Guide to Space Archives

October 20, 2015November 6, 2017 by Matt Williams

Saturn’s Icy Moon Enceladus

"Tiger stripes" -- sources of ice spewing -- in this image of Saturn's Enceladus taken by the Cassini spacecraft in 2009. Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA

In the ongoing drive to unlock the secrets of Saturn and its system of moons, some truly fascinating and awe-inspiring things have been discovered. In addition to things like methane lakes and propane-rich atmospheres (Titan) to moon’s that resemble the Death Star (Mimas), it is also becoming abundantly clear that planet’s beyond Earth may harbor interior oceans and even the extra-terrestrial organisms.

Nowhere is this more apparent than on Enceladus, Saturn’s sixth largest moon, which also possesses some of the most interesting characteristics in the outer Solar System. These include long veins of blue ice that resemble stripes, not to mention amazing plumes of water ice that have been spotted periodically blasting out of the moon’s southern pole. These, in turn, raise the possibility of liquid water beneath the surface, and possibly even life!

Discovery and Naming:

Discovered in 1789 by William Herschel, Enceladus is named after one of the giants in Greek mythology. In fact, all of the large moons of Saturn are named after the Titans, as suggested by William Herschel’s son, John Herschel. He chose these names because Saturn (known in Greek mythology as Kronos) was the father of the Titans.

In contrast, in accordance with the IAU naming conventions for Enceladus, features are named after characters and places from the classic story One Thousand and One Nights (aka. Arabian Nights). Impact craters are named after characters, whereas other feature types – such as fossae (long, narrow depressions), dorsa (ridges), planitia (plains), and sulci (long parallel grooves), are named after places.

iameter comparison of the Saturnian moon Enceladus, Moon, and Earth. Credit: NASA/JPL-Caltech/Tom Reding — *Size comparison between the Cronian moon Enceladus, the Moon, and Earth. Credit: NASA/JPL-Caltech/Tom Reding*

Size, Mass and Orbit:

With a mean radius of 252 km, Enceladus is equivalent in size to 0.0395 Earths (or 0.1451 Moons). But with a mass of 1.08 ×10²⁰ kg, it is only 0.000018 as massive. The planet has a very minor eccentricity (0.0047) and orbits Saturn at an average distance (semi-major axis) of 237,948 km, between the orbits of Mimas and Tethys.

Enceladus takes 32.9 hours (1.37 days) to complete a single orbit around Saturn, and is currently in a 2:1 mean-motion orbital resonance with Dione; meaning that it completes two orbits of Saturn for every orbit completed by Dione. This forced resonance is what maintains Enceladus’s orbital eccentricity and results in tidal deformation, and the resulting heat dissipation is the main heating source for Enceladus’s geologic activity.

Like most of the larger natural satellites of Saturn, Enceladus rotates synchronously with its orbital period, keeping one face pointed toward Saturn. The planet also experiences forced libration, where it appears to oscillate relative to Saturn’s other moons – which may also provide Enceladus with an internal heat source.

Composition and Surface Features:

Enceladus has a density of 1.61 g/cm³, which is higher than Saturn’s other mid-sized, icy satellites, suggesting a composition that includes a greater percentage of silicates and iron. It is also believed to be largely differentiated between a geologically active core and an icy mantle, with a liquid water ocean nestled between.

Gravity measurements by NASA's Cassini spacecraft and Deep Space Network suggest that Saturn's moon Enceladus, which has jets of water vapor and ice gushing from its south pole, also harbors a large interior ocean beneath an ice shell, as this illustration depicts. Image Credit: NASA/JPL-Caltech — *Gravity measurements by NASA’s Cassini spacecraft and Deep Space Network suggest that Saturn’s moon Enceladus harbors a large interior ocean beneath it’s south pole.* *Credit:* *NASA/JPL-Caltech*

The existence of this liquid water ocean has been the subject of scientific debate since 2005, when scientists first observed plumes containing water vapor spewing from Enceladus’s south polar surface. These jets are capable of dispensing 250 kg of water vapor every second at speeds of up to 2,189 km/h (1,360 mph), and reaching 500 km into space.

In 2006, it was determined that Enceladus’s plumes are the source of Saturn’s E Ring and actively replenish it. According to measurements made by the Cassini-Huygens probe, these emissions are composed mostly of water vapor, as well as minor components like molecular nitrogen, methane, and carbon dioxide. Further observations noted the presence of simple hydrocarbons such as methane, propane, acetylene and formaldehyde.

The combined analysis of imaging, mass spectrometry, and magnetospheric data suggests that the observed south polar plume emanates from pressurized subsurface chambers. The intensity of the eruptions varies significantly due to changes in Enceladus’s orbit. Basically, the plumes are about four times brighter when Enceladus is at apoapsis (farthest from Saturn), which is consistent with geophysical calculations that predict that the south polar fissures will be under less compression, thus opening them wider.

The existence of subsurface water was confirmed thanks to evidence provided by the Cassini mission in 2014. This included gravity measurements obtained during the flybys of 2010-2012, which confirmed the existence of a south polar subsurface ocean of liquid water within Enceladus with a thickness of around 10 km.

Artist's rendering of possible hydrothermal activity that may be taking place on and under the seafloor of Enceladus. Image Credit: NASA/JPL — *Artist’s rendering of possible hydrothermal activity that may be taking place on and under the seafloor of Enceladus. Credit: NASA/JPL*

In addition, during the July 14, 2005 flyby, the Cassini probe also detected the presence of escaping internal heat in the southern polar region. These temperatures were too high to be attributed to solar heating, and combined with the geyser activity, seemed to indicate that the interior of the planet is still geologically active.

Further studies from measurements of Enceladus’s libration as it orbits Saturn strongly suggest that the entire icy crust is detached from the rocky core, which would mean that the ocean beneath its surface is planet-wide. The amount of libration implies that this global ocean is about 26 to 31 kilometers in depth (compared to Earth’s average ocean depth of 3.7 kilometers).

Observations of Enceladus’ surface has revealed five types of terrain – cratered terrain, smooth (young) terrain, ridged terrain (often bordering on smooth areas), linear cracks, scarps, troughs, and grooves. Surveys of the cratered terrain, smooth plains, and other features indicate a level of resurfacing that suggests that tectonics are an important factor in the geological history of Enceladus.

