Saturn's moon, Mimas, captured by NASA's Cassini spacecraft in 2010. (Credit: NASA/JPL-Caltech/Space Science Institute)
A recent study published in Nature presents a groundbreaking discovery that Saturn’s moon, Mimas, commonly known as the “Death Star” moon due to its similarities with the iconic Star Wars space station, possesses an internal ocean underneath its rocky crust. This study was conducted by an international team of researchers and holds the potential to help planetary geologists better understand the conditions for a planetary body to possess an internal ocean, which could also possess the conditions for life as we know it. While Mimas was photographed on several occasions by NASA’s Cassini spacecraft, including a close flyby in February 2010, what was the motivation behind this recent study regarding finding an internal ocean on Mimas?
The moons of Saturn, from left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan in the background; Iapetus (top) and irregularly shaped Hyperion (bottom). Some small moons are also shown. All to scale. Credit: NASA/JPL/Space Science Institute
Saturn is best known for two things: its iconic ring structures and its large system of natural satellites. Currently, 146 moons and moonlets have been discovered orbiting the ringed giant, 24 of which are regular satellites. These include the seven largest moons, Titan, Rhea, Iapetus, Dione, Tethys, Enceladus, and Mimas, which are icy bodies believed to have interior oceans. In addition, there are unresolved questions about the age of these satellites, with some suspecting that they formed more recently (like Saturn’s rings, which are a few hundred million years old).
To address these questions, an international team of astronomers created a series of high-resolution simulations coupled with improved estimates of Trans-Neptunian Object (TNO) populations. This allowed them to construct a chronology of impacts for Saturn’s most heavily cratered regular satellites – Mimas, Enceladus, Tethys, Dione, and Rhea. This established age limits of 4.1 and 4.4 billion years for all five, with the two innermost moons appearing more youthful than the outer three. These results could have significant implications for our understanding of the formation and tidal evolution of moons in the outer Solar System.
Mimas, as imaged by NASA's Cassini spacecraft and processed by @kevinmgill
Data from the Cassini mission keeps fuelling discoveries. The latest discovery is that Saturn’s tiny moon Mimas may have an internal ocean. If it does, the moon joins a growing list of natural satellites in our Solar System that may harbour liquid water under their surfaces.
Mimas, as imaged by NASA's Cassini spacecraft and processed by @kevinmgill
The Cassini mission to Saturn took many images of Mimas, one of the smallest moons in the solar system. And now you can view it in all its icy, cratered glory, thanks to the work of Kevin Gill.
Saturn's rings and moons have been the subject of scientific debate. A 2019 study showed that the migration of Saturn's moons has widened the Cassini Division in Saturn's rings. Image Credit: Cassini, Dante, Baillié and Noyelles
Saturn’s moon Mimas is the smallest of the gas giant’s major moons. (Saturn has 62 moons, but some of them are tiny moonlets less than 1 km in diameter.) Two new studies show how Mimas acted as a kind of snow-plow, widening the Cassini division between Saturn’s rings.
Mosaic view of Mimas, created using images taken by the Cassini probe (and illuminated to show the full surface). Credit: NASA/JPL-Caltech/Space Science Institute
Since the Cassini mission arrived in the Saturn system in 2004, it has provided some stunning images of the gas giant and its many moons. And in the course of capturing new views of Titan’s dense atmosphere, Iapetus’ curious “yin-yang” coloration, and the water plumes and “tiger stripes” of Enceladus, it snapped the most richly-detailed images of Mimas ever seen.
But like all good things, Cassini’s days of capturing close-up images of Mimas are coming to an end. As of January 30th, 2017, the probe made its final close approach to the moon, and took the last of it’s close-up pictures in the process. In the future, all observations (and pictures) of Mimas will take place at roughly twice this distance – and will therefore be less detailed.
