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.
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).
Saturn's moon Dione, with Saturn's rings visible in the background. Credit: NASA/JPL
Thanks to the Cassini mission, a great deal has been learned about Saturn’s system of moons (aka. the Cronian system) in the past decade. Thanks to the presence of an orbiter in the system, astronomers and space exploration enthusiasts have been treated to a seemingly endless stream of images and data, which in turn has enabled us to learn many interesting things about these moons’ appearances, surface features, composition, and history of formation.
This is certainly true of Saturn’s bright moon of Dione. In addition to being the 15th largest moon in the Solar System, and more massive than all known moons smaller than itself combined, it has much in common with other Cronian satellites – like Tethys, Iapetus and Rhea. This includes being mainly composed of ice, having a synchronous rotation with Saturn, and an unusual coloration between its leading and trailing hemispheres.
Discovery and Naming:
Dione was first observed by Italian astronomer Giovanni Domenico Cassini on in 1684 using a large aerial telescope he set up on the grounds of the Paris Observatory. Along with the moons of Iapetus, Rhea and Tethys – which he had discovered in 1671, 1672 and 1684, respectively – he named these moons Sidera Lodoicea (“Stars of Louis”, after his patron, King Louis XIV of France).
These names, however, did not catch on outside of France. By the end of the 17th century, astronomers instead fell into the habit of naming Saturn’s then-known moons as Titan and Saturn I through V, in order of their observed distance from the planet. Being the second most-distant (behind Tethys) Dione came to be known as Saturn II for over a century.
An engraving of the Paris Observatory during Cassini’s time. Credit: Public Domain
The modern names were suggested in 1847 by John Herschel (the son of famed astronomer William Herschel), who suggested all the moons of Saturn be named after Titans – the sons and daughters of Cronos in the Greek mythology (the equivalent of the Roman Saturn).
In his 1847 publication, Results of Astronomical Observations made at the Cape of Good Hope, he suggested the name Dione, an ancient oracular Titaness who was the wife of Zeus and the mother of Aphrodite. Dione is featured in Homer’s The Iliad, and geological features – such as craters and cliffs – take their names from people and places in Virgil’s Aeneid.
Size, Mass and Orbit:
With a mean radius of 561.4 ± 0.4 km and a mass of about 1.0954 × 1021 kg, Dione is equivalent in size to 0.088 Earths and 0.000328 times as massive. It orbits Saturn at an average distance (semi-major axis) of 377,396 km, with a minor eccentricity of 0.0022 – ranging from 376,566 km at periapsis and 378,226 km at apoapsis.
Dione’s semi-major axis is about 2% less than that of the Moon. However, reflecting Saturn’s greater mass, Dione’s orbital period is one tenth that of the Moon (2.736915 days compared to 28). Dione is currently in a 1:2 mean-motion orbital resonance with Saturn’s moon Enceladus, completing one orbit of Saturn for every two orbits completed by Enceladus.
Size comparison between Earth, the Moon, and Saturn’s moon Dione. Credit: NASA/JPL/Space Science Institute
This resonance maintains Enceladus’s orbital eccentricity (0.0047) and provides tidal flexing that powers Enceladus’ extensive geological activity (which in turn powers its cryovolcanic jets). Dione has two co-orbital (aka. trojan) moons: Helene and Polydeuces. They are located within Dione’s Lagrangian points, 60 degrees ahead of and behind it, respectively.
Composition and Surface Features:
With a mean density of 1.478 ± 0.003 g/cm³, Dione is composed mainly of water, with a small remainder likely consisting of a silicate rock core. Though somewhat smaller and denser than Rhea, Dione is otherwise very similar in terms of its varied terrain, albedo features, and the different between its leading and trailing hemisphere.
Overall, scientists recognize five classes of geological features on Dione – Chasmata (chasms), dorsa (ridges), fossae (long, narrow depressions), craters, and catenae (crater chains). Craters are the most common feature, as with many Cronian moons, and can be distinguished in terms of heavily cratered terrain, moderately cratered plains, and lightly cratered plains.
The heavily cratered terrain has numerous craters greater than 100 km (62 mi) in diameter, whereas the plains areas tend to have craters less than 30 km (19 mi) in diameter (with some areas being more heavily cratered than others).