Recent observations by Cassini have provided a closer look at the crater distribution and size. These features have been named by the IAU after characters and places from Burton’s translation of The Book of One Thousand and One Nights – i.e. the Shahrazad crater, the Diyar plains, the Anbar depression.

Artist impression of the view of Saturn from its moon Enceladus (Michael Carroll) — *Artist impression of the view of Saturn from Enceladus, with geysers erupting at the right in the foreground. Credit: Michael Carroll*

The smooth plains are dominated by fresh clean ice, which gives Enceladus what is possibly the most reflective surface in the Solar System (with a visual geometric albedo of 1.38). These areas have few craters, which indicate that they are likely younger than a few hundred million years old. In addition, the relative youthfulness of these regions are an indication that cryovolcanism and other processes actively renew the surface.

The older terrain is not only cratered, but numerous fractures have also been observed – suggesting that the surface has been subject to extensive deformation since the craters formed. Some areas show regions with no craters, indicating major resurfacing events in the geologically recent past. The fissures, plains, corrugated terrain and other crustal deformations also indicate that Enceladus is geologically active.

One of the more dramatic types of tectonic features found on Enceladus are its rift canyons. These canyons can be up to 200 km long, 5–10 km wide, and 1 km deep. Such features are geologically young, because they cut across other tectonic features and have sharp topographic relief with prominent outcrops along the cliff faces.

Evidence of tectonics on Enceladus is also derived from grooved terrain, consisting of lanes of curved formations and ridges that often separate smooth plains from cratered regions. Deep fractures are another, which are often found in bands cutting across cratered terrain, and which were probably influenced by the formation of weakened regolith produced by impact craters.

Enceladus. Credit: NASA/JPL/Space Science Institute — *Enceladus, showing the famous “Tiger Stripes” feature – a series of fractures bound on either side by colorful ice. Credit: NASA/JPL/Space Science Institute*

Linear grooves can also be seen cutting across other terrain types, like the groove and ridge belts. Like the deep rifts, they are among the youngest features on Enceladus. However, some linear grooves have been softened like the craters nearby, suggesting that they are older. Ridges have also been observed on Enceladus, though they are relatively limited in extent and are up to one kilometer tall.

Other interesting features include the “Tiger stripes“: a series of fractures bounded on either side by ridges in the southern polar region that are are surrounded by mint-green-colored, coarse-grained water ice. These fractures appear to be the youngest features in this region, and combined with a lack of impact craters in this area, are further evidence of geological activity.

Atmosphere:

Saturn’s moon Enceladus has an atmosphere greater than that of all others in the Solar System, with the exception of Titan. The source of the atmosphere is attributed to the periodic cryovolcanism, which leads to gases and vapor escaping from the surface or the interior. Evidence of a tenuous atmosphere came from magnetometer readings provided by the Cassini‘s probe in 2005.

This consisted of an increased detection in the power of ion cyclotron waves, which are produced by the interaction of ionized particles and magnetic fields. During the next two encounters, the magnetometer team determined that gases in Enceladus’s atmosphere are concentrated over the south polar region, with atmospheric density away from the pole being much lower.

Water vapour geysers erupting from Enceladus' south pole. Credit: NASA/JPL — *Water vapour geysers erupting from Enceladus’ south pole. Credit: NASA/JPL*

Much like the content of the jet plumes, this atmosphere is composed primarily of water vapor (91%), but also shows signs of minor components like molecular nitrogen (4%) and carbon dioxide (3.2%). There has also been evidence of simple hydrocarbons, which take the form of methane (1.7%) as well as trace amounts of propane, acetylene and formaldehyde.

Habitability:

Ever since the discovery of Enceladus’s geysers and evidence that suggested an interior ocean, scientists have speculated about the possibility of there being life on Enceladus. Because it reflects so much sunlight, the mean surface temperature at noon only reaches -198 °C, making it somewhat colder than other Cronian satellites. However, within the core, multiple indications of life exist.

It’s resonance with Dione excites its orbital eccentricity, which tidal forces damp, resulting in tidal heating of its interior. This offers a possible explanation for its geological activity, and also suggests that its interior oceans are warmer closer to the core. In addition, geological models have indicated that the large rocky core is porous, allowing water to flow through it to pick up heat.

A model of Enceladus’s ocean created by Christopher R. Glein et al. (2015) suggests that it has an alkaline pH of 11 to 12. This high pH (alkaline) is interpreted to be a consequence of serpentinization of chondritic rock, which leads to the generation of molecular hydrogen (H²). This geochemical source of energy can be metabolized by methanogen microbes to provide energy for life.

The presence of an internal salty ocean with an energy source and simple organic compounds are all strong indications that microbes may exist closer to the core, where the water is warm and the basic building blocks of life exist.

Exploration:

Although it was first discovered in the late 18th century, astronomers didn’t know much about this moon for many centuries. It was not until it was first visited in a series of flybys by NASA’s two Voyager spacecraft in the 1980’s that certain things began to become apparent about Enceladus.

Voyager 1 has traveled far past the realm of the gas or even ice giants and is now in uncharted territory where scientists are learning more and more about the dynamic environment at the far-flung edges of our solar system. Image Credit: NASA/JPL - Caltech — *Artist’s impression of Voyager 1 reaching Saturn and its system of moons. Image Credit: NASA/JPL – Caltech*

For starters, the Voyager missions showed that the planet has a diameter of only 500 km (310 miles), which makes it less than one-tenth the diameter of Saturn’s largest moon of Titan. They also noted that most of the surface is covered in fresh, clean ice; giving it a pure, snow-white appearance that also attracts close to 100% of the sunlight that strikes its surface.

The Voyager 1 mission also confirmed that Enceladus was embedded in the densest part of Saturn’s diffuse E-ring. Combined with the apparent youthful appearance of the surface, Voyager scientists suggested that the E-ring consisted of particles vented from Enceladus’s surface. The Voyager 2 mission provided better photographs than its predecessor, confirming the presence of a youthful surface, but also other features.

