Review of Saturn's icy moons following the Cassini mission

Mimas is the sm allest (diameter of 394 km) and innermost of the classical regular or intermediate sized satellites. Its orbit lies between the tenuous G and E rings at a radial distance of 3.08Rs from Saturn (Herschel 1790). Its surface is extremely heavily cratered and there is little indication of any major endogenic modifications and it has a fairly uniform albedo (or reflectivity). Its most striking feature is a well-preserved crater, named Herschel, 130 km in extent and nearly centered on its leading hemisphere as well as covering a third of the surface. It is thought to be the result of an impact which nearly split the moon apart (Moore et al 2004).

Enceladus is slightly larger than Mimas with a diameter of 502 km and orbits Saturn at a radial distance of 3.95Rs. Both Enceladus and Mimas were discovered by Herschel in 1789 (Herschel 1790). The remarkably high surface albedo of Enceladus has been known since ground-based observations of it were made (Slipher and Slipher 1914, Franz and Millis 1975) and this was confirmed by the Voyager flybys. It is deficient in large craters, a striking indication of large scale endogenic activity (especially when compared to Mimas close by) although there are some craters in its north polar region. The mid-latitudes and equatorial regions have vast crater-free smooth plains with a system of peculiar curvilinear ridges up to 1 km in height (Smith et al 1982). Photometric properties of its surface are remarkably uniform (Buratti 1988) and it appears to have been extensively and recently resurfaced. Water ice was found to be the dominant spectrally active feature of the surface. It was postulated that the satellite may be the source of the E ring which has a peak in density near the orbit of Enceladus (Haff et al 1983) and some of the questions raised following the Voyager flybys included whether the E ring is a recent phenomenon associated with the moon and whether there is an unexpectedly large internal heat source (Schenk et al 2018).

Tethys discovered by Cassini in 1684 (Cassini 1686) orbits at 4.88Rs and is over twice the diameter of Enceladus (1060 km). Like Dione and Rhea the moon exhibits terrains of different geological ages, for example north of the equator around a longitude of 60° the surface seems to be rough, hilly and densely cratered whereas a portion of the trailing hemisphere is clearly less rugged and lightly cratered (Stone and Miner 1982. Moore et al 2004). It seems to be much less active geologically than Enceladus, with heavily cratered terrain covering the majority of the surface and two outstanding topographic features, a large 500 km diameter crater, known as Odysseus and a huge trough or rift zone (Ithaca Chasma) which extends over 270° across the surface (Stone and Miner 1981). The crater is the largest in our solar system and covers at least three quarters of the circumference of the moon.

Dione, also discovered by Cassini in the same year as Tethys (Cassini 1686), orbits at a distance of 6.26Rs from Saturn and is slightly larger than Tethys (with a diameter of 1120 km) but a higher bulk density (the highest well determined density amongst the inner satellites). Its surface has the highest brightness contrasts outside of Iapetus with the albedo being higher on the leading than trailing hemisphere with some evidence of wispy terrain (where the surface appears mottled with patches of different albedo, Morrison et al (1984)). Some of these wispy markings appear more like bright narrow lines which extend into the leading hemisphere, some of which are associated with topographic features. Dione shows considerably more evidence of internal activity and a lower crater population than Rhea, the moon beyond it. There are extensive resurfaced plains and troughs or valleys hundreds of kilometers in length. The real surprise at Dione was the clear evidence for levels of endogenic activity exceeding in intensity and duration that observed on nearby and larger Rhea. The lack of correlation between observed geological history and expected thermal relaxation times represents one of the major geological challenges of the Voyager data for the regular satellites at Saturn (Morrison et al 1984).

Rhea is the largest of the intermediate sized satellites (close in size to Iapetus, both of which were discovered by Cassini in 1671 (Cassini 1686)), with a diameter of 1530 km and it orbits at a radial distance of 8.73Rs from Saturn. Images from Voyager revealed a bright heavily cratered surface (Moore et al 2004) whose leading side is fairly uniform in brightness, with large craters (>40 km across) although in parts of the polar and equatorial regions the craters are smaller. These well-formed impact craters are similar to the highland provinces of the Moon and Mercury. The difference in size of the craters in the two areas may be an indication of a major resurfacing event at some stage in in Rhea's past (Morrison et al 1984).