To be fair, these close approaches were a pretty rare event during the Cassini mission. Over the course of the thirteen years that the probe has been in the Saturn system, only seven flybys have taken place, occurring at distances of less 50,000 km (31,000 mi). At its closest approach, Cassini passed within 41,230 km (25,620 mi) of Mimas.
Second mosaic view of Mimas, showing illumination on only the Sun-facing side. Credit: NASA/JPL-Caltech/Space Science Institute
During this time, the probe managed to take a series of images that allowed for the creation of a beautiful mosaic. This mosaic was made from ten combined narrow-angle camera images, and is one of the highest resolution views ever captured of the icy moon. It also comes in two versions. In one, the left side of Mimas is illuminated by the Sun and the picture is enhanced to show the full moon (seen at top).
In the second version (shown above), natural illumination shows only the Sun-facing side of the moon. They also created an animation that allows viewers to switch between mosaics, showing the contrast. And as you can see, these mosaics provide a very detailed look at Mimas heavily-cratered surface, a well as the large surface fractures that are believed to have been caused by the same impact that created the Herschel Crater.
This famous crater, from which Mimas gets it’s “Death Star” appearance, was photographed during Cassini’s first flyby – which occurred on February 13th, 2010. Named in honor of William Herschel (the discoverer of Uranus, its moons Oberon, and Titania, and Saturn’s moons Enceladus and Mimas), this crater measures 130 km (81 mi) across, almost a third of Mimas’ diameter.
This mosaic, created from images taken by NASA’s Cassini spacecraft during its closest flyby of Saturn’s moon Mimas, looks straight at the moon’s huge Herschel Crater Credit: NASA/JPL
Its is also quite deep, as craters go, with walls that are approximately 5 km (3.1 mi) high. Parts of its floor reach as deep as 10 km (6.2 mi), and it’s central peak rises 6 km (3.7 mi) above the crater floor. The impact that created this crater is believed to have nearly shattered Mimas, and also caused the fractures visible on the opposite side of the moon.
It’s a shame we won’t be getting any more close ups of the moon’s many interesting features. However, we can expect a plethora of intriguing images of Saturn’s rings, which it will be exploring in depth as part of the final phase of its mission. The mission is scheduled to end on September 15th, 2017, which will culminate with the crash of the probe in Saturn’s atmosphere.
Ask me my favorite object in the Solar System, especially to see through a telescope, and my answer is always the same: Saturn.
Saturn is this crazy, ringed world, different than any other place we’ve ever seen. And in a small telescope, you can really see the ball of the planet, you can see its rings. It’s one thing to see a world like this from afar, a tiny jumping image in a telescope. To really appreciate and understand a place like Saturn, you’ve got to visit.
And thanks to NASA’s Cassini spacecraft, that’s just what we’ve been doing for the last 13 years. Take a good close look at this amazing ringed planet and its moons, and studying it from every angle.
Cassini orbiting Saturn. Credit: NASA
Throughout this article, I’m going to regale you with the amazing discoveries made by Cassini at Saturn. What it taught us, and what new mysteries it uncovered.
NASA’s Cassini spacecraft was launched from Earth on October 15, 1997. Instead of taking the direct route, it made multiple flybys of Venus, a flyby of Earth and a flyby of Jupiter. Each one of these close encounters boosted Cassini’s velocity, allowing it to make the journey with less escape velocity from Earth.
It arrived at Saturn on July 1st, 2004 and began its science operations shortly after that. The primary mission lasted 4 years, and then NASA extended its mission two more times. The first ending in 2010, and the second due to end in 2017. But more on that later.
Before Cassini, we only had flybys of Saturn. NASA’s Pioneer 11, and Voyagers 1 and 2 both zipped past the planet and its moons, snapping pictures as they went.
But Cassini was here to stay. To orbit around and around the planet, taking photos, measuring magnetic fields, and studying chemicals.
For Saturn itself, Cassini was able to make regular observations of the planet as it passed through entire seasons. This allowed it to watch how the weather and atmospheric patterns changed over time. The spacecraft watched lightning storms dance through the cloudtops at night.