By 2005, the Cassini spacecraft began performing multiple close flybys of Enceladus, revealing its surface and environment in greater detail. In particular, Cassini discovered the water-rich plumes venting from the south polar region of Enceladus, which became the subject of much research and speculation.

Cassini has provided strong evidence that Enceladus has an ocean with an energy source, nutrients and organic molecules, making Enceladus one of the best places for the study of potentially habitable environments for extraterrestrial life. By contrast, the water thought to be on Jupiter’s moon Europa is located under a much thicker layer of ice.

Cassini-Huygens Mission — *An artist illustration of the Cassini spacecraft. Image Credit: NASA/JPL*

Cassini’s latest flyby took place on October 14th, 2015, passing the moon at an altitude of 1,839 kilometers (1,142 miles) above the northern polar region. This was the first time that Cassini had been able to observe the northern polar region, due to the fact that on all previous occasions, the northern region was experiencing its winter cycle and was concealed by darkness.

Cassini’s instruments took pictures of multiple surface features, including craters (many of which look like they are melting), fractures and wrinkles. The latter features are believed to be an indication that the moon’s spin rate has changed, which may be another indication that the surface has undergone multiple episodes of geologic activity over the course of much of its lifetime.

The discoveries Cassini has made at Enceladus have prompted studies into follow-up mission concepts. In 2013, NASA proposed a possible sample-return mission to Enceladus that would involve a low-cost orbiter. This mission would launch during the 2020s and last 15 years.

Another proposal for a probe flyby, known as Journey to Enceladus and Titan (JET) would analyze plume contents in-situ. Proposed in response to NASA’s 2010 Discovery Announcement of Opportunity, the mission would involve an orbiter conducting high-resolution mass spectroscopy surveys of Enceladus and Titan, assessing them for biological potential.

The German Aerospace Center has also proposed studying the habitability of Enceladus’s subsurface ocean using an Enceladus Explorer, and two astrobiology-oriented mission concepts (the Enceladus Life Finder and Life Investigation For Enceladus). In 2007, the European Space Agency (ESA) proposed sending a probe to Enceladus in a mission to be combined with studies of Titan – known as TandEM (Titan and Enceladus Mission).

Additionally, there’s the Titan Saturn System Mission (TSSM), a joint NASA/ESA flagship-class proposal to explore Saturn’s moons (with a focus on Enceladus). TSSM was competing against the Europa Jupiter System Mission (EJSM) proposal for funding. In February 2009, it was announced that NASA/ESA had given the EJSM mission priority ahead of TSSM, although TSSM will continue to be studied and evaluated.

Enceladus is a tempting target for future research and exploration, and for good reason. For starters, it is one of the few Solar System bodies (alongside with Earth, Io, and Triton) to have confirmed contemporary volcanic activity. Second is the distinct possibility that life exists beneath its icy surface, much like Europa. But with Enceladus, getting to a place where we could study that life would be much easier.

As such, it is almost certain that any missions to Saturn and/or the outer Solar System in the coming years will likely involve a close flyby of Enceladus. Maybe we’ll even pop in a lander and an aquatic explorer to examine the surface and peak underneath it!

We’ve written many articles about Enceladus for Universe Today. Here’s an article about salt found in the plumes from Enceladus, and the possibility of a liquid ocean on Enceladus.

And here is a rundown of Cassini’s Most Interesting Discoveries.

If you’d like more information on Enceladus, check out NASA’s Solar System Exploration Guide, and here’s a link to a cool mosaic image of Enceladus.

We’ve recorded an episode of Astronomy Cast all about Saturn’s moons. Listen here, Episode 61: Saturn’s Moons.

Sources:

October 19, 2015February 23, 2017 by Fraser Cain

Is the Universe Perfect for Life?

Doesn’t it feel like the Universe is perfectly tuned for life? Actually, it’s a horrible hostile place, delivering the bare minimum for human survival.

Consider that incomprehensible series of events that brought you to this moment. In a way that we still don’t understand, a complex mix of chemicals came together in just the right combination to kick off the evolution of life.

Generation after generation of bacteria, insects, fish, lizards, mammals and eventually humans somehow successfully found a buddy and passed along their genetic material to another generation. Clever humans invented computers, the internet, YouTube, and somehow you found your way to this exact video, to hear these words. Whoa.

It’s amazing to consider the Universe we live in, and how it’s perfectly tuned for life. If just a single variable was a little bit different, life as we know it probably wouldn’t exist. Gravity might be a repulsive force. Pokemons might catch you.

Doesn’t it feel like the Universe was created especially for us? I mean, didn’t I already tell you that we’re all the center of the Universe?

I’m sad to say, but this couldn’t be further from the truth. The reality is that the Universe is 100% completely inhospitable. Well, apart from a thin layer on the surface of our Earth, but that’s got to be a rounding error. A fraction of a fraction of a fraction of the teeniest percent of the volume of the Universe. The rest of the Universe is bunk.

If I was plucked out of our cozy environment and dropped into the near vacuum of pretty much anywhere else, the only resource would be a handful of hydrogen atoms. And what can you do with a few hydrogen atoms? Nothing. It might even give Bear Grylls a run for his money. He might have a little more trouble on a star’s surface, crisping up in a heartbeat.

Into a black hole? Surface of a neutron star? Near an exploding supernova? Please enjoy the crushing pressures and hellish temperatures of Venus, or the freezing irradiated surface of Mars.

Earth itself is mostly a deathtrap. Travel down a few kilometers and you’d bake and crush from the rising temperatures of the Earth’s interior. Travel up and the air gets thin, cold and killy. In fact, without our technology heating, cooling, or helping us breathe, we wouldn’t last more than a few days on most of the planet.

Panorama of one area of Mars, from Sol 173. Credit: NASA/JPL/Caltech/Malin Space Science Systems. Image editing by — Panorama of the part of Mars, from Sol 173. Credit: NASA/JPL/Caltech/Malin Space Science Systems. Image editing by

When you think about the landscape of time, we even live in a brief thumbnail of a moment when Earth is hospitable. Over the next few billion years, the Sun is going to heat up to the point that the surface of Earth will resemble the surface of Venus. And then the last hospitable hidey-hole in the entire Universe, that we know of, will wink out. The Universe is as inhospitable as it could possibly be. That is, without being completely inhospitable.