Iapetus, the last of the intermediate sized satellites, with a diameter of 1460 km, has a large nearly circular high inclination (14.7°) varying orbit around Saturn (at a radial distance of 59Rs). Its orbital variation is due to the combined action from the Sun and Saturn's oblateness. It is best known for the puzzling large amplitude of its light variation (Morrison et al 1975), with a leading hemisphere almost 25 times darker than its trailing hemisphere, similar in darkness to cometary nuclei and other primitive bodies. The dark material is reddish in color and carbon bearing (Morrison et al 1984). There is virtually perfect longitudinal symmetry of dark material with respect to its direction of orbital motion which the Voyager data confirmed as well as the bright polar regions which were suggested from earlier photometric measurements. There is a large dark equatorial ring, some 300 km in diameter, which interrupts the symmetry of the albedo boundary and the bright side of the surface is heavily cratered, especially the north polar region (Porco et al 2005, Denk et al 2010, Spencer et al 2010). The density of Iapetus is very similar to the other icy satellites of Saturn and consistent with models where water ice is the primary constituent. As a consequence of the poor Voyager coverage of the leading hemisphere the nature and origin of the dark material remains uncertain. However, the strong symmetry with the direction of orbital motion points to some form of external control, such as dust from Phoebe or perhaps from an as yet undiscovered external satellite in a retrograde orbit?

Given its small size, large distance from the sun, and no atmosphere to insulate it, scientists thought Enceladus should have frozen solid long ago (Spencer et al 2009). Instead, Cassini's flybys of Enceladus revealed a surprisingly active world. Geysers shoot from its south pole forming a huge plume of water vapor and icy particles (Porco et al 2006). Some of these particles leave Enceladus and form the E ring (Mitchell et al 2015). Enceladus is the smallest body found so far to have active cryovolcanism on its surface.

Early images of Enceladus revealed a bright, icy world much as photographed by Voyager. During the Voyager flybys Enceladus' south pole was in darkness while Cassini's early flybys showed a fully sunlit, and remarkable south polar region. Detailed Cassini images from the imaging sub-system (ISS) of Enceladus' south pole were first obtained in July 2005, (Porco et al 2006), see figure 3. These images showed a complex, youthful terrain that is almost entirely free of impact craters. This region is cut by large tectonic fractures and is the youngest surface on Enceladus. It includes a system of four nearly parallel fissures, nicknamed 'tiger stripes', that may also be tectonic in origin and are centered near the pole (Helfenstein 2010). The fissures are flanked on each side by 100 m tall ridges. Each 'tiger stripe' is about 130 km long, 2–4 km wide, and each is separated by about 35 km. The fractures are about 300 meters deep, with V-shaped inner walls (Helfenstein 2010).

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High-resolution images showed widespread deposits of fine-grained material along the ridges and terrain containing boulder-sized chunks of ice tens of meters in size (Martens et al 2015). The tiger stripes are named after cities in the Arabian nights, Alexandria, Cairo, Bagdad and Damascus. Some high-resolution images also showed signs that the south polar icy crust has been spreading with time, much like some of the tectonic spreading we see on Earth (Yin and Pappalardo 2015). On Enceladus, however, the spreading seems to be primarily in one direction.

The Cassini composite infrared spectrometer (CIRS) revealed another remarkable aspect of the south polar region, southward of 70° south latitude. The CIRS instrument created a thermal or heat map of the south polar region and found it was much warmer than anticipated if this region was heated solely from sunlight (Spencer et al 2006), see figure 4. CIRS measured Enceladus emission at mid- and far-infrared wavelengths to determine that the temperature in the 'tiger stripe' region was hottest of all (Howett et al 2011a). The bulk of the heat is focused along the 'tiger stripes' with temperatures of 115–190 K, much warmer than the expected 65 K for this area (Goguen et al 2013, Spencer et al 2013). The temperatures cool off quickly away from the main fissures. The heat source is coming from the interior of Enceladus, with the continuous release of over 16 gigawatts of energy, which is enough to power a city of about six million people.