This series of images from NASA’s Cassini spacecraft shows the development of the largest storm seen on the planet since 1990. These true-color and composite near-true-color views chronicle the storm from its start in late 2010 through mid-2011, showing how the distinct head of the storm quickly grew large but eventually became engulfed by the storm’s tail. Credit: NASA/JPL-Caltech/Space Science Institute
Two highlights. In 2010, Cassini watched a huge storm erupt in the planet’s northern hemisphere. This storm dug deep into Saturn’s lower atmosphere, dredging up ice from a layer 160 kilometers below and mixing it onto the surface. This was the first time that astronomers were able to directly study this water ice on Saturn, which is normally in a layer hidden from view.
Natural color images taken by NASA’s Cassini wide-angle camera, showing the changing appearance of Saturn’s north polar region between 2012 and 2016.. Credit: NASA/JPL-Caltech/Space Science Institute/Hampton University
The second highlight, of course, is the massive hexagonal storm churning away in Saturn’s northern pole. This storm was originally seen by Voyager, but Cassini brought its infrared and visible wavelength instruments to bear.
Why a hexagon? That’s still a little unclear, but it seems like when you rotate fluids of different speeds, you get multi-sided structures like this.
Cassini showed how the hexagonal storm has changed in color as Saturn moved through its seasons.
This is one of my favorite images sent back by Cassini. It’s the polar vortex at the heart of the hexagon. Just look at those swirling clouds.
The polar vortex, in all its glory. Credit: NASA/JPL-Caltech/Space Science Institute
Now, images of Saturn itself are great and all, but there was so much else for Cassini to discover in the region.
Cassini studied Saturn’s rings in great detail, confirming that they’re made up of ice particles, ranging in size as small a piece of dust to as large as a mountain. But the rings themselves are actually quite thin. Just 10 meters thick in some places. Not 10 kilometers, not 10 million kilometers, 10 meters, 30 feet.
The spacecraft helped scientists uncover the source of Saturn’s E-ring, which is made up of fresh icy particles blasting out of its moon Enceladus. More on that in a second too.
Vertical structures, among the tallest seen in Saturn’s main rings, rise abruptly from the edge of Saturn’s B ring to cast long shadows on the ring in this image taken by NASA’s Cassini spacecraft two weeks before the planet’s August 2009 equinox. Credit: NASA/JPL/Space Science Institute
Here’s another one of my favorite images of the mission. You’re looking at strange structures in Saturn’s B-ring. Towering pillars of ring material that rise 3.5 kilometers above the surrounding area and cast long shadows. What is going on here?
They’re waves, generated in the rings and enhanced by nearby moons. They move and change over time in ways we’ve never been able to study anywhere else in the Solar System.
Daphnis, one of Saturn’s ring-embedded moons, is featured in this view, kicking up waves as it orbits within the Keeler gap. Credit: NASA/JPL-Caltech/Space Science Institute
Cassini has showed us that Saturn’s rings are a much more dynamic place than we ever thought. Some moons are creating rings, other moons are absorbing or distorting them. The rings generate bizarre spoke patterns larger than Earth that come and go because of electrostatic charges.
Speaking of moons, I’m getting to the best part. What did Cassini find at Saturn’s moons?
Let’s start with Titan, Saturn’s largest moon. Before Cassini, we only had a few low resolution images of this fascinating world. We knew Titan had a dense atmosphere, filled with nitrogen, but little else.
Cassini was carrying a special payload to assist with its exploration of Titan: the Huygens lander. This tiny probe detached from Cassini just before its arrival at Saturn, and parachuted through the cloudtops on January 14, 2005, analyzing all the way. Huygens returned images of its descent through the atmosphere, and even images of the freezing surface of Titan.