Especially when you consider the timeframes, and the long future when all the stars have died, where there’s nothing but black holes and frozen matter, and the Universe finally ditches that rounding error, and becomes 100% purely inhospitable.

Cosmologists use a term known as the anthropic principle to explain this very special moment we find ourselves in. There’s the greater anthropic principle that says the Universe wouldn’t be here without us to observe it, but that seems nutty and egotistical.

The lesser anthropic principle says that if the Universe turned out any differently, we wouldn’t be here to observe it.

First ever image of Earth Taken by Mars Color Camera aboard India’s Mars Orbiter Mission (MOM) spacecraft while orbiting Earth and before the Trans Mars Insertion firing on Dec. 1, 2013. Image is focused on the Indian subcontinent. Credit: ISRO

Imagine you threw a dart out the window of an airplane and it landed in a tiny spot on the surface of the Earth. What were the chances that it would land there? Almost zero. What a lucky spot.

You can imagine all kinds of other even more inhospitable Universes, where the conditions were never good enough for life to evolve, and so intelligent civilizations could never even ask the question, “Is Our Universe Perfect for Life.”

So when you look out across a meadow in the springtime. The birds are chirping, and there’s new growth everywhere, don’t forget about the boiling rock magma beneath your feet, the frigid air and then vacuum above your head, and the whole Universe of burning, radiating, impacting objects trying their best to kill you.

Of all the extreme environments in the Universe, which ones do you find most fascinating? Tell us in the comments below.

October 17, 2015April 21, 2016 by Matt Williams

Jupiter’s Moon Callisto

Callisto has many more craters than Europa and a thicker icy crust. Image credit: NASA/JPL

With 67 confirmed satellites, Jupiter has the largest system of moons in the Solar System. The greatest of these are the four major moons of Io, Europa, Ganymede and Callisto – otherwise known as the Galilean Moons. Named in honor of their founder, these moons are not only comparable in size to some planets (such as Mercury), they are also some of the few places outside of Earth where liquid water exists, and perhaps even life.

But it is Callisto, the fourth and farthest moon of Jupiter, that may be the most rewarding when it comes to scientific research. In addition to the possibility of a subsurface ocean, this moon is the only Galilean far enough outside of Jupiter’s powerful magnetosphere that it does not experience harmful levels of radiation. This, and the prospect of finding life, make Callisto a prime candidate for future exploration.

Discovery and Naming:

Along with Io, Europa and Ganymede, Callisto was discovered in January of 1610 by Galileo Galilei using a telescope of his own design. Like all the Galilean Moons, it takes its name from one of Zeus’ lovers in classic Greek mythology. Callisto was a nymph (or the daughter of Lycaon) who was associated with the goddess of the hunt, Artemis.

The name was suggested by German astronomer Simon Marius, apparently at the behest of Johannes Kepler. However, Galileo initially refused to use them, and the moons named in his honor were designed as Jupiter I through IV, based on their proximity to their parent planet. Being the farthest planet from Jupiter, Callisto was known as Jupiter IV until the 20th century, by which time, the names suggested by Marius were adopted.

Galilean Family Portrait — *The Galilean moons to scale, with Callisto in the bottom left corner. Credit: NASA/JPL*

Size, Mass and Orbit:

With a mean radius of 2410.3 ± 1.5 km (0.378 Earths) and a mass of 1.0759 × 10²³ kg (0.018 Earths), Callisto is the second largest Jupiter’s moons (after Ganymede) and the third largest satellite in the solar system. Much like Ganymede, it is comparable in size to Mercury – being 99% as large – but due to its mixed composition, it has less than one-third of Mercury mass.

Callisto orbits Jupiter at an average distance (semi-major axis) of 1,882,700 km. It has a very minor eccentricity (0.0074) and ranges in distance from 1,869,000 km at periapsis to 1,897,000 km at apoapsis. This distance, which is far greater than Ganymede’s, means that Callisto does not take part in the mean-motion resonance that Io, Europa and Ganymede do.

Much like the other Galileans, Callisto’s rotation is synchronous with its orbit. This means that it takes the same amount of time (16.689 days) for Callisto to complete a single orbit of Jupiter and a single rotation on its axis. Its orbit is very slightly eccentric and inclined to the Jovian equator, with the eccentricity and inclination changing over the course of centuries due to solar and planetary gravitational perturbations.

*Size comparison of Earth, Moon and Callisto. Credit: NASA/JPL/DLR/Gregory H. Revera*

Unlike the other Galileans, Callisto’s distant orbit means that it has never experienced much in the way of tidal-heating, which has had a profound impact on its internal structure and evolution. Its distance from Jupiter also means that the charged particles from Jupiter’s magnetosphere have had a very minor influence on its surface.

Composition and Surface Features:

The average density of Callisto, at 1.83 g/cm³, suggests a composition of approximately equal parts of rocky material and water ice, with some additional volatile ices such as ammonia. Ice is believed to constitute 49-55% of the moon, with the rock component likely made up of chondrites, silicates and iron oxide.

Callisto’s surface composition is thought to be similar to its composition as a whole, with water ice constituting 25-50% of its overall mass. High-resolution, near-infrared and UV spectra imaging have revealed the presence of various non-ice materials, such as magnesium and iron-bearing hydrated silicates, carbon dioxide, sulfur dioxide, and possibly ammonia and various organic compounds.

Model of Callisto's internal structure showing a surface ice layer, a possible liquid water layer, and an ice–rock interior. Credit: NASA/JPL — *Model of Callisto’s internal structure showing a surface ice layer, a possible liquid water layer, and an ice–rock interior. Credit: NASA/JPL*

Beneath the surface is an icy lithosphere that is between 80-150 m thick. A salty ocean 50–200 km deep is believed to exist beneath this, thanks to the presence of radioactive elements and the possible existence of ammonia. Evidence of this ocean include Jupiter’s magnetic field, which shows no signs of penetrating Callisto’s surface. This suggests a layer of highly conductive fluid that is at least 10 km in depth. However, if this water contains ammonia, which is more likely, than it could be up to 250-300 km.