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Tidal heating and tidal stress provide sources for this heat (Tobie et al 2008). Enceladus' orbit around Saturn is slightly elliptical as a result of its 2:1 orbital resonance with Dione. During its elliptical orbit Enceladus moves closer to, and then farther away from Saturn. Enceladus feels a stronger gravitational tug from Saturn when it is closer to the planet compared to when it is farther away. This oscillating gravitational tug or tidal forcing on Enceladus causes it to flex slightly, which produces tidal heating. These gravitational tides also produce stresses that crack the icy crust in places like the south pole. Tidal stress can pull these cracks open and closed during Enceladus' orbit around Saturn (Nimmo et al 2014), generating friction, which could release some extra heat at the surface. Using current models, the total heat produced by tidal heating and stress can only explain a portion of the excess heat emanating from Enceladus' south pole. If the Dione–Enceladus interaction changes with time, more intense periods of tidal heating may be separated by more quiescent periods perhaps providing a mechanism to generate the necessary heat (Bland et al 2012).

The north pole of Enceladus does not exhibit activity similar to its south pole. The north pole contains a large fracture that goes through the pole and exhibits numerous craters as well. Perhaps the crust is thicker in the north polar region which might prohibit similar activity. Other areas of Enceladus include smooth plains that typically have low relief and fewer craters than the cratered terrains (Kirchoff and Schenk 2009) such as the north polar region. Some of these plains exhibit troughs and ridges as well as a lower crater density, indicating a younger surface than the cratered regions.

A giant plume of water vapor gas and icy grains, fed by discrete jets and curtains of material emanating from each tiger stripe, extends thousands of kilometers into space (Porco et al 2014, Spitale et al 2015). The material shoots out of the tiger stripes at about 1.25 km s⁻¹. A fair fraction of the material eventually re-impacts Enceladus after it escapes into Saturn orbit but some of the tiniest grains escape Enceladus' gravity and form Saturn's extended E ring (Spahn et al 2006, Kempf et al 2010, Mitchell et al 2015). Cassini found that more than 90% of the material in the plume is water vapor. This gas lofts the particles into space where sunlight scatters off them and makes them visible to Cassini's cameras and spectrometers.

The Cassini imaging instrument, ISS, took the first images of Enceladus' plume in early 2005 (Porco et al 2006) although concern about possible scattering inside the instrument at high phase angles delayed the announcement of plume detection until November 2005 when Cassini images clearly showed jets of icy particles spraying from Enceladus' south polar region (Spitale and Porco 2007). These images showed the plume's detailed structure, revealing a number of individual jets within a larger plume that extended well above Enceladus' surface, see figure 5.

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Detailed analysis of Cassini images showed that the origin of some of the strongest jets is near the hottest spots CIRS measured along the tiger stripes. All of the jets appear to originate from locations along the four tiger stripes, and to date, one hundred individual jets have been mapped (Porco et al 2014). Another source of Enceladus activity is curtain eruptions from material spouting along the length of the tiger stripe (Spitale et al 2015). Some of the apparent jets may actually be phantom jets resulting from images taken along a fold or kink in the curtain that gives the optical illusion of a jet. Curtain eruptions occur on Earth also when molten lava gushes out of a deep fracture and creates stunning curtains of fire such as those seen from some of the active Hawaiian volcanoes. The geysers on Enceladus are most likely a combination of jets and curtains. Output from these individual jets and curtains combine above Enceladus to form a single large plume.

The combined analysis of imaging, ultraviolet and mass spectrometer data suggests that the jets originate from pressurized chambers beneath the surface, similar to Earth's geysers. The intensity of the eruptions varies significantly as a function of the location of Enceladus in its orbit (Hedman et al 2013, Nimmo et al 2014). Visual and infrared mapping spectrometer (VIMS) data first showed that the particle plume is about three times brighter when Enceladus is at its most distant point in its orbit (apoapse) than at its closest point (periapse), consistent with the predicted opening of cracks near apoapse and closing of cracks near peripase (Hedman et al 2013). Ultraviolet imaging spectrograph (UVIS) stellar occultation data, however, do not show this same large variation for the gas emission. UVIS saw only see about a 20% increase in the total amount of gas from apoapse to periapse of Enceladus' orbit. However, gas emission associated with a jet was four times higher for the same geometries. This four-fold increase in the jet's activity is what can loft more particles into space.

The first indication that the Enceladus plume might be emanating from a liquid water source beneath its icy crust rather than from sublimation at its surface came from gravity measurements based on the change in the Doppler signal as Cassini flew close to Enceladus (Iess et al 2014). The slight change in Doppler frequency indicates how strongly Cassini is affected by Enceladus' gravity. The initial gravity results pointed toward a liquid water source underneath the south pole.