Global map of Dione, showing dark red in the trailing hemisphere (left), which is due to radiation and charged particles from Saturn’s. Credit: NASA/JPL/Space Science Institute
Much of the heavily cratered terrain is located on the trailing hemisphere, with the less cratered plains areas present on the leading hemisphere. This is the opposite of what many scientists expected, and suggests that during the period of Heavy Bombardment, Dione was tidally locked to Saturn in the opposite orientation.
Because Dione is relatively small, it is theorized that an impact large enough to cause a 35 km crater would have been sufficient to spin the satellite in the opposite direction. Because there are many craters larger than 35 km (22 mi), Dione could have been repeatedly spun during its early history. The pattern of cratering since then and the leading hemisphere’s bright albedo suggests that Dione has remained in its current orientation for several billion years.
Dione is also known for its differently colored leading and trailing hemispheres, which are similar to Tethys and Rhea. Whereas its leading hemisphere is bright, its trailing hemisphere is darker and redder in appearance. This is due to the leading hemisphere picking up material from Saturn’s E-Ring, which is fed by Enceladus’ cryovolcanic emissions.
Meanwhile, the trailing hemisphere interacts with radiation from Saturn’s magnetosphere, which causes organic elements contained within its surface ice to become dark and redder in appearance.
Dione’s trailing hemisphere, pictured by the Cassini orbiter, which shows its patches of “wispy terrain”. Credit: NASA/JPL
Another prominent feature is Dione’s “wispy terrain“, which covers its trailing hemisphere and is composed entirely of high albedo material that is also thin enough as to not obscure the surface features beneath. The origin of these features are unknown, but an earlier hypothesis suggested that that Dione was geologically active shortly after its formation, a process which has since ceased.
During this time of geological activity, endogenic resurfacing could have pushed material from the interior to the surface, with streaks forming from eruptions along cracks that fell back to the surface as snow or ash. Later, after the internal activity and resurfacing ceased, cratering continued primarily on the leading hemisphere and wiped out the streak patterns there.
This hypothesis was proven wrong by the Cassini probe flyby of December 13th, 2004, which produced close-up images. These revealed that the ‘wisps’ were, in fact, not ice deposits at all, but rather bright ice cliffs created by tectonic fractures (chasmata). During this flyby, Cassini also captured oblique images of the cliffs which showed that some of them are several hundred meters high.
Atmosphere:
Dione also has a very thin atmosphere of oxygen ions (O+²), which was first detected by the Cassini space probe in 2010. This atmosphere is so thin that scientists prefer to call it an exosphere rather than a tenuous atmosphere. The density of molecular oxygen ions determined from the Cassini plasma spectrometer data ranges from 0.01 to 0.09 per cm3 .
Dione viewed by Cassini on October 11th, 2005, showing the Alcander crater (top) and the larger Prytanis crater to its left. Credit: NASA/JPL/SSI
Unfortunately, the prevalence of water molecules in the background (from Saturn’s E-Ring) obscured detection of water ice on the surface, so the source of oxygen remains unknown. However, photolysis is a possible cause (similar to what happens on Europa), where charged particles from Saturn’s radiation belt interact with water ice on the surface to create hydrogen and oxygen, the hydrogen being lost to space and the oxygen retained.
Exploration:
Dione was first imaged by the Voyager 1 and 2 space probes as they passed by Saturn on their way to the Outer Solar System in 1980 and 1981, respectively. Since that time, the only probe to conduct a flyby or close-up imaging of Dione has been the Cassini orbiter, which conducted five flybys of the moon between 2005 and 2015.
The first close flyby took place on October 11th, 2005, at a distance of 500 km (310 mi), followed by another on April 7th, 2010, (again at a distance of 500 km). A third flyby was performed on December 12th, 2011, and was the closest, at an distance of 99 km (62 mi). The fourth and fifth flybys took place on June 16th and August 17th, 2015, at a distance of 516 km (321 mi) and 474 km (295 mi), respectively.
In addition to obtaining images of Cassini’s cratered and differently-colored surface, the Cassini mission was also responsible for detecting the moon’s tenuous atmosphere (exosphere). Beyond that, Cassini also provided scientists with new evidence that Dione could be more geologically active than previously predicted.
Based on models constructed by NASA scientists, it is now believed that Dione’s core experiences tidal heating, which increases the closer it gets to Saturn. Because of this, scientists also believe that Dione may also have a liquid water ocean at its core-mantle boundary, thus joining moons like Enceladus, Europa and others in being potential environments where extra-terrestrial life could exist.