Huygen’s view of Titan. Credit: ESA/NASA/JPL/University of Arizona
But Cassini’s own observations of Titan took the story even further. Instead of a cold, dead world, Cassini showed that it has active weather, as well as lakes, oceans and rivers of hydrocarbons. It has shifting dunes of pulverized rock hard water ice.
If there’s one place that needs exploring even further, it’s Titan. We should return with sailboats, submarines and rovers to better explore this amazing place.
A view of Mimas from the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute
We learned, without a shadow of a doubt, that Mimas absolutely looks like the Death Star. No question. But instead of a megalaser, this moon has a crater a third of its own size.
Saturn’s moon Iapetus. Image credit: NASA/JPL/SSI
Cassini helped scientists understand why Saturn’s moon Iapetus has one light side and one dark side. The moon is tidally locked to Saturn, its dark side always leading the moon in orbit. It’s collecting debris from another Saturnian moon, Phoebe, like bugs hitting the windshield of a car.
Perhaps the most exciting discovery that Cassini made during its mission is the strange behavior of Saturn’s moon Enceladus. The spacecraft discovered that there are jets of water ice blasting out of the moon’s southern pole. An ocean of liquid water, heated up by tidal interactions with Saturn, is spewing out into space.
And as you know, wherever we find water on Earth, we find life. We thought that water in the icy outer Solar System would be hard to reach, but here it is, right at the surface, venting into space, and waiting for us to come back and investigate it further.
Icy water vapor geysers erupting from fissures on Enceladus. Credit: NASA/JPL
On September 15, 2017, the Cassini mission will end. How do we know it’s going to happen on this exact date? Because NASA is going to crash the spacecraft into Saturn, killing it dead.
That seems a little harsh, doesn’t it, especially for a spacecraft which has delivered so many amazing images to us over nearly two decades of space exploration? And as we’ve seen from NASA’s Opportunity rover, still going, 13 years longer than anticipated. Or the Voyagers, out in the depths of the void, helping us explore the boundary between the Solar System and interstellar space. These things are built to last.
The problem is that the Saturnian system contains some of the best environments for life in the Solar System. Saturn’s moon Enceladus, for example, has geysers of water blasting out into space.
Cassini spacecraft is covered in Earth-based bacteria and other microscopic organisms that hitched a ride to Saturn, and would be glad to take a nice hot Enceladian bath. All they need is liquid water and a few organic chemicals to get going, and Enceladus seems to have both.
NASA feels that it’s safer to end Cassini now, when they can still control it, than to wait until they lose communication or run out of propellant in the future. The chances that Cassini will actually crash into an icy moon and infect it with our Earth life are remote, but why take the risk?
For the last few months, Cassini has been taking a series of orbits to prepare itself for its final mission. Starting in April, it’ll actually cross inside the orbit of the rings, getting closer and closer to Saturn. And on September 15th, it’ll briefly become a meteor, flashing through the upper atmosphere of Saturn, gone forever.
This graphic illustrates the Cassini spacecraft’s trajectory, or flight path, during the final two phases of its mission. The view is toward Saturn as seen from Earth. The 20 ring-grazing orbits are shown in gray; the 22 grand finale orbits are shown in blue. The final partial orbit is colored orange. Image credit: NASA/JPL-Caltech/Space Science Institute
Even in its final moments, Cassini is going to be sciencing as hard as it can. We’ll learn more about the density of consistency of the rings close to the planet. We’ll learn more about the planet’s upper atmosphere, storms and clouds with the closest possible photographs you can take.
And then it’ll all be over. The perfect finale to one of the most successful space missions in human history. A mission that revealed as many new mysteries about Saturn as it helped us answer. A mission that showed us not only a distant alien world, but our own planet in perspective in this vast Solar System. I can’t wait to go back.
How have the photos from Cassini impacted your love of astronomy? Let me know your thoughts in the comments.