Careful measurements on Enceladus images taken over the last decade showed a very slight libration, or wobble as Enceladus orbits Saturn (Thomas et al 2016). The magnitude of that wobble can only be explained if Enceladus' outer icy shell is decoupled from its rocky core by a global ocean. The ocean is about 10 km deep underneath the south polar region, beneath an ice shell that is about 26–31 km thick. Enceladus' plume is being fed from this liquid water reservoir under the icy crust.

Enceladus' plume is a mixture of gas and icy particles. Seven of Cassini's closest Enceladus flybys were targeted to fly through the plume to directly sample the plume material and determine its composition. Cassini's ion and neutral mass spectrometer (INMS) and UVIS measure gas and detected abundant water vapor. INMS discovered an unexpected mix of volatile gasses including small amounts of carbon dioxide, and simple hydrocarbons such as methane, propane, and acetylene (Waite et al 2006, 2009). During Cassini's deepest dive through the plume in October 2015, the spacecraft flew through just 49 km above the surface where INMS discovered molecular hydrogen (H₂) in the plume (Waite et al 2017).

Cassini's cosmic dust analyzer (CDA) probes the icy particles and along with water found evidence for salt-rich ice grains containing sodium and potassium that are probably frozen droplets from a saltwater ocean underneath Enceladus' icy crust (Postberg et al 2011). The salty minerals are created when liquid water is in contact with a mineral-rich rocky core and the salts are dissolved in the water. These grains also contain carbonates that provide a slightly alkaline pH value for the ocean (Postberg et al 2009, Glein et al 2015).

CDA also discovered tiny grains of silica, only 6–9 nm in size, even before Cassini entered Saturn's orbit in 2004 (Hsu et al 2015). After careful study, the CDA team concluded that these tiny grains must originate inside Enceladus' ocean. The most common way to form silica grains this tiny on Earth is hydrothermal activity. Cold slightly alkaline and salty water is heated almost to the boiling point inside Enceladus' rocky core where it becomes super-saturated with silica and other minerals. This hot water undergoes a large temperature drop as it shoots out of hydrothermal vents and encounters the cold water. Tiny silica grains condense, ultimately being ejected into Enceladus' plume. Deep in Earth's oceans, hydrothermal vents called 'white smokers' like 'Lost City' also form tiny grains of silica in this way.

Other byproducts of hydrothermal activity, including methane and molecular hydrogen, have also been found in the Cassini data. Methane and molecular hydrogen are particularly abundant in Enceladus' plume. At the high pressures expected in Enceladus' oceans icy materials called clathrates can form and imprison molecules of methane or hydrogen within a crystalline structure of water ice. This clathrate process is so efficient at removing methane (Bouquet et al 2014) and hydrogen (Waite et al 2017) from the ocean that another explanation is needed to explain the abundance of methane and hydrogen observed by Cassini in the plume. Active hydrothermal processes could produce methane and hydrogen faster than it is being converted into clathrates and produce the excesses being observed. Molecular hydrogen provides a geochemical energy source by reacting with carbon dioxide to release a burst of energy that can be used by microbes, if any exist in Enceladus' ocean.

The plume material that escapes Enceladus's gravity leaves the moon and forms Saturn's E ring (Mitchell et al 2015). The E ring consists of tiny micron and sub-micron particles of water ice, containing silicates, carbon dioxide and ammonia (Spahn et al 2006, Kempf et al 2010). The E ring is densest at Enceladus, and is spread throughout the Saturnian system, between the orbits of Mimas and Titan. Unlike Saturn's main rings, the E ring is more than 2000 km thick and increases in thickness with its distance from Enceladus. Tendril-like structures have been observed within the E ring and can be traced back to jets emanating from the south pole (Mitchell et al 2015). The tiny particles in the E ring are unstable, with a lifespan short enough that they must be continuously replenished for the E ring to continue to exist (Spahn et al 2006).

E ring particles accumulate on the moons that orbit within it, including Mimas, Enceladus, Tethys, Dione and Rhea (Schenk et al 2011). Bright E ring grains coat the surfaces of the tiny Trojan moons Telesto, Calypso, Helene and Polydeuces, and smooth out their surface features. E ring grains bombard the leading sides of Tethys and Dione, and the trailing side of Mimas (Hendrix et al 2012). In the ultraviolet, the hemispheres bombarded by E ring grains are brighter than the opposite hemispheres, so Tethys and Dione have brighter leading hemispheres, and Mimas' trailing hemisphere is brighter. Enceladus itself displays variations in the thickness of its surface deposits (Hendrix et al 2012). Global color mosaics show regions of enhanced deposits that are in agreement with CDA models of icy grain deposition (Schenk et al 2011).