This, as well as Dione’s geological history and the nature of its surface (which could be what gives rise to its atmosphere) make Dione a suitable target for future research. Though no missions to study the moon are currently being planned, any mission to the Saturn system in the coming years would likely include a flyby or two!
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.
Saturn's moon Rhea, as imaged by the Cassini-Huygens spaceprobe. Credit: NASA/JPL-Caltech
The Cronian system (i.e. Saturn and its system of rings and moons) is breathtaking to behold and intriguing to study. Besides its vast and beautiful ring system, it also has the second-most satellites of any planet in the Solar System. In fact, Saturn has an estimated 150 moons and moonlets – and only 53 of them have been officially named – which makes it second only to Jupiter.
For the most part, these moons are small, icy bodies that are believed to house interior oceans. And in all cases, particularly Rhea, their interesting appearances and compositions make them a prime target for scientific research. In addition to being able to tell us much about the Cronian system and its formation, moons like Rhea can also tell us much about the history of our Solar System.
Discovery and Naming:
Rhea was discovered by Italian astronomer Giovanni Domenico Cassini on December 23rd, 1672. Together with the moons of Iapetus, Tethys and Dione, which he discovered between 1671 and 1672, he named them all Sidera Lodoicea (“the stars of Louis”) in honor of his patron, King Louis XIV of France. However, these names were not widely recognized outside of France.
In 1847, John Herschel (the son of famed astronomer William Herschel, who discovered Uranus, Enceladus and Mimas) suggested the name Rhea – which first appeared in his treatise Results of Astronomical Observations made at the Cape of Good Hope. Like all the other Cronian satellites, Rhea was named after a Titan from Greek mythology, the “mother of the gods” and one the sisters of Cronos (Saturn, in Roman mythology).
The moons of Saturn, from left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan (background), Iapetus (top), and Hyperion (bottom). Credit: NASA/JPL/Space Science Institute
Size, Mass and Orbit:
With a mean radius of 763.8±1.0 km and a mass of 2.3065 ×1021 kg, Rhea is equivalent in size to 0.1199 Earths (and 0.44 Moons), and about 0.00039 times as massive (or 0.03139 Moons). It orbits Saturn at an average distance (semi-major axis) of 527,108 km, which places it outside the orbits of Dione and Tethys, and has a nearly circular orbit with a very minor eccentricity (0.001).
With an orbital velocity of about 30,541 km/h, Rhea takes approximately 4.518 days to complete a single orbit of its parent planet. Like many of Saturn’s moons, its rotational period is synchronous with its orbit, meaning that the same face is always pointed towards it.
Composition and Surface Features:
With a mean density of about 1.236 g/cm³, Rhea is estimated to be composed of 75% water ice (with a density of roughly 0.93 g/cm³) and 25% of silicate rock (with a density of around 3.25 g/cm³). This low density means that although Rhea is the ninth-largest moon in the Solar System, it is also the tenth-most massive.
In terms of its interior, Rhea was originally suspected of being differentiated between a rocky core and an icy mantle. However, more recent measurements would seem to indicate that Rhea is either only partly differentiated, or has a homogeneous interior – likely consisting of both silicate rock and ice together (similar to Jupiter’s moon Callisto).
Views of Saturn’s moon Rhea, with false-color image showing elevation data at the right. Credit: NASA/JPL/Space Science Institute
Models of Rhea’s interior also suggest that it may have an internal liquid-water ocean, similar to Enceladus and Titan. This liquid-water ocean, should it exist, would likely be located at the core-mantle boundary, and would be sustained by the heating caused by from decay of radioactive elements in its core.
Rhea’s surface features resemble those of Dione, with dissimilar appearances existing between their leading and trailing hemispheres – which suggests that the two moons have similar compositions and histories. Images taken of the surface have led astronomers to divide it into two regions – the heavily cratered and bright terrain, where craters are larger than 40 km (25 miles) in diameter; and the polar and equatorial regions where craters are noticeably smaller.
Another difference between Rhea’s leading and trailing hemisphere is their coloration. The leading hemisphere is heavily cratered and uniformly bright while the trailing hemisphere has networks of bright swaths on a dark background and few visible craters. It had been thought that these bright areas (aka. wispy terrain) might be material ejected from ice volcanoes early in Rhea’s history when its interior was still liquid.