The moons of Saturn, from left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan in the background; Iapetus (top) and irregularly shaped Hyperion (bottom). Some small moons are also shown. All to scale. Credit: NASA/JPL/Space Science Institute
Continuing with our “Definitive Guide to Terraforming“, Universe Today is happy to present our guide to terraforming Saturn’s Moons. Beyond the inner Solar System and the Jovian Moons, Saturn has numerous satellites that could be transformed. But should they be?
Around the distant gas giant Saturn lies a system of rings and moons that is unrivaled in terms of beauty. Within this system, there is also enough resources that if humanity were to harness them – i.e. if the issues of transport and infrastructure could be addressed – we would be living in an age a post-scarcity. But on top of that, many of these moons might even be suited to terraforming, where they would be transformed to accommodate human settlers.
As with the case for terraforming Jupiter’s moons, or the terrestrial planets of Mars and Venus, doing so presents many advantages and challenges. At the same time, it presents many moral and ethical dilemmas. And between all of that, terraforming Saturn’s moons would require a massive commitment in time, energy and resources, not to mention reliance on some advanced technologies (some of which haven’t been invented yet).
A view of Mimas from the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute
Much has been learned about Saturn’s system of moons in recent decades, thanks to the Voyager missions and the more recent surveys conducted by the Cassini spaceprobe. Between its estimated 150 moons and moonlets (only 53 of which have been identified and named) there is no shortage of scientific curiosities, and enough mysteries to keep astronomers here on Earth busy for decades.
Consider Mimas, which is often referred to as Saturn’s “Death Star Moon” on a count of its unusual appearance. Much like Saturn’s moons Tethys and Rhea, Mimas’ peculiar characteristics represents something of a mystery. Not only is it almost entirely composed ice, it’s coloration and surface features reveal a great deal about the history of the Saturnian (aka. Cronian) system. On top of that, it may even house an interior, liquid-water ocean.
Discovery and Naming:
Saturn’s moon Mimas was discovered by William Herschel in 1789, more than 100 years after Saturn’s larger moons were discovered by Christian Huygens and Giovanni Cassini. As with all the seven then-known satellites of Saturn, Mimas’ name was suggested by William Herschel’s son John in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope.
Mimas takes its name from one of the Titans of Greek mythology, who were the sons and daughters of Cronus (the Greek equivalent to Jupiter). Mimas was an offspring of Gaia, born from the blood of the castrated Uranus, who eventually died during the struggle with the Olympian Gods for control of the universe.
A replica of the telescope which William Herschel used to observe Uranus. Credit: Alun Salt/Wikimedia Commons
Size, Mass and Orbit:
With a mean radius of 198.2 ± 0.4 km and a mass of about 3.75 ×1019 kg, Mimas is equivalent in size to 0.0311 Earths and 0.0000063 times as massive. Orbiting Saturn at an average distance (semi-major axis) of 185,539 km, it is the innermost of Saturn’s larger moons, and the 8th moon orbiting Saturn. It’s orbit also has a minor eccentricity of 0.0196, ranging from 181,902 km at periapsis and 189,176 km at apoapsis.
With an estimated orbital velocity of 14.28 km/s, Mimas takes 0.942 days to complete a single orbit of Saturn. Like many of Saturn’s moons. Mimas rotation period is synchronous to its orbital period, which means it keeps one face constantly pointing towards the planet. Mimas is also in a 2:1 mean-motion resonance with the larger moon Tethys, and in a 2:3 resonance with the outer F Ring shepherd moonlet, Pandora.
Composition and Surface Features:
Mimas’ mean density of 1.1479 ± 0.007 g/cm³ is just slightly higher than that of water (1 g/cm³), which means that Mimas is mostly composed of water ice, with just a small amount of silicate rock. In this respect, Mimas is much like Tethys, Rhea, and Dione – moon’s of Saturn that are primarily composed of water ice.
Due to the tidal forces acting on it, Mimas is noticeably prolate – i.e. its longest axis is about 10% longer than the shortest, giving it its egg-shaped appearance. In fact, with a diameter of 396 km (246 mi), Mimas is just barely large and massive enough to achieve hydrostatic equilibrium (i.e. to become rounded in shape under the force of its own gravitation). Mimas is the smallest known astronomical body to have achieved this.