As Cassini first approached Saturn, UVIS found that the Saturn system was filled with oxygen atoms (Esposito et al 2005). At that time, early in the mission, the source of oxygen was a puzzling. After discovering the plume, we now know that the Enceladus water vapor in Saturn's magnetosphere is broken down into oxygen and hydrogen, providing both to the Saturn system (Waite et al 2006, 2009). These materials react with other moons in the Saturn system, influencing their surfaces.

Before the Cassini mission very little was known about the interior of Enceladus. A number of Enceladus flybys were dedicated to radio science and provided the effects of Enceladus' gravity on Cassini, yielding a mass for Enceladus (Iess et al 2014). The mass and shape provided a density of 1.61 g cm⁻³, which is much higher than Saturn's other mid-sized icy satellites indicating that Enceladus contains larger amounts of silicates and iron. Gravity measurements also showed that the density of the core is low, indicating that the core contains water in addition to silicates and iron (Iess et al 2014). High-resolution observations of the surface including images and spectra, indicated internal activity and thermal evolution (Howett et al 2011, Spencer et al 2013), providing additional evidence for a differentiated interior. Cassini observations have shown that Enceladus is a differentiated body with a liquid water ocean (McKinnon et al 2015, Thomas et al 2016). This combination provides the potential for ocean water to circulate within the rock. The heated water could pick up elements like silica, potassium, phosphorous, and sulfur, elements that are key for life as we know it.

Enceladus provides a unique laboratory for monitoring the unusual behavior of plasma, or hot ionized gas, in its vicinity. Enceladus is a major source of the ionized material that fills Saturn's magnetosphere (Coates et al 2010, Hill et al 2012). About 100 kilograms of water vapor per second spray out from the cracks at the south pole (Hansen et al 2006, 2011). The water vapor is converted into charged particles that interact with the plasma in Saturn's magnetosphere.

Cassini's fields and particles instruments have shown that the usual heavy and light charged particles in normal plasma are actually reversed near Enceladus' plume and that the dust grains are negatively charged (Morooka et al 2011, Hill et al 2012). The nature of this unique gas-dust-plasma mixture has been revealed over the course of Cassini's 13 year mission with data from multiple instruments, including the Cassini plasma spectrometer (CAPS), magnetometer (MAG), magnetospheric imaging instrument (MIMI), and the radio and plasma wave science instrument (RPWS).

Using its ultraviolet imaging spectrograph (UVIS) Cassini was able to measure the amount of water vapor erupting from the geysers by monitoring a bright star as it passed behind the plume of gas and dust spewing from Enceladus (Hansen et al 2006, 2011). These measurements offered new insights on geologic activity happening beneath Enceladus' surface.

Other instruments on Cassini have observed that the number of water ice grains being ejected from the small moon was three times greater when Enceladus was farthest from Saturn in its orbit compared to when it was closest (Hedman et al 2013, Nimmo et al 2014). The UVIS instrument only measured a 20% increase with orbital position in the total amount of gas. However, when Cassini flew over one of Enceladus' active jets, UVIS measured that the jet was four times more active than the background water vapor. The active jets may be responsible for greater increase in particle emission as Enceladus orbits Saturn. Tidal forcing from Saturn may be responsible for slightly opening or closing the jets as Enceladus orbits the planet leading to increasing and decreasing particle flow.

Enceladus discoveries have changed the direction of planetary science. Multiple discoveries have increased our understanding of Enceladus, including the plume venting from its south pole; hydrocarbons in the plume; a global, salty ocean and hydrothermal vents on the seafloor. They all point to the possibility of a habitable ocean world well beyond Earth's habitable zone. Planetary scientists now have Enceladus to consider as a possible habitat for life. However, the Cassini spacecraft does not carry instruments to look for life so the search for life will require a future mission.