However, observations of Dione, which has an even darker trailing hemisphere and similar but more prominent bright streaks, has cast this into doubt. It is now believed that the wispy terrain are tectonically-formed ice cliffs (chasmata) which resulted from extensive fracturing of the moon’s surface. Rhea also has a very faint “line” of material at its equator which was thought to be deposited by material deorbiting from its rings (see below).
Hemispheric color differences on Saturn’s moon Rhea are apparent in this false-color view of the anti-Cronian side, from NASA’s Cassini spacecraft. Image Credit: NASA/JPL/SSI
Rhea has two particularly large impact basins, both of which are situated on Rhea’s anti-Cronian side (aka. the side facing away from Saturn). These are known as Tirawa and Mamaldi basins, which measure roughly 360 and 500 km (223.69 and 310.68 mi) across. The more northerly and less degraded basin of Tirawa overlaps Mamaldi – which lies to its southwest – and is roughly comparable to the Odysseus crater on Tethys (which gives it its “Death-Star” appearance).
Atmosphere:
Rhea has a tenuous atmosphere (exosphere) which consists of oxygen and carbon dioxide, which exists in a 5:2 ratio. The surface density of the exosphere is from 105 to 106 molecules per cubic centimeter, depending on local temperature. Surface temperatures on Rhea average 99 K (-174 °C/-281.2 °F) in direct sunlight, and between 73 K (-200 °C/-328 °F) and 53 K (-220 °C/-364 °F) when sunlight is absent.
The oxygen in the atmosphere is created by the interaction of surface water ice and ions supplied from Saturn’s magnetosphere (aka. radiolysis). These ions cause the water ice to break down into oxygen gas (O²) and elemental hydrogen (H), the former of which is retained while the latter escapes into space. The source of the carbon dioxide is less clear, and could be either the result of organics in the surface ice being oxidized, or from outgassing from the moon’s interior.
Saturn’s second-largest moon Rhea, pictured by the Cassini probe on March 29, 2012. Credit: NASA/JPL
Rhea may also have a tenuous ring system, which was inferred based on observed changes in the flow of electrons trapped by Saturn’s magnetic field. The existence of a ring system was temporarily bolstered by the discovered presence of a set of small ultraviolet-bright spots distributed along Rhea’s equator (which were interpreted as the impact points of deorbiting ring material).
However, more recent observations made by the Cassini probe have cast doubt on this. After taking images of the planet from multiple angles, no evidence of ring material was found, suggesting that there must be another cause for the observed electron flow and UV bright spots on Rhea’s equator. If such a ring system were to exist, it would be the first instance where a ring system was found orbiting a moon.
Exploration:
The first images of Rhea were obtained by the Voyager 1 and 2 spacecraft while they studied the Cronian system, in 1980 and 1981, respectively. No subsequent missions were made until the arrival of the Cassini orbiter in 2005. After it’s arrival in the Cronian system, the orbiter made five close targeted fly-bys and took many images of Saturn from long to moderate distances.
The Cronian system is definitely a fascinating place, and we’ve really only begun to scratch its surface in recent years. In time, more orbiters and perhaps landers will be traveling to the system, seeking to learn more about Saturn’s moons and what exists beneath their icy surfaces. One can only hope that any such mission includes a closer look at Rhea, and the other “Death Star Moon”, Dione.
Enhanced-color image from Cassini showing red streaks on Tethys (NASA/JPL-Caltech/Space Science Institute)
Resembling what the skin on my arms looks like after giving my cat a bath, the surface of Saturn’s moon Tethys is seen above in an extended-color composite from NASA’s Cassini spacecraft showing strange long red streaks. They stretch for long distances across the moon’s surface following the rugged terrain, continuing unbroken over hills and down into craters… and their cause isn’t yet known.
According to a NASA news release, “The origin of the features and their reddish color is currently a mystery to Cassini scientists. Possibilities being studied include ideas that the reddish material is exposed ice with chemical impurities, or the result of outgassing from inside Tethys. The streaks could also be associated with features like fractures that are below the resolution of the available images.”
The images were taken by Cassini during a flyby of the 660-mile-wide (1,062 km) Tethys on April 11, 2015 at a resolution of about 2,300 feet (700 meters) per pixel. They were acquired in visible green, infrared, and ultraviolet light wavelengths and so the composite image reveals colors our eyes can’t directly perceive. The combination of this and the solar illumination needed to image this particular area as the spacecraft passed by are why these features haven’t been seen so well until now.
“The red arcs really popped out when we saw the new images,” said Cassini participating scientist Paul Schenk of the Lunar and Planetary Institute in Houston. “It’s surprising how extensive these features are.”
Extended color mosaic of Tethys from Cassini images acquired on April 11, 2015. The region where the streaks are is outlined. Click for original hi-res version. (NASA/JPL-Caltech/SSI)
While the nature of Tethys’ streaks isn’t understood, the observations do indicate a relatively young age compared to the surrounding surface.
“The red arcs must be geologically young because they cut across older features like impact craters, but we don’t know their age in years.” said Paul Helfenstein, a Cassini imaging scientist at Cornell University in Ithaca. “If the stain is only a thin, colored veneer on the icy soil, exposure to the space environment at Tethys’ surface might erase them on relatively short time scales.”
Reprocessed Galileo image of Europa’s streaked surface by Ted Stryk (NASA/JPL/Ted Stryk)
Could these arcs be signs of an underground ocean or reservoir of briny liquid, like Enceladus’ tiger stripes (aka sulcae) or the streaks that crisscross Europa’s ice? Or are they the results of infalling material from one of Saturn’s other moons? More observations with Cassini, now in its eleventh year in orbit at Saturn, are being planned to “study the streaks.”
“We are planning an even closer look at one of the Tethys red arcs in November to see if we can tease out the source and composition of these unusual markings,” said Linda Spilker, Cassini project scientist at JPL.
Topographic elevation map of Ceres showing some newly-named craters. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
Ceres’ topography is revealed in full (but false) color in a new map created from elevation data gathered by NASA’s Dawn spacecraft, now nearly five months in orbit around the dwarf planet orbiting the Sun within the main asteroid belt.
With craters 3.7 miles (6 km) deep and mountains rising about the same distance from its surface, Ceres bears a resemblance to some of Saturn’s frozen moons.
“The craters we find on Ceres, in terms of their depth and diameter, are very similar to what we see on Dione and Tethys, two icy satellites of Saturn that are about the same size and density as Ceres,” said Paul Schenk, Dawn science team member and a geologist at the Lunar and Planetary Institute (LPI) in Houston, TX. “The features are pretty consistent with an ice-rich crust.”
Check out a rotation video of Ceres’ topography below:
In addition to elevation mapping Ceres has also had some of its more prominent craters named. No longer just “bright spot crater” and “Spot 1,” these ancient impact scars now have official IAU monikers… from the Roman Occator to the Hawaiian Haulani to the Hopi Kerwan, craters on Ceres are named after agriculture-related gods and goddesses of mythologies from around the world.
Ceres’ famous “bright spot” crater is now named Occator, after the Roman god of harrowing. (NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)
Dawn is currently moving closer toward Ceres into its third mapping orbit. By mid-August it will be 900 miles (1448 km) above Ceres’ surface and will proceed with acquiring data from this lower altitude, three times closer than it has been previously.
At 584 miles (940 km) in diameter Ceres is about 40 percent the size of Pluto.
NASA’s Dawn spacecraft is the first to successfully enter orbit around two different mission targets and the first to orbit a dwarf planet. Its first target was the asteroid Vesta, which it orbited from July 2011 to September 2012. Dawn arrived in orbit at Ceres on March 6, 2015 and there it will remain during its primary science phase and beyond; Ceres is now Dawn’s permanent home.
Ceres (left, Dawn image) compared to Tethys (right, Cassini image) at comparative scale sizes. (Credits: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA and NASA/JPL-Caltech/SSI. Comparison by J. Major.)
Tethys is mostly obscured behind Rhea as the moons orbit Saturn. The picture was captured by the Cassini spacecraft in April 2012 and highlighted in December 2014. Credit: NASA/JPL-Caltech/Space Science Institute
Peekaboo! Tethys makes a (mostly in vain) attempt to hide behind Rhea in this picture taken by the Cassini spacecraft a couple of years ago, but highlighted by NASA in a recent picture essay. Besides the neat view of the orbital dance, one thing that is clearly visible between the two moons is the different colors — a product of their different surfaces. It turns out that Tethys’ bright surface is due to geysers from another moon.
“Scientists believe that Tethys’ surprisingly high albedo is due to the water ice jets emerging from its neighbor, Enceladus,” NASA stated. “The fresh water ice becomes the E ring [of Saturn] and can eventually arrive at Tethys, giving it a fresh surface layer of clean ice.”
Saturn has an astounding number of moons — 62 moons discovered so far, and 53 of them named, if you don’t count the spectacular ring that surrounds the planet. The collection of celestial bodies includes Titan, the second-biggest moon in the Solar System. It’s so big that it includes a thick atmosphere. (Ganymede, around Jupiter, is the biggest.)
Below are some other pictures of moons dancing around Saturn — some harder to spot than others. All images were taken by the Cassini spacecraft since it arrived at the planet in 2004.
Titan peeks from behind two of Saturn’s rings. Another small moon Epimetheus, appears just above the rings. Credit: NASA/JPL/Space Science Institute
Saturn’s moons Dione and Rhea appear conjoined in this optical illusion-like image taken by the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute
Saturn’s rings, made dark in part as the planet casts its shadow across them, cut a striking figure before Saturn’s largest moon, Titan. Credit: NASA/JPL/Space Science Institute
Three of Saturn’s moons bunch together in this image by Cassini. Credit: NASA/JPL/Space Science Institute. Click for larger image.
Saturn’s rings with Saturn’s moon Mimas in the foreground (credit: NASA)
Titan and Tethys line up for a portrait of ‘sibling’ moons. Credit: NASA/JPL/Space Science Institute
This global map of Dione, a moon of Saturn, shows dark red in the trailing hemisphere, which is due to radiation and charged particles from Saturn's intense magnetic environment. Credit: NASA/JPL/Space Science Institute
If you hang out in Saturn’s intense magnetic environment for a while, it’s going to leave a mark. That’s one conclusion from scientists who proudly released new maps yesterday (Dec. 9) of the planet’s icy moons, showing dark blotches on the surfaces of Dione, Rhea, and Tethys.
Cassini has been at Saturn for more than 10 years, and compared to the flyby Voyager mission has given us a greater understanding of what these moons contain. You can see the difference clearly in the maps below; look under the jump and swipe back and forth to see the difference.
So what do these maps yield? Radiation-burned hemispheres in Dione, Tethys, and Rhea. Icy deposits building up on Enceladus from eruptions, which you can see in yellow and magenta, as well as fractures in blue. Dust from Saturn’s E-ring covering several of the moons, except for Iapetus and Tethys.
“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
Could these be used by future explorers seeking life in some of these moons? In the meantime, enjoy the difference between Voyager’s view in the 1980s, and Cassini’s view for the past decade, in the comparison maps below.
A caution about the maps: they are a little more enhanced than human vision, showing some features in infrared and ultraviolet wavelengths. “Differences in color across the moons’ surfaces that are subtle in natural-color views become much easier to study in these enhanced colors,” NASA stated.
When Voyager first imaged the huge Herschel Crater on Mimas, scientists could not help comparing the small and battered moon to the Death Star in George Lucas’ science-fiction adventure Star Wars. But Saturn’s moon Tethys is also home to a massive crater; the remains of an ancient impact that nearly destroyed the tiny moon. Odysseus Crater dominates the surface of Tethys covering two-thirds of the surface. The tiny moon is just 1062 kilometers, or 660 miles, across. Using information from Voyager and Cassini, scientists found that the heavily cratered and fractured moon is made up of mostly water ice with a small amount of rock.
Odysseus Crater takes up the entire left side of this image.
With the Sun over Cassini’s shoulder, the spacecraft took this image of the northern part of Odysseus June 28, 2012 while the spacecraft zipped along just 72,000 kilometers (45,000 miles) above Tethys. If you’re interested, the resolution of this image is about 430 meters (1,409 feet) per pixel; meaning that one pixel takes up 430 meters in the image.
John Williams is a science writer and owner of TerraZoom, a Colorado-based web development shop specializing in web mapping and online image zooms. He also writes the award-winning blog, StarryCritters, an interactive site devoted to looking at images from NASA’s Great Observatories and other sources in a different way. A former contributing editor for Final Frontier, his work has appeared in the Planetary Society Blog, Air & Space Smithsonian, Astronomy, Earth, MX Developer’s Journal, The Kansas City Star and many other newspapers and magazines.