Mosaic image of Mimas, created from images taken by NASA’s Cassini spacecraft, showing the Herschel crater in the center. Credit: NASA/JPL
Three types of geological features are officially recognized on Mimas: craters, chasmata (chasms) and catenae (crater chains). Of these, craters are the most common, and it is believed that many of them have existed since the beginning of the Solar System. Mimas surface is saturated with craters, with every part of the surface showing visible depressions, and newer impacts overwriting older ones.
Mimas’ most distinctive feature is the giant impact crater Herschel, named in honor of William Herschel (the discoverer of Uranus, its moons Oberon, and Titania, and the Cronian moons Enceladus and Mimas). This large crater gives Mimas the appearance of the “Death Star” from Star Wars. At 130 km (81 mi) in diameter, Herschel’s is almost a third of Mimas’ own diameter.
Its walls are approximately 5 km (3.1 mi) high, parts of its floor measure 10 km (6.2 mi) deep, and its central peak rises 6 km (3.7 mi) above the crater floor. If there were a crater of an equivalent scale on Earth, it would be over 4,000 km (2,500 mi) in diameter, which would make it wider than the continent of Australia.
The impact that made this crater must have nearly shattered Mimas, and is believed to have created the fractures on the opposite side of the moon by sending shock waves through Mimas’s body. In this respect, Mimas’ surface closely resembles that of Tethys, with its massive Odysseus crater on its western hemisphere and the concentric Ithaca chasma, which is believed to have formed as a result of the impact that created Odysseus.
Color map of Mimas, created using data provided by the Cassini spaceprobe. Credit: NASA/JPL/Caltech/SSI/LPI
Mimas’ surface is also saturated with smaller impact craters, but no others are anywhere near the size of Herschel. The cratering is also not uniform, with most of the surface being covered with craters larger than 40 km (25 mi) in diameter. However, in the south polar region, there are generally no craters larger than 20 km (12 mi) in diameter.
Data obtained in 2014 from the Cassini spacecraft has also led to speculation about a possible interior ocean. Due to the planet’s libration (oscillation in its orbit), scientists believe that the planet’s interior is not uniform, which could be the result of a rocky interior or an interior ocean at the core-mantle boundary. This ocean would likely be maintained thanks to tidal flexing caused by Mimas’ orbital resonances with Tethys and Pandora.
A number of features in Saturn’s rings are also related to resonances with Mimas. Mimas is responsible for clearing the material from the Cassini Division, which is the gap between Saturn’s two widest rings – the A Ring and B Ring. The repeated pulls by Mimas on the Cassini Division particles, always in the same direction, forces them into new orbits outside the gap.
Particles in the Huygens Gap at the inner edge of the Cassini division are in a 2:1 resonance with Mimas. In other words, they orbit Saturn twice for each orbit competed by Mimas. The boundary between the C and B ring is meanwhile in a 3:1 resonance with Mimas; and recently, the G Ring was found to be in a 7:6 co-rotation eccentricity resonance with Mimas.
This figure illustrates the unexpected and bizarre pattern of daytime temperatures found on Saturn’s small inner moon Mimas. Credit: NASA/JPL/GSFC/SWRI/SSI
Exploration:
The first mission to study Mimas up close was Pioneer 11, which flew by Saturn in 1979 and made its closest approach on Sept. 1st, 1979, at a distance of 104,263 km. The Voyager 1 and 2 missions both flew by Mimas in 1980 and 1981, respectively, and snapped pictures of Saturn’s atmosphere, its rings, its system of moons. Images taken by Voyager 1 probe were the first ever of the Herschel crater.
Mimas has been imaged several times by the Cassini orbiter, which entered into orbit around Saturn in 2004. A close flyby occurred on February 13, 2010, when Cassini passed Mimas at a distance of 9,500 km (5,900 mi). In addition to providing multiple images of Mimas’ cratered surface, it also took measurements of Mimas’ orbit, which led to speculation about a possible interior ocean.
The Saturn system is truly a wonder. So many moons, so many mysteries, and so many chances to learn about the formation of the Solar System and how it came to be. One can only hope that future missions are able to probe some of the deeper ones, like what might be lurking beneath Mimas’ icy, imposing “Death Star” surface!
We’ve written many great articles about Mimas and Saturn’s moons here at Universe Today. Here’s one about the Herschel Crater, one about the first detailed look Cassini made, and one about it’s “Death Star” appearance.
Another great resource about Mimas is Solar Views, and you can get even more info from the Nine Planets.
Saturn's moon Tethys, imaged by Cassini on April 14, 2012.
Thanks the Voyager missions and the more recent flybys conducted by the Cassini space probe, Saturn’s system of moons have become a major source of interest for scientists and astronomers. From water ice and interior oceans, to some interesting surface features caused by impact craters and geological forces, Saturn’s moons have proven to be a treasure trove of discoveries.
This is particularly true of Saturn’s moon Tethys, also known as a “Death Star Moon” (because of the massive crater that marks its surface). In addition to closely resembling the space station out of Star Wars lore, it boasts the largest valleys in the Solar System and is composed mainly of water ice. In addition, it has much in common with two of its Cronian peers, Mimas and Rhea, which also resemble a certain moon-size space station.
Discovery and Naming:
Originally discovered by Giovanni Cassini in 1684, Tethys is one of four moons discovered by the great Italian mathematician, astronomer, astrologer and engineer between the years of 1671 and 1684. These include Rhea and Iapetus, which he discovered in 1671-72; and Dione, which he discovered alongside Tethys.
Cassini observed all of these moons using a large aerial telescope he set up on the grounds of the Paris Observatory. At the time of their discovery, he named the four new moons “Sider Lodoicea” (“the stars of Louis”) in honor of his patron, king Louis XIV of France.
An engraving of the Paris Observatory during Cassini’s time. Credit: Public Domain
Size, Mass and Orbit: With a mean radius of 531.1 ± 0.6 km and a mass of 6.1745 ×1020 kg, Tethys is equivalent in size to 0.083 Earths and 0.000103 times as massive. Its size and mass also mean that it is the 16th-largest moon in the Solar System, and more massive than all known moons smaller than itself combined. At an average distance (semi-major axis) of 294,619 km, Tethys is the third furthest large moon from Saturn and the 13th most distant moon over all.
Tethys’ has virtually no orbital eccentricity, but it does have an orbital inclination of about 1°. This means that the moon is locked in an inclination resonance with Saturn’s moon Mimas, though this does not cause any noticeable orbital eccentricity or tidal heating. Tethys has two co-orbital moons, Telesto and Calypso, which orbit near Tethys’s Lagrange Points.
Diameter comparison of the Saturnian moon Tethys, Moon, and Earth. Credit: NASA/JPL/USGS/Tom Reding
Tethys’ orbit lies deep inside the magnetosphere of Saturn, which means that the plasma co-rotating with the planet strikes the trailing hemisphere of the moon. Tethys is also subject to constant bombardment by the energetic particles (electrons and ions) present in the magnetosphere.
Composition and Surface Features: Tethys has a mean density of 0.984 ± 0.003 grams per cubic centimeter. Since water is 1 g/cm3, this means that Tethys is comprised almost entirely of water ice. In essence, if the moon were brought closer to the Sun, the vast majority of the moon would sublimate and evaporate away.
It is not currently known whether Tethys is differentiated into a rocky core and ice mantle. However, given the fact that rock accounts for less 6% of its mass, a differentiated Tethys would have a core that did not exceed 145 km in radius. On the other hand, Tethys’ shape – which resembles that of a triaxial ellipsoid – is consistent with it having a homogeneous interior (i.e. a mix of ice and rock).
This ice is also very reflective, which makes Tethys the second-brightest of the moons of Saturn, after Enceladus. There are two different regions of terrain on Tethys. One portion is ancient, with densely packed craters, while the other parts are darker and have less cratering. The surface is also marked by numerous large faults or graben.
The Odysseus Crater, the 400 km surface feature that gives Tethys it’s “Death Star” appearance. Credit: NASA/JPL/SSI
The western hemisphere of Tethys is dominated by a huge, shallow crater called Odysseus. At 400 km across, it is the largest crater on the surface, and roughly 2/5th the size of Tethys itself. Due to its position, shape, and the fact that a section in the middle is raised, this crater is also responsible for lending the moon it’s “Death Star” appearance.
The largest graben, Ithaca Chasma, is about 100 km wide and more than 2000 km long, making it the second longest valley in the Solar System. Named after the island of Ithaca in Greece, this valley runs approximately three-quarters of the way around Tethys’ circumference. It is also approximately concentric with Odysseus crater, which has led some astronomers to theorize that the two features might be related.
Scientists also think that Tethys was once internally active and that cryovolcanism led to endogenous resurfacing and surface renewal. This is due to the fact that a small part of the surface is covered by smooth plains, which are devoid of the craters and graben that cover much of the planet. The most likely explanation is that subsurface volcanoes deposited fresh material on the surface and smoothed out its features.
Cassini closeup of the southern end of Ithaca Chasma. Credit: NASA/JPL/Space Science Institute.
Like all other regular moons of Saturn, Tethys is believed to have formed from the Saturnian sub-nebula – a disk of gas and dust that surrounded Saturn soon after its formation. As this dust and gas coalesced, it formed Tethys and its two co-orbital moons: Telesto and Calypso. Hence why these two moons were captured into Tethys’ Lagrangian points, with one orbiting ahead of Tethys and the other following behind.
Exploration: Tethys has been approached by several space probes in the past, including Pioneer 11 (1979), Voyager 1 (1980) and Voyager 2 (1981). Although both Voyager spacecraft took images of the surface, only those taken by Voyager 2 were of high enough resolution to truly map the surface. While Voyager 1 managed to capture an image of Ithaca Chasma, it was the Voyager 2 mission that revealed much about the surface and imaged the Odysseus crater.
Tethys has also been photographed multiple times by the Cassini orbiter since 2004. By 2014, all of the images taken by Cassini allowed for a series of enhanced-color maps that detailed the surface of the entire planet (shown below). The color and brightness of Tethys’ surface have since become sources of interest to astronomers.
On the leading hemisphere of the moon, spacecraft have found a dark bluish band spanning 20° to the south and north from the equator. The band has an elliptical shape getting narrower as it approaches the trailing hemisphere, which is similar to the one found on Mimas.
Global, color mosaics of Saturn’s moon Tethys, as produced from images taken by NASA’s Cassini spacecraft between 2004-2014. Credit: NASA/JPL-Caltech/Space Science Institute/ Lunar and Planetary Institute
The band is likely caused by the influence of energetic electrons from Saturn’s magnetosphere, which drift in the direction opposite to the rotation of the planet and impact areas on the leading hemisphere close to the equator. Temperature maps of Tethys obtained by Cassini have shown this bluish region to be cooler at midday than surrounding areas.
At present, Tethys’ water-rich composition remains unexplained. One of the most interesting explanations proposed is that the rings and inner moons accreted from the ice-rich crust of a much larger, Titan-sized moon before it was swallowed up by Saturn. This, and other mysteries, will likely be addressed by future space probe missions.
We have many great articles about Tethys here at Universe Today. Here’s one about the story about Tethys, with a photograph taken by NASA’s Cassini spacecraft, and another about a feature on the surface of Tethys called Ithaca Chasma.
Want more info on Tethys? Check out this article from Solar Views, and this one from Nine Planets.