Beneath this hypothetical ocean, Callisto’s interior appears to be composed of compressed rocks and ices, with the amount of rock increasing with depth. This means, in effect, that Callisto is only partially differentiated, with a small silicate core no larger than 600 km (and a density of 3.1-3.6 g/cm³) surrounded by a mix of ice and rock.

Spectral data has also indicated that Callisto’s surface is extremely heterogeneous at the small scale. Basically, the surface consists of small, bright patches of pure water ice, intermixed with patches of a rock–ice mixture, and extended dark areas made of a non-ice material.

Compared to the other Galilean Moons, Callisto’s surface is quite dark, with a surface albedo of about 20%. Another difference is the nature of its asymmetric appearance. Whereas with the other Galileans, the leading hemisphere is lighter than the trailing one, with Callisto the opposite is true.

*Interior density structures created by an outer solar system late heavy bombardment onto Ganymede (top row) and Callisto (bottom row). Credit: SwRI*

An immediately obvious feature about Callisto’s surface is the ancient and heavily cratered nature of it. In fact, the surface is the most cratered in the Solar System and is almost entirely saturated by craters, with newer ones having formed over older ones. What’s more, impact craters and their associated structures are the only large features on the surface. There are no mountains, volcanoes or other endogenic tectonic features.

Callisto’s impact craters range in size from 0.1 km to over 100 km, not counting the multi-ring structures. Small craters, with diameters less than 5 km, have simple bowl or flat-floored shapes, whereas those that measure 5–40 km usually have a central peak.

Larger impact features, with diameters that range from 25–100 km have central pits instead of peaks. Those with diameters over 60 km can have central domes, which are thought to result from central tectonic uplift after an impact.

The largest impact features on Callisto’s surface are multi-ring basins, which probably originated as a result of post-impact concentric fracturing which took place over a patch of lithosphere that overlay a section of soft or liquid material (possibly a patch of the interior ocean). The largest of these are Valhalla and Asgard, whose central, bright regions measure 600 and 1600 km in diameter (respectively) with rings extending farther outwards.

*Voyager 1 image of Valhalla, a multi-ring impact structure 3800 km in diameter. Credit: NASA/JPL*

The relative ages of the different surface units on Callisto can be determined from the density of impact craters on them – the older the surface, the denser the crater population. Based on theoretical considerations, the cratered plains are thought to be ~4.5 billion years old, dating back almost to the formation of the Solar System.

The ages of multi-ring structures and impact craters depend on chosen background cratering rates, and are estimated by different researchers to vary between 1 and 4 billion years of age.

Atmosphere:

Callisto has a very tenuous atmosphere composed of carbon dioxide which has an estimated surface pressure of 7.5 × 10-¹² bar (0.75 micro Pascals) and a particle density of 4 × 10⁸ cm-³. Because such a thin atmosphere would be lost in only about 4 days, it must be constantly replenished, possibly by slow sublimation of carbon dioxide ice from Callisto’s icy crust.

While it has not been directly detected, it is believed that molecular oxygen exists in concentrations 10-100 times greater than CO². This is evidenced by the high electron density of the planet’s ionosphere, which cannot be explained by the photoionization of carbon dioxide alone. However, condensed oxygen has been detected on the surface of Callisto, trapped within its icy crust.

Habitability:

Much like Europa and Ganymede, and Saturn’s moons of Enceladus, Mimas, Dione, Titan, the possible existence of a subsurface ocean on Callisto has led many scientists to speculate about the possibility of life. This is particularly likely if the interior ocean is made up of salt-water, since halophiles (which thrive in high salt concentrations) could live there.

In addition, the possibility of extra-terrestrial microbial life has also been raised with respect to Callisto. However, the environmental conditions necessary for life to appear (which include the presence of sufficient heat due to tidal flexing) are more likely on Europa and Ganymede. The main difference is the lack of contact between the rocky material and the interior ocean, as well as the lower heat flux in Callisto’s interior.

In essence, while Callisto possesses the necessary pre-biotic chemistry to host life, it lacks the necessary energy. Because of this, the most likely candidate for the existence of extra-terrestrial life in Jupiter’s system of moons remains Europa.

Exploration:

The first exploration missions to Callisto were the Pioneer 10 and 11 spacecrafts, which conducted flybys of the Galilean moon in 1973 and 1974, respectively, But these missions provided little additional information beyond what had already learned through Earth-based observations. In contrast, the Voyager 1 and 2 spacecraft, which conducted flybys of the moon in 1979, managed to image more than half the surface and precisely measured Callisto’s temperature, mass and shape.

Capturing Callisto — *New Horizons Long Range Reconnaissance Imager (LORRI) captured these two images of Jupiter’s outermost large moon, Callisto, during its flyby in February 2007. Credit: NASA/JPL*

Further exploration took place between 1994 and 2003, when the Galileo spacecraft performed eight close flybys with Callisto. The orbiter completed the global imaging of the surface and delivered a number of pictures with a resolution as high as 15 meters. In 2000, while en route to Saturn, the Cassini spacecraft acquired high-quality infrared spectra of the Galilean satellites, including Callisto.

In February–March 2007, while en route to Pluto, the New Horizons probe obtained new images and spectra of Callisto. Using its Linear Etalon Imaging Spectral Array (LEISA) instrument, the probe was able to reveal how lighting and viewing conditions affect infrared spectrum readings of its surface water ice.

The next planned mission to the Jovian system is the European Space Agency’s Jupiter Icy Moon Explorer (JUICE), due to launch in 2022. Ostensibly geared towards exploring Europa and Ganymede, the mission profile also includes several close flybys of Callisto.

Colonization:

Compared to the other Galileans, Callisto presents numerous advantages as far as colonization is concerned. Much like the others, the moon has an abundant supply of water in the form of surface ice (but also possibly liquid water beneath the surface). But unlike the others, Callisto’s distance from Jupiter means that colonists would have far less to worry about in terms of radiation.

In 2003, NASA conducted a conceptual study called Human Outer Planets Exploration (HOPE) regarding the future human exploration of the outer Solar System. The target chosen to consider in detail was Callisto, for the purposes of investigating the possible existence of life forms embedded in the ice crust on this moon and on Europa.

Artist's impression of a base on Callisto. Credit: NASA — *Artist’s impression of a base on the icy surface of Callisto. Credit: NASA*

The study proposed a possible surface base on Callisto where a crew could “teleoperate a Europa submarine and excavate Callisto surface samples near the impact site”. In addition, this base could extract water from Callisto’s ample supply of water ices to produce rocket propellant for further exploration of the Solar System.

The advantages of a base on Callisto include low radiation (due to its distance from Jupiter) and geological stability. Such a base could facilitate exploration on other Galilean Moons, and be an ideal location for a Jovian system way station, servicing spacecraft heading farther into the outer Solar System – which would likely take the form of craft using a gravity assist from a close flyby of Jupiter.

Reports filed by NASA’s Glenn Research Center and Langley Research Center – in December and February of 2003, respectively – both outlined possible manned missions to Callisto, as envisioned by HOPE. According to these reports, a mission that would likely involve a ship using a Mangetoplasmadynamic (MPD) or Nuclear-Electric Propulsion (NEP) drive system, and equipped to generate artificial gravity, could be mounted in the 2040s.

So while Callisto may not be the best target in the search for extra-terrestrial life, it may be the most hospitable of Jupiter’s moons for human life. In either case, any future missions to Jupiter will likely include a stopovers to Callisto, with the intent of investigating both of these possibilities.

We have many great articles on Callisto, Jupiter, and its system of moons here at Universe Today. Here’s one about how impacts effected Callisto’s interior, And here is one on all of the Galilean Moons.

For more information, check out NASA’s Solar System Exploration page on Callisto.

Astronomy Cast offers has a good episode on the subject, titled Episode 57: Jupiter’s Moons.

October 15, 2015April 21, 2016 by Matt Williams

Jupiter’s Moon Ganymede

In 1610, Galileo Galilei looked up at the night sky through a telescope of his own design. Spotting Jupiter, he noted the presence of several “luminous objects” surrounding it, which he initially took for stars. In time, he would notice that these “stars” were orbiting the planet, and realized that they were in fact Jupiter’s moons – which would come to be named Io, Europa, Ganymede and Callisto.

Of these, Ganymede is the largest, and boasts many fascinating characteristics. In addition to being the largest moon in the Solar System, it is also larger than even the planet Mercury. It is the only satellite in the Solar System known to possess a magnetosphere, has a thin oxygen atmosphere, and (much like its fellow-moons, Europa and Callisto) is believed to have an interior ocean.

Continue reading “Jupiter’s Moon Ganymede”

October 15, 2015February 23, 2017 by Fraser Cain

Why Do Red Giants Expand?

We know that the Sun will last another 5 billion years and then expand us a red giant. What will actually make this process happen?

One of the handy things about the Universe, apart from the fact that it exists, is that it lets us see crazy different configurations of everything, including planets, stars and galaxies.

We see stars like our Sun and dramatically unlike our Sun. Tiny, cool red dwarf stars with a fraction of the mass of our own, sipping away at their hydrogen juice boxes for billions and even trillions of years. Stars with way more mass than our own, blasting out enormous amounts of radiation, only lasting a few million years before they detonate as supernovae.

There are ones younger than the Sun; just now clearing out the gas and dust in their solar nebula with intense ultraviolet radiation. Stars much older than ours, bloated up into enormous sizes, nearing the end of their lives before they fade into their golden years as white dwarfs.

The Sun is a main sequence star, converting hydrogen into helium at its core, like it’s been doing for more than 4.5 billion years, and will continue to do so for another 5 or so. At the end of its life, it’s going to bloat up as a red giant, so large that it consumes Mercury and Venus, and maybe even Earth.

What’s the process going on inside the Sun that makes this happen? Let’s peel away the Sun and take a look at the core. After we’re done screaming about the burning burning hands, we’ll see that the Sun is this enormous sphere of hydrogen and helium, 1.4 million kilometers across, the actual business of fusion is happening down in the core, a region that’s a delicious bubblegum center a tiny 280,000 kilometers across.

The core is less than one percent of the entire volume, but because the density of hydrogen in the chewy center is 150 times more than liquid water, it accounts for a freakishly huge 35% of its mass.

It’s thanks to the mass of the entire star, 2 x 10^30 kg, bearing down on the core thanks to gravity. Down here in the core, temperatures are more than 15 million degrees Celsius. It’s the perfect spot for nuclear fusion picnic.

There are a few paths fusion can take, but the main one is where hydrogen atoms are mushed into helium. This process releases enough gamma radiation to make you a planet full of Hulks.

Proton-proton fusion in a sun-like star. Credit: Borb

While the Sun has been performing hydrogen fusion, all this helium has been piling up at its core, like nuclear waste. Terrifyingly, it’s still fuel, but our little Sun just doesn’t have the temperature or pressure at its core to be able to use it.

Eventually, the fusion at the core of the Sun shuts down, choked off by all this helium and in a last gasp of high pitched mickey mouse voice terror the helium core begins to contract and heat up. At this point, an amazing thing happens. It’s now hot enough for a layer of hydrogen just around the core to heat up and begin fusion again. The Sun now gets a second chance at life.

As this outer layer contains a bigger volume than the original core of the Sun, it heats up significantly, releasing far more energy. This increase in light pressure from the core pushes much harder against gravity, and expands the volume of the Sun.

Even this isn’t the end of the star’s life. Dammit, Harkness, just stay down. Helium continues to build up, and even this extra shell around the core isn’t hot and dense enough to support fusion. So the core dies again. The star begins to contract, the gravitational energy heats up again, allowing another shell of hydrogen to have the pressure and temperature for fusion, and then we’re back in business!

Our Sun will likely go through this process multiple times, each phase taking a few years to complete as it expands and contracts, heats and cools. Our Sun becomes a variable star.

Eventually, we run out of usable hydrogen, but fortunately, it’s able to switch over to using helium as fuel, generating carbon and oxygen as byproducts. This doesn’t last long, and when it’s gone, the Sun gets swollen to hundreds of times its size, releasing thousands of times more energy.

This is when the Sun becomes that familiar red giant, gobbling up the tasty planets, including, quite possibly the Earth.The remaining atmosphere puffs out from the Sun, and drifts off into space creating a beautiful planetary nebula that future alien astronomers will enjoy for thousands of years. What’s left is a carbon oxygen core, a white dwarf.

The Sun is completely out of tricks to make fusion happen any more, and it’ll now cool down to the background temperature of the Universe. Our Sun will die in a dramatic way, billions of years from now when it bloats up 500 times its original volume.

What do you think future alien astronomers will call the planetary nebula left behind by the Sun? Give it a name in the comments below.

October 12, 2015February 23, 2017 by Fraser Cain

How Are Galaxies Moving Away Faster Than Light?

So, how can galaxies be traveling faster than the speed of light when nothing can travel faster than light?

I’m a little world of contradictions. “Not even light itself can escape a black hole”, and then, “black holes and they are the brightest objects in the Universe”. I’ve also said “nothing can travel faster than the speed of light”. And then I’ll say something like, “ galaxies are moving away from us faster than the speed of light.” There’s more than a few items on this list, and it’s confusing at best. Thanks Universe!

So, how can galaxies be traveling faster than the speed of light when nothing can travel faster than light? Warp speed galaxies come up when I talk about the expansion of the Universe. Perhaps it’s dark energy acceleration, or the earliest inflationary period of the Universe when EVERYTHING expanded faster than the speed of light.

Imagine our expanding Universe. It’s not an explosion from a specific place, with galaxies hurtling out like cosmic jetsam. It’s an expansion of space. There’s no center, and the Universe isn’t expanding into anything.

I’d suggested that this is a terribly oversimplified model for our Universe expanding. Unfortunately, it’s also terribly convenient. I can steal it from my children whenever I want.

Imagine you’re this node here, and as the toy expands, you see all these other nodes moving away from you. And if you were to move to any other node, you’d see all the other nodes moving away from you.

Here’s the interesting part, these nodes over here, twice as far away as the closer ones, appear to move more quickly away from you. The further out the node is, the faster it appears to be moving away from you.

This is our freaky friend, the Hubble Constant, the idea that for every megaparsec of distance between us and a distant galaxy, the speed separating them increases by about 71 kilometers per second.

Galaxies separated by 2 parsecs will increase their speed by 142 kilometers every second. If you run the mathatron, once you get out to 4,200 megaparsecs away, two galaxies will see each other traveling away faster than the speed of light. How big Is that, is it larger than the Universe?

The first light ever, the cosmic microwave background radiation, is 46 billion light-years away from us in all directions. I did the math and 4,200 megaparsecs is a little over 13.7 billion light-years.There’s mountains of room for objects to be more than 4,200 megaparsecs away from each other. Thanks Universe?!?

Most of the Universe we can see is already racing away at faster than the speed of light. So how it’s possible to see the light from any galaxies moving faster than the speed of light. How can we even see the Cosmic Microwave Background Radiation? Thanks Universe.

WMAP data of the Cosmic Microwave Background. Credit: NASA

Light emitted by the galaxies is moving towards us, while the galaxy itself is traveling away from us, so the photons emitted by all the stars can still reach us. These wavelengths of light get all stretched out, and duckslide further into the red end of the spectrum, off to infrared, microwave, and even radio waves. Given time, the photons will be stretched so far that we won’t be able to detect the galaxy at all.

In the distant future, all galaxies and radiation we see today will have faded away to be completely undetectable. Future astronomers will have no idea that there was ever a Big Bang, or that there are other galaxies outside the Milky Way. Thanks Universe.

I stand with Einstein when I say that nothing can move faster than light through space, but objects embedded in space can appear to expand faster than the speed of light depending on your perspective.

What aspects about cosmology still give you headaches? Give us some ideas for topics in the comments below.

October 8, 2015February 23, 2017 by Fraser Cain

What are White Holes?

Black holes are created when stars die catastrophically in a supernova. So what in the universe is a white hole?

It’s imagination day, and we’re going to talk about fantasy creatures. Like unicorns, but even rarer. Like leprechauns, but even more fantastical!

Today, we’re going to talk about white holes. Before we talk about white holes, let’s talk about black holes. And before we talk about Black Holes, what’s is this thing you have with holes exactly?

Black holes are places in the Universe where matter and energy are compacted so densely together that their escape velocity is greater than the speed of light. We’ve done at least a million videos on them, but if you still want more info, you can start here with our Black Hole playlist.

Fully describing a black hole requires a lot of fancy math, but these are real objects in our Universe. They were predicted by Einstein’s theory of relativity, and actually discovered over the last few decades.

Black holes are created when stars, much more massive than our Sun, die catastrophically in a supernova.
So then what’s a white hole?

White holes are created when astrophysicists mathematically explore the environment around black holes, but pretend there’s no mass within the event horizon. What happens when you have a black hole singularity with no mass?

White holes are completely theoretical mathematical concepts. In fact, if you do black hole mathematics for a living, I’m told, ignoring the mass of the singularity makes your life so much easier.

They’re not things that actually exist. It’s not like astronomers detected an unusual outburst of radiation and then developed hypothetical white hole models to explain them.

As my good friend and sometimes Guide to Space contributor, Dr. Brian Koberlein says, “If you start with five cupcakes and start giving them away, you eventually run out. At that point you can’t give away any more. In this case you can’t count down past zero. Sure, you can hand out slips of paper with “I O U ONE cupcake.” written on them, but it would be ridiculous to use the existence of negative numbers to claim that “negative cupcakes” exist and can be handed out to people.”

Now if white holes did exist, which they probably don’t, they would behave like reverse black holes – just like the math predicts. Instead of pulling material inward, a white hole would blast material out into space like some kind of white chocolate fountain. So generous, these white holes and their chocolate.

One of the other implications of white hole math, is that they only theoretically exist as long as there isn’t a single speck of matter within the event horizon. As soon as single atom of hydrogen drifted into the region, the whole thing would collapse. Even if white holes were created back at the beginning of the Universe, they would have collapsed long ago, since our Universe is already filled with stray matter.

That said, there are a few physicists out there who think white holes might be more than theoretical. Hal Haggard and Carlo Rovelli of Aix-Marseille University in France are working to explain what happens within black holes using a branch of theoretical physics called loop quantum gravity.

Artistic view of a radiating black hole. Credit: NASA

In theory, a black hole singularity would compress down until the smallest possible size predicted by physics. Then it would rebound as a white hole. But because of the severe time dilation effect around a black hole, this event would take billions of years for even the lowest mass ones to finally get around to popping.

If there were microscopic black holes created after the Big Bang, they might get around to decaying and exploding as white holes any day now. Except, according to Stephen Hawking, they would have already evaporated.

Another interesting idea put forth by physicists, is that a white hole might explain the Big Bang, since this is another situation where a tremendous amount of matter and energy spontaneously appeared.

In all likelihood, white holes are just fancy math. And since fancy math rarely survives contact with reality, white holes are probably just imaginary.

What other highly theoretical theories in space and physics would you like us to investigate? Tell us in the comments below.

October 5, 2015February 23, 2017 by Fraser Cain

Is Jupiter Our Friend Or Enemy?

Like me, you’re probably a little ego-geocentric about the importance of Earth. It’s where you were born, it’s where you keep all your stuff. It’s even where you’re going to die – I know, I know, not you Elon Musk, you’re going to “retire” on Mars, right after you nuke the snot out of it.

For the rest of us, Earth is the place. But in reality, when it comes to planets, this is somebody else’s racket. This is Jupiter’s Solar System, and we all sleep on its couch.

Jupiter accounts for 75% of the mass of the planets of the Solar System, nearly 318 times more massive than Earth, and isn’t just the name of everyone’s favorite secret princess. It’s the 1.9 × 10^27 kilogram gorilla in the room. Whatever Jupiter wants, Jupiter gets. Jupiter hungry? JUPITER HUNGRY.

What Jupiter apparently wants is to throw our stuff around the Solar System. Thanks to its immense gravity, Jupiter yanks material around in the asteroid belt, preventing the poor space rocks from ever forming up into anything larger than Ceres.

Jupiter gobbles up asteroids, comets, and spacecraft, and hurtles others on wayward trajectories. Who knows how much mayhem and destruction Jupiter has gotten into over the course of its 4.5 billion years in the Solar System.

Some scientists think we owe our existence to Jupiter’s protective gravity. It greedily vacuums up dangerous asteroids and comets in the Solar System.

Other scientists totally disagree and think that Jupiter is a bully, perturbing perfectly safe comets and asteroids into dangerous trajectories and flushing earth’s head in the toilet during recess.

Which is it? Is Jupiter our friend and protector, or evil enemy. We’ve already figured out how to dismantle you Jupiter, don’t make us put our plans into action.

Some of the most dangerous objects in the Solar System are long-period comets. These balls of rock and ice come from the deepest depths of the Oort cloud. Some event nudges these death missiles into trajectories that bring them into the inner Solar System, to shoot past the Sun and maybe, just maybe, smash into a planet and kill 99.99999% of the life on it.

There’s a pretty good chance some of the biggest extinctions in the history of the Earth were caused by impacts by long period comets.

As these comets make their way through the Solar System, they interact with Jupiter’s massive gravity, and get pushed this way and that. As we saw with Comet Shoemaker-Levy, some just get consumed entirely, like a tasty ice-rock sandwich.

The theory goes that Jupiter pushes these dangerous comets out of their murder orbits so they don’t smash into Earth and kill us all.

But a competing theory says that Jupiter actually diverts comets that would have completely missed our planet into deadly, Earth-killing trajectories.

Will the Sailor Scouts provide us any clues? Who can say?

Fragmentation of comets is common. Many sungrazers are broken up by thermal and tidal stresses during their perihelions. At top, an image of the comet Shoemaker-Levy 9 (May 1994) after a close approach with Jupiter which tore the comet into numerous fragments. An image taken by Andrew Catsaitis of components B and C of Comet 73P/Schwassmann–Wachmann 3 as seen together on 31 May 2006 (Credit: NASA/HST, Wikipedia, A.Catsaitis) — At top, an image of the comet Shoemaker-Levy 9 (May 1994) after a close approach with Jupiter which tore the comet into numerous fragments. An image taken by Andrew Catsaitis of components B and C of Comet 73P/Schwassmann–Wachmann 3 as seen together on 31 May 2006 (Credit: NASA/HST, Wikipedia, A.Catsaitis)

Here’s friend of the show, Dr. Kevin Grazier, a planetary scientist and scientific advisor for many of your favorite sci-fi TV shows and movies.

… [ see video for Interview with Dr. Grazier about Jupiter]

So which is it? Is Jupiter our friend or enemy? We’ll need to run more simulations and figure this out with more accuracy. And until then, it’s probably best if we just tremble in fear and worship Jupiter as a dark and capricious god until the evidence proves otherwise. It’s what Pascal would wager.

What are some other theories you’ve heard about and you’d like us to dig in further? Make some suggestions in the comments below.

Thanks for watching! Click subscribe, never miss an episode.
If you’re into other facts about our Solar system here’s a link to our Solar system playlist. Thanks to Ben Johnson and Tal Ghengis, and the members of the Guide to Space community who keep these shows rolling. Love space science? Want to see episodes before anyone else? Get extras, contests, and shenanigans with Jay, myself and the rest of the team. Get in on the action. Click here.

October 3, 2015March 3, 2017 by Matt Williams

Saturn’s Moon Titan

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, Moon, and Earth. Credit: NASA/GJPL/regory H. Revera — *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.

The seas of Titan — *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) NASA/JPL-Caltech/ASI/Cornell — 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 colour image of Titan's atmosphere. Credit: NASA/JPL/Space Science Institute/ESA — *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 Aug. 23, 1981 from a range of 2.3 million kilometers (1.4 million miles), shows some detail in the cloud systems on this Saturnian moon. Credit: NASA/JPL — *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.

On the left is TALISE (Titan Lake In-situ Sampling Propelled Explorer), the ESA proposal. This would have it's own propulsion, in the form of paddlewheels. Credit: bisbos.com — *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 — *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.

Here’s NASA’s Cassini mission to Saturn and Titan, and here’s the ESA’s version.

We have recorded two episodes of Astronomy Cast just about Saturn. The first is Episode 59: Saturn, and the second is Episode 61: Saturn’s Moons.

September 29, 2015March 13, 2022 by Matt Williams

Jupiter’s Moon Europa

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.