There have been four untargeted and therefore rather distant flybys of this icy moon whose main feature is that of the prominent crater Herschel (which has a diameter ~120 km and a depth ~11 km). In addition the surface is crossed by number of linear almost parallel troughs that are thought to potentially be the consequence of fractures on a global scale as a result of the Herschel impact event (McKinnon 1985). However these features do not form a simple radial or concentric pattern which would be expected if they were related to an impact event and so they may simply be a random feature arising as a result of the freeze expansion of the interior (Jaumann et al 2009) It will take closer, more targeted flybys of Mimas in the future for a better understanding of these features as well as the interior of the moon to be gained. The Cassini surface spectra of Mimas confirm that the surface is dominated by water ice.

Figure 6 reveals an ISS image of Mimas, as well as a temperature map of the surface from CIRS. Instead of the smoothly varying temperatures peaking in the early afternoon near the equatorial region as one might expect, the warmest region was found to arise in the morning, along one edge of the disk of the moon, in a Pac-Man shape (Howett et al 2011b). The peak temperature is of order 92 K with the rest of the moon being much colder around 77 K. There is a small warm dot (84 K) close to the Herschel crater which can be explained as a result of heat being trapped by the tall crater walls. The hot sharp and V-shaped pattern of the thermal anomaly is more difficult to interpret however and is potentially revealing different textures on the surface. The boundary does not correspond to any clear change in surface albedo suggesting therefore that the thermal inertia variations are probably responsible for the anomaly (Spencer et al 2010). Global color maps of Mimas (Schenk et al 2011) reveal an unusual blue region that is similar in shape and size and may have the same cause, and it was noted that the shape is consistent with its formation being caused by irradiation of Mimas' surface by high-energy electrons. What the mechanism is that would enable such high thermal inertias to be produced by electron irradiation is as yet unknown.

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At Tethys, there has been one targeted and seven non-targeted flybys. The surface of this moon is dominated by 2 large features, a large valley known as Ithaca Chasma (Smith et al 1982) which spans three quarters of the globe of the moon and a giant impact basin known as Odysseus which is of order 400 km in diameter. The size of such an impact would have shattered a solid body and so it is likely that Tethys was still partially molten when the impact took place. The large valley feature is 100 km wide, 3 to 5 km deep, and extends 2000 km. There are a number of theories to explain its formation including that it may have been caused by expansion of internal liquid water as it froze into ice after the surface had already frozen or an alternate theory is that the impact that created the Odysseus Crater also generated forces that created Ithaca Chasma, especially since the chasm is on the opposite side of Tethys from the Odysseus Crater. The chasm and surrounding area are heavily cratered, indicating that it was formed a long time ago. (Jaumann et al 2009). More recent work from the Cassini era (Giese et al 2007) suggests that Itahca Chasma is older than Oydessus and hence could not have influenced its formation. This work concluded that Ithaca Chasma is an endogenic tectonic feature but the theory is unable to explain why it is confined to such a narrow circular zone.

Tethys has a high geometric albedo, a density close to that of liquid water with many of its crater floors being bright, all of which are suggestive of a composition largely of water ice. Its bright surface is probably also as a result of bombardment of E-ring water ice particles (Verbischer et al 2007).

Cassini observations revealed that Tethys (like Mimas) has a region of surprisingly high thermal inertia at low latitudes centered on the leading hemisphere (Howett et al (2012), see figure 7). This region is correlated spatially with a decrease in the IR/UV surface coloration and the discovery supports the hypothesis that high-energy electrons, which preferentially bombard the leading hemispheres on both Tethys and Mimas, produce clear alterations in texture of the surface. The region is reminiscent of 'Pac-Man' just as it is at Mimas. At Tethys, unlike Mimas, the Pac-Man pattern is also observable in visible-light images of the surface, as a dark lens-shaped region and this was first observed by Voyager observations in 1980 (Burns 1984) but it took higher resolution and the additional instrumentation on Cassini to better understand the implications.

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An unusual observation from images taken of Tethys in April 2015, see figure 8, is that of arc-shaped, reddish streaks which cut across the surface as observed below in the enhanced-color mosaic. These red streaks are narrow, curved lines on the moon's surface, only a few kilometers) wide but several hundred kilometers in length. The origin of these streaks and their color is still a mystery with some ideas including exposed ice with chemical impurities, or outgassing from inside Tethys (Schenk 2015). The yellowish tones on the left side of the view are a result of alteration of the moon's surface by high-energy particles from Saturn's magnetosphere.

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There have been 2 targeted flybys of Dione by the Cassini spacecraft and 4 untargeted ones.

Magnetic field observations from the close Dione flyby in October 2005 provided hints of a plasