of the mutable elements (earth, water, air and fire), and the heavenly stars – an influential philosophy that would dominate for centuries.
In the 2nd century BCE, Seleucus of Seleucia correctly theorized that tides were due to the attraction of the Moon, and that their height depends on the Moon’s position relative to the Sun. In the same century, Aristarchus computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the radius of Earth for the distance. These figures were greatly improved by Ptolemy (90–168 BCE), who’s values of a mean distance of 59 times Earth’s radius and a diameter of 0.292 Earth diameters were close to the correct values (60 and 0.273 respectively).
By the 4th century BCE, the Chinese astronomer Shi Shen gave instructions for predicting solar and lunar eclipses. By the time of the Han Dynasty (206 BCE – 220 CE), astronomers recognized that moonlight was reflected from the Sun, and Jin Fang (78–37 BC) postulated that the Moon was spherical in shape.
In 499 CE, the Indian astronomer Aryabhata mentioned in his Aryabhatiya that reflected sunlight is the cause of the shining of the Moon. The astronomer and physicist Alhazen (965–1039) found that sunlight was not reflected from the Moon like a mirror, but that light was emitted from every part of the Moon in all directions.
Shen Kuo (1031–1095) of the Song dynasty created an allegory to explain the waxing and waning phases of the Moon. According to Shen, it was comparable to a round ball of reflective silver that, when doused with white powder and viewed from the side, would appear to be a crescent.
During the Middle Ages, before the invention of the telescope, the Moon was increasingly recognized as a sphere, though many believed that it was “perfectly smooth”. In keeping with medieval astronomy, which combined Aristotle’s theories of the universe with Christian dogma, this view would later be challenged as part of the Scientific Revolution (during the 16th and 17th century) where the Moon and other planets would come to be seen as being similar to Earth.
Using a telescope of his own design, Galileo Galilei drew one of the first telescopic drawings of the Moon in 1609, which he included in his book Sidereus Nuncius (“Starry Messenger). From his observations, he noted that the Moon was not smooth, but had mountains and craters. These observations, coupled with observations of moons orbiting Jupiter, helped him to advance the heliocentric model of the universe.
Telescopic mapping of the Moon followed, which led to the lunar features being mapped in detail and named. The names assigned by Italian astronomers Giovannia Battista Riccioli and Francesco Maria Grimaldi are still in use today. The lunar map and book on lunar features created by German astronomers Wilhelm Beer and Johann Heinrich Mädler between 1834 and 1837 were the first accurate trigonometric study of lunar features, and included the heights of more than a thousand mountains.
Lunar craters, first noted by Galileo, were thought to be volcanic until the 1870s, when English astronomer Richard Proctor proposed that they were formed by collisions. This view gained support throughout the remainder of the 19th century; and by the early 20th century, led to the development of lunar stratigraphy – part of the growing field of astrogeology.
Exploration:
With the beginning of the Space Age in the mid-20th century, the ability to physically explore the Moon became possible for the first time. And with the onset of the Cold War, both the Soviet and American space programs became locked in an ongoing effort to reach the Moon first. This initially consisted of sending probes on flybys and landers to the surface, and culminated with astronauts making manned missions.
The Soviet Luna 1 Robotic space probe. Credit: RIA Novosti/ Alexander Mokletsov/Public Domain
Exploration of the Moon began in earnest with the Soviet Luna program. Beginning in earnest in 1958, the programmed suffered the loss of three unmanned probes. But by 1959, the Soviets managed to successfully dispatch fifteen robotic spacecraft to the Moon and accomplished many firsts in space exploration. This included the first human-made objects to escape Earth’s gravity (Luna 1), the first human-made object to impact the lunar surface (Luna 2), and the first photographs of the far side of the Moon (Luna 3).
Between 1959 and 1979, the program also managed to make the first successful soft landing on the Moon (Luna 9), and the first unmanned vehicle to orbit the Moon (Luna 10) – both in 1966. Rock and soil samples were brought back to Earth by three Luna sample return missions – Luna 16 (1970), Luna 20 (1972), and Luna 24 (1976).
Two pioneering robotic rovers landed on the Moon – Luna 17 (1970) and Luna 21 (1973) – as a part of Soviet Lunokhod program. Running from 1969 to 1977, this program was primarily designed to provide support for the planned Soviet manned moon missions. But with the cancellation of the Soviet manned moon program, they were instead used as remote-controlled robots to photograph and explore the lunar surface.
NASA began launching probes to provide information and support for an eventual Moon landing in the early 60s. This took the form of the Ranger program, which ran from 1961 – 1965 and produced the first close-up pictures of the lunar landscape. It was followed by the Lunar Orbiter program which produced maps of the entire Moon between 1966-67, and the Surveyor program which sent robotic landers to the surface between 1966-68.
In 1969, astronaut Neil Armstrong made history by becoming the first person to walk on the Moon. As the commander of the American mission Apollo 11, he first sett foot on the Moon at 02:56 UTC on 21 July 1969. This represented the culmination of the Apollo program (1969-1972), which sought to send astronauts to the lunar surface to conduct research and be the first human beings to set foot on a celestial body other than Earth.
The Apollo 11 to 17 missions (save forApollo 13, which aborted its planned lunar landing) sent a total of 13 astronauts to the lunar surface and returned 380.05 kilograms (837.87 lb) of lunar rock and soil. Scientific instrument packages were also installed on the lunar surface during all the Apollo landings. Long-lived instrument stations, including heat flow probes, seismometers, and magnetometers, were installed at the Apollo 12, 14, 15, 16, and 17 landing sites, some of which are still operational.
After the Moon Race was over, there was a lull in lunar missions. However, by the 1990s, many more countries became involve in space exploration. In 1990, Japan became the third country to place a spacecraft into lunar orbit with its Hiten spacecraft, an orbiter which released the smaller Hagoroma probe.
In 1994, the U.S. sent the joint Defense Department/NASA spacecraft Clementine to lunar orbit to obtain the first near-global topographic map of the Moon and the first global multispectral images of the lunar surface. This was followed in 1998 by the Lunar Prospector mission, whose instruments indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few meters of the regolith within permanently shadowed craters.
Mosaic of the Chang’e-3 moon lander and the lunar surface, taken by the Yutu rover during Lunar Day 3. Credit: CNSA/SASTIND/Xinhua/Marco Di Lorenzo/Ken Kremer
Since the year 2000, exploration of the moon has intensified, with a growing number of parties becoming involved. The ESA’s SMART-1spacecraft, the second ion-propelled spacecraft ever created, made the first detailed survey of chemical elements on the lunar surface while in orbit from November 15th, 2004, until its lunar impact on September 3rd, 2006.
China has pursued an ambitious program of lunar exploration under their Chang’e program. This began with Chang’e 1, which successfully obtained a full image map of the Moon during its sixteen month orbit (November 5th, 2007 – March 1st, 2009) of the Moon. This was followed in October of 2010 with the Chang’e 2 spacecraft, which mapped the Moon at a higher resolution before performing a flyby of asteroid 4179 Toutatis in December of 2012, then heading off into deep space.
On 14 December 2013, Chang’e 3 improved upon its orbital mission predecessors by landing a lunar lander onto the Moon’s surface, which in turn deployed a lunar rover named Yutu (literally “Jade Rabbit”). In so doing, Chang’e 3 made the first soft lunar landing since Luna 24 in 1976, and the first lunar rover mission since Lunokhod 2 in 1973.
Between October 4th, 2007, and June 10th, 2009, the Japan Aerospace Exploration Agency‘s (JAXA) Kaguya (“Selene”) mission – a lunar orbiter fitted with a high-definition video camera and two small radio-transmitter satellites – obtained lunar geophysics data and took the first high-definition movies from beyond Earth orbit.
The Indian Space Research Organisation (ISRO) first lunar mission, Chandrayaan I, orbited the Moon between November 2008 and August 2009 and created a high resolution chemical, mineralogical and photo-geological map of the lunar surface, as well as confirming the presence of water molecules in lunar soil. A second mission was planned for 2013 in collaboration with Roscosmos, but was cancelled.
NASA has also been busy in the new millennium. In 2009, they co-launched the Lunar Reconnaissance Orbiter (LRO) and the Lunar CRater Observation and Sensing Satellite(LCROSS) impactor. LCROSS completed its mission by making a widely observed impact in the crater Cabeus on October 9th, 2009, while the LRO is currently obtaining precise lunar altimetry and high-resolution imagery.
Two NASA Gravity Recovery And Interior Library (GRAIL) spacecraft began orbiting the Moon in January 2012 as part of a mission to learn more about the Moon’s internal structure.
Upcoming lunar missions include Russia’s Luna-Glob – an unmanned lander with a set of seismometers, and an orbiter based on its failed Martian Fobos-Grunt mission. Privately funded lunar exploration has also been promoted by the Google Lunar X Prize, which was announced on September 13th, 2007, and offers US$20 million to anyone who can land a robotic rover on the Moon and meet other specified criteria.
Under the terms of the Outer Space Treaty, the Moon remains free to all nations to explore for peaceful purposes. As our efforts to explore space continue, plans to create a lunar base and possibly even a permanent settlement may become a reality. Looking to the distant future, it wouldn’t be far fetched at all to imagine native-born humans living on the Moon, perhaps known as Lunarians (though I imagine Lunies will be more popular!)
We have many interesting articles about the Moon here at Universe Today. Below is a list that covers just about everything we know about it today. We hope you find what you are looking for:
In addition to being the birthplace of humanity and the cradle of human civilization, Earth is the only known planet in our Solar System that is capable of sustaining life. As a terrestrial planet, Earth is located within the Inner Solar System between between Venus and Mars (which are also terrestrial planets). This place Earth in a prime location with regards to our Sun’s Habitable Zone.
Earth has a number of nicknames, including the Blue Planet, Gaia, Terra, and “the world” – which reflects its centrality to the creation stories of every single human culture that has ever existed. But the most remarkable thing about our planet is its diversity. Not only are there an endless array of plants, animals, avians, insects and mammals, but they exist in every terrestrial environment. So how exactly did Earth come to be the fertile, life-giving place we all know and love?
Illustration of Jupiter and the Galilean satellites. Credit: NASA
Jupiter was appropriately named by the Romans, who chose to name it after the king of the gods. In addition to being the largest planet in our Solar System – with two and a half times the mass of all the other planets combined – it also has the most moons of any Solar planet. So far, 67 natural satellites have been discovered around the gas giant, and more could be on the way.
The moons of Jupiter are so numerous and so diverse that they are broken down into several groups. First, there are the largest moons known as the Galileans, or Main Group. Together with the smaller Inner Group, they make up Jupiter’s Regular Satellites. Beyond them, there are the many Irregular Satellites that circle the planet, along with its debris rings. Here’s what we know about them…
Discovery and Naming:
Using a telescope of his own design, which allowed for 20 x normal magnification, Galileo Galilei was able to make the first observations of celestial bodies that were not visible to the naked eye. In 1610, he made the first recorded discovery of moons orbiting Jupiter, which later came to be known as the Galilean Moons.
At the time, he observed only three objects, which he believed to be fixed stars. However, between January and March of 1610, he continued to observe them, and noted a fourth body as well. In time, he realized that these four bodies did not behave like fixed stars, and were in fact objects that orbited Jupiter.
Portrait of Galileo Galilei by Giusto Sustermans, 1636 . Credit: Royal Museum Greenwhich
These discoveries proved the importance of using the telescope to view celestial objects that had previously remained unseen. More importantly, by showing that planets other than Earth had their own system of satellites, Galileo dealt a significant blow to the Ptolemaic model of the universe, which was still widely accepted.
Seeking the patronage of the Grand Duke of Tuscany, Cosimo de Medici, Galileo initially sought permission to name the moons the “Cosmica Sidera” (or Cosimo’s Stars). At Cosimo’s suggestion, Galileo changed the name to Medicea Sidera (“the Medician stars”), honouring the Medici family. The discovery was announced in the Sidereus Nuncius (“Starry Messenger”), which was published in Venice in March 1610.
However, German astronomer Simon Marius had independently discovered these moons at the same time as Galileo. At the behest of Johannes Kepler, he named the moons after the lovers of Zues (the Greek equivalent of Jupiter). In his treatise titled Mundus Jovialis (“The World of Jupiter”, published in 1614) he named them Io, Europa, Ganymede, and Callisto.
Galileo steadfastly refused to use Marius’ names and instead invented the numbering scheme that is still used today, alongside proper moon names. In accordance with this scheme, moons are assigned numbers based on their proximity to their parent planet and increase with distance. Hence, the moons of Io, Europa, Ganymede and Callisto were designated as Jupiter I, II, III, and IV, respectively.
Drawing of Jupiter made on Nov. 1, 1880 by French artist and astronomer Etienne Trouvelot showing transiting moon shadows and a much larger Great Red Spot. Credit: E.L. Trouvelot, New York Public Library
After Galileo made the first recorded discovery of the Main Group, no additional satellites were discovered for almost three centuries – not until E. E. Barnard observed Amalthea in 1892. In fact, it was not until the 20th century, and with the aid of telescopic photography and other refinements, that most of the Jovian satellites began to be discovered.
Himalia was discovered in 1904, Elara in 1905, Pasiphaë in 1908, Sinope in 1914, Lysithea and Carme in 1938, Ananke in 1951, and Leda in 1974. By the time Voyager space probes reached Jupiter around 1979, 13 moons had been discovered, while Voyager herself discovered an additional three – Metis, Adrastea, and Thebe.
Between October 1999 and February 2003, researchers using sensitive ground-based detectors found and later named another 34 moons, most of which were discovered by a team led by Scott S. Sheppard and David C. Jewitt. Since 2003, 16 additional moons have been discovered but not yet named, bringing the total number of known moons of Jupiter to 67.
Though the Galilean moons were named shortly after their discovery in 1610, the names of Io, Europa, Ganymede and Callisto fell out of favor until the 20th century. Amalthea (aka. Jupiter V) was not so named until an unofficial convention took place in 1892, a name that was first used by the French astronomer Camille Flammarion.
Jupiter and its largest moons. Image credit: NASA/JPL
The other moons, in the majority of astronomical literature, were simply labeled by their Roman numeral (i.e. Jupiter IX) until the 1970s. This began in 1975 when the International Astronomical Union’s (IAU) Task Group for Outer Solar System Nomenclature granted names to satellites V–XIII, thus creating a formal naming process for any future satellites discovered. The practice was to name newly discovered moons of Jupiter after lovers and favorites of the god Jupiter (Zeus); and since 2004, also after their descendants.
Regular Satellites:
Jupiter’s Regular Satellites are so named because they have prograde orbits – i.e. they orbit in the same direction as the rotation of their planet. These orbits are also nearly circular and have a low inclination, meaning they orbit close to Jupiter’s equator. Of these, the Galilean Moons (aka. the Main Group) are the largest and the most well known.
These are Jupiter’s largest moons, not to mention the Solar System’s fourth, sixth, first and third largest satellites, respectively. They contain almost 99.999% of the total mass in orbit around Jupiter, and orbit between 400,000 and 2,000,000 km from the planet. They are also among the most massive objects in the Solar System with the exception of the Sun and the eight planets, with radii larger than any of the dwarf planets.
They include Io, Europa, Ganymede, and Callisto, and were all discovered by Galileo Galilei and named in his honor. The names of the moons, which are derived from the lovers of Zeus in Greek mythology, were prescribed by Simon Marius soon after Galileo discovered them in 1610. Of these, the innermost is Io, which is named after a priestess of Hera who became Zeus’ lover.
This global view of Jupiter’s moon, Io, was obtained during the tenth orbit of Jupiter by NASA’s Galileo spacecraft. Credit: NASA
With a diameter of 3,642 kilometers, it is the fourth-largest moon in the Solar System. With over 400 active volcanoes, it is also the most geologically active object in the Solar System. Its surface is dotted with over 100 mountains, some of which are taller than Earth’s Mount Everest.
Unlike most satellites in the outer Solar System (which are covered with ice), Io is mainly composed of silicate rock surrounding a molten iron or iron sulfide core. Io has an extremely thin atmosphere made up mostly of sulfur dioxide (SO2).
The second innermost Galilean moon is Europa, which takes its name from the mythical Phoenician noblewoman who was courted by Zeus and became the queen of Crete. At 3121.6 kilometers in diameter, it is the smallest of the Galileans, and slightly smaller than the Moon.
Europa’s surface consists of a layer of water surrounding the mantle which is thought to be 100 kilometers thick. The uppermost section is solid ice while the bottom is believed to be liquid water, which is made warm due to heat energy and tidal flexing. If true, then it is possible that extraterrestrial life could exist within this subsurface ocean, perhaps near a series of deep-ocean hydrothermal vents.
The surface of Europa is also one of the smoothest in the Solar System, a fact which supports the idea of liquid water existing beneath the surface. The lack of craters on the surface is attributed to the surface being young and tectonically active. Europa is primarily made of silicate rock and likely has an iron core, and a tenuous atmosphere composed primarily of oxygen.
Next up is Ganymede. At 5262.4 kilometers in diameter, Ganymede is the largest moon in the Solar System. While it is larger than the planet Mercury, the fact that it is an icy world means that it has only half of Mercury’s mass. It is also the only satellite in the Solar System known to possess a magnetosphere, likely created through convection within the liquid iron core.
Ganymede is composed primarily of silicate rock and water ice, and a salt-water ocean is believed to exist nearly 200 km below Ganymede’s surface – though Europa remains the most likely candidate for this. Ganymede has a high number of craters, most of which are now covered in ice, and boasts a thin oxygen atmosphere that includes O, O2, and possibly O3 (ozone), and some atomic hydrogen.
Callisto is the fourth and farthest Galilean moon. At 4820.6 kilometers in diameter, it is also the second largest of the Galileans and third largest moon in the Solar System. Callisto is named after the daughter of the Arkadian King, Lykaon, and a hunting companion of the goddess Artemis.
Composed of approximately equal amounts of rock and ices, it is the least dense of the Galileans, and investigations have revealed that Callisto may also have an interior ocean at depths greater than 100 kilometers from the surface.
Callisto is also one of the most heavily cratered satellites in the Solar System – the greatest of which is the 3000 km wide basin known as Valhalla. It is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably molecular oxygen. Callisto has long been considered the most suitable place for a human base for future exploration of the Jupiter system since it is furthest from the intense radiation of Jupiter.
This natural color view of Ganymede was taken from the Galileo spacecraft during its first encounter with the Jovian moon. Credit: NASA/JPL
The Inner Group (or Amalthea group) are four small moons that have diameters of less than 200 km, orbit at radii less than 200,000 km, and have orbital inclinations of less than half a degree. This groups includes the moons of Metis, Adrastea, Amalthea, and Thebe.
Along with a number of as-yet-unseen inner moonlets, these moons replenish and maintain Jupiter’s faint ring system – Metis and Adrastea helping Jupiter’s main ring, while Amalthea and Thebe maintain their own faint outer rings.
Metis is the closest moon to Jupiter at a distance of 128,000 km. It is roughly 40 km in diameter, tidally-locked, and highly-asymmetrical in shape (with one of the diameters being almost twice as large as the smallest one). It was not discovered until the 1979 flyby of Jupiter by the Voyager 1 space probe. It was named in 1983 after the first wife of Zeus.
The second closest moon is Adrastea, which is about 129,000 km from Jupiter and 20 km in diameter. Also known as Jupiter XV, Amalthea is the second by distance, and the smallest of the four inner moons of Jupiter. It was discovered in 1979 when the Voyager 2 probe photographed it during a flyby.
A schema of Jupiter’s ring system showing the four main components. For simplicity, Metis and Adrastea are depicted as sharing their orbit. Credit: NASA/JPL/Cornell University
Amalthea, also known as Jupiter V, is the third moon of Jupiter in order of distance from the planet. It was discovered on September 9, 1892, by Edward Emerson Barnard and named after a nymph in Greek mythology. It is thought to consist of porous water ice with unknown amounts of other materials. Its surface features include large craters and ridges.
Thebe (aka. Jupiter XIV) is the fourth and final inner moon of Jupiter. It is irregularly shaped and reddish in colour, and is thought like Amalthea to consist of porous water ice with unknown amounts of other materials. Its surface features also include large craters and high mountains – some of which are comparable to the size of the moon itself.
Irregular Satellites:
The Irregular Satellites are those that are substantially smaller and have more distant and eccentric orbits than the Regular Satellites. These moons are broken down into families that have similarities in orbit and composition. It is believed that these were at least partially formed as a result of collisions, most likely by asteroids that were captured by Jupiter’s gravitational field.
Amalthea, as photographed by the Galileo spacecraft. The left is from August 12, 1999 at a range of 446,000 km, the right from November 26, 1999 at a range of 374,000. Credit: NASA/JPL
Those that are grouped into families are all named after their largest member. For example, the Himalia group is named after Himalia – a satellite with a mean radius of 85 km, making it the fifth largest moon orbiting Jupiter. It is believed that Himalia was once an asteroid that was captured by Jupiter’s gravity, which then experienced a impact that formed the moons of Leda, Lysithea, and Elara. These moons all have prograde orbits, meaning they orbit in the same direction as Jupiter’s rotation.
The Carme group takes its name from the Moon of the same name. With a mean radius of 23 km, Carme is the largest member of a family of Jovian satellites which have similar orbits and appearance (uniformly red), and are therefore thought to have a common origin. The satellites in this family all have retrograde orbits, meaning they orbit Jupiter in the opposite direction of its rotation.
The Ananke group is named after its largest satellite, which has a mean radius of 14 km. It is believed that Ananke was also an asteroid that was captured by Jupiter’s gravity and then suffered a collision which broke off a number of pieces. Those pieces became the other 15 moons in the Ananke group, all of which have retrograde orbits and appear gray in color.
This image shows the Themis Main Belt which sits between Mars and Jupiter. Credit: Josh Emery/University of Tennessee, Knoxville
The Pasiphae group is a very diverse group which ranges in color from red to grey – signifying the possibility of it being the result of multiple collisions. Named after Paisphae, which has a mean radius of 30 km, these satellites are retrograde, and are also believed to be the result of an asteroid that was captured by Jupiter and fragmented due to a series of collisions.
There are also several irregular satellites that are not part of any particular family. These include Themisto and Carpo, the innermost and outermost irregular moons, both of which have prograde orbits. S/2003 J 12 and S/2011 J 1 are the innermost of the retrograde moons, while S/2003 J 2 is the outermost moon of Jupiter.
Structure and Composition:
As a rule, the mean density of Jupiter’s moons decrease with their distance from the planet. Callisto, the least dense of the four, has an intermediate density between ice and rock, whereas Io has a density that indicates its made of rock and iron. Callisto’s surface also has a heavily cratered ice surface, and the way it rotates indicates that its density is equally distributed.
This suggests that Callisto has no rocky or metallic core, but consists of a homogeneous mix of ice and rock. The rotation of the three inner moons, in contrast, indicates differentiation between a core of denser matter (such as silicates, rock and metals) and a mantle of lighter material (water ice).
Surface features of the four members at different levels of zoom in each row. Credit: NASA/JPL
The distance from Jupiter also accords with significant alterations in the surface structure of its moons. Ganymede reveals past tectonic movement of the ice surface, which would mean that the subsurface layers underwent partial melting at once time. Europa reveals more dynamic and recent movement of this nature, suggesting a thinner ice crust. Finally, Io, the innermost moon, has a sulfur surface, active volcanism, and no sign of ice.
All this evidence suggests that the nearer a moon is to Jupiter, the hotter its interior – with models suggesting that the level of tidal heating is in inverse proportion to the square of their distance from the planet. It is believed that all of Jupiter’s moons may have once had an internal composition similar to that of modern-day Callisto, while the rest changed over time as a result of tidal heating caused by Jupiter’s gravitational field.
What this means is that for all of Jupiter’s moons, except Callisto, their interior ice melted, allowing rock and iron to sink to the interior and water to cover the surface. In Ganymede, a thick and solid ice crust then formed while in warmer Europa, a thinner more easily broken crust formed. On Io, the closest planet to Jupiter, the heating was so extreme that all the rock melted and the water boiled out into space.
Jupiter, a gas giant of immense proportions, was appropriately named after the king of the Roman pantheon. It is only befitting that such a planet has many, many moons orbiting it. Given the discovery process, and how long it has taken us, it would not be surprising if there are more satellites around Jupiter just waiting to be discovered. Sixty-seven and counting!
Saturn is well known for being a gas giant, and for its impressive ring system. But would it surprise you to know that this planet also has the second-most moons in the Solar System, second only to Jupiter? Yes, Saturn has at least 150 moons and moonlets in total, though only 53 of them have been given official names.
Most of these moons are small, icy bodies that are little more than parts of its impressive ring system. In fact, 34 of the moons that have been named are less than 10 km in diameter while another 14 are 10 to 50 km in diameter. However, some of its inner and outer moons are among the largest and most dramatic in the Solar System, measuring between 250 and 5000 km in diameter and housing some of greatest mysteries in the Solar System.
Discovery and Naming:
Prior to the invention of telescopic photography, eight of Saturn’s moons were observed using simple telescopes. The first to be discovered was Titan, Saturn’s largest moon, which was observed by Christiaan Huygens in 1655 using a telescope of his own design. Between 1671 and 1684, Giovanni Domenico Cassini discovered the moons of Tethys, Dione, Rhea and Iapetus – which he collectively named the “Sider Lodoicea” (Latin for “Louisian Stars”, after King Louis XIV of France).
In 1789, William Herschel discovered Mimas and Enceladus, while father-and-son astronomers W.C Bond and G.P. Bond discovered Hyperion in 1848 – which was independently discovered by William Lassell that same year. By the end of the 19th century, the invention of long-exposure photographic plates allowed for the discovery of more moons – the first of which Phoebe, observed in 1899 by W.H. Pickering.
Saturn’s moons (from left to right) Janus, Pandora, Enceladus, Mimas and Rhea. Rhea is on top of Saturn. Credit: NASA/JPL-Caltech/Space Science Institute
In 1966, the tenth satellite of Saturn was discovered by French astronomer Audouin Dollfus, which was later named Janus. A few years later, it was realized that his observations could only be explained if another satellite had been present with an orbit similar to that of Janus. This eleventh moon was later named Epimetheus, which shares the same orbit with Janus and is the only known co-orbital in the Solar System.
By 1980, three additional moons were discovered and later confirmed by the Voyager probes. They were the trojan moons (see below) of Helene (which orbits Dione) as well as Telesto and Calypso (which orbit Tethys).
The study of the outer planets has since been revolutionized by the use of unmanned space probes. This began with the arrival of the Voyager spacecraft to the Cronian system in 1980-81, which resulted in the discovery of three additional moons – Atlas, Prometheus, and Pandora – bringing the total to 17. By 1990, archived images also revealed the existence of Pan.
This was followed by the Cassini-Huygens mission, which arrived at Saturn in the summer of 2004. Initially, Cassini discovered three small inner moons, including Methone and Pallene between Mimas and Enceladus, as well as the second Lagrangian moon of Dione – Polydeuces. In November of 2004,Cassini scientists announced that several more moons must be orbiting within Saturn’s rings. From this data, multiple moonlets and the moons of Daphnis and Anthe have been confirmed.
The moons of Saturn, from left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan in the background; Iapetus (top) and Hyperion (bottom). Credit: NASA/JPL/Space Science Institute
The study of Saturn’s moons has also been aided by the introduction of digital charge-coupled devices, which replaced photographic plates by the end of the 20th century. Because of this, ground-based telescopes have begun to discovered several new irregular moons around Saturn. In 2000, three medium-sized telescopes found thirteen new moons with eccentric orbits that were of considerable distance from the planet.
In 2005, astronomers using the Mauna Kea Observatory announced the discovery of twelve more small outer moons. In 2006, astronomers using Japan’s Subaru Telescope at Mauna Kea reported the discovery of nine more irregular moons. In April of 2007, Tarqeq (S/2007 S 1) was announced, and in May of that same year, S/2007 S 2 and S/2007 S 3 were reported.
The modern names of Saturn’s moons were suggested by John Herschel (William Herschel’s son) in 1847. In keeping with the nomenclature of the other planets, he proposed they be named after mythological figures associated with the Roman god of agriculture and harvest – Saturn, the equivalent of the Greek Cronus. In particular, the seven known satellites were named after Titans, Titanesses and Giants – the brothers and sisters of Cronus.
In 1848, Lassell proposed that the eighth satellite of Saturn be named Hyperion after another Titan. When in the 20th century, the names of Titans were exhausted, the moons were named after different characters of the Greco-Roman mythology, or giants from other mythologies. All the irregular moons (except Phoebe) are named after Inuit and Gallic gods and Norse ice giants.
Saturn’s Inner Large Moons:
Saturn’s moons 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.
Saturn’s moon of Enceladus. Credit: NASA/JPL/Space Science Institute
Saturn’s Inner Large Moons, which orbit within the E Ring (see below), include 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.
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.
Dione’s heavily cratered surface, as observed by the Cassini flyby in June, 2015. Credit: NASA/JPL
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.
Saturn’s Large Outer Moons:
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.
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.
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
The surface of Titan, which is difficult to observe due to persistent atmospheric haze, shows only a few impact craters, evidence of cryovolcanoes, 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.
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.
The two sides of Iapetus, “Saturn’s yin yang moon”. Credit: NASA/JPL
Saturn’s Irregular 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.
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.
Saturn’s rings and moons Credit: NASA
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.
Moon Formation:
It is thought that Saturn’s moon of Titan, its mid-sized moons and rings developed in a way that is closer to the Galilean moons of Jupiter. In short, this would mean that the regular moons formed from a circumplanetary disc, a ring of accreting gas and solid debris similar to a protoplanetary disc. Meanwhile, the outer, irregular moons are believed to have been objects that were captured by Saturn’s gravity and remained in distant orbits.
However, there are some variations on this theory. In one alternative scenario, two Titan-sized moons were formed from an accretion disc around Saturn; the second one eventually breaking up to produce the rings and inner mid-sized moons. In another, two large moons fused together to form Titan, and the collision scattered icy debris that formed to create the mid-sized moons.
However, the mechanics of how the moon’s formed remains a mystery for the time being. With additional missions mounted to study the atmospheres, compositions and surfaces of these moons, we may begin to understand where they truly came from.
Much like Jupiter, and all the other gas giants, Saturn’s system of satellites is extensive as it is impressive. In addition to the larger moons that are believed to have formed from a massive debris field that once orbited it, it also has countless smaller satellites that were captured by its gravitational field over the course of billions of years. One can only imagine how many more remain to be found orbiting the ringed giant.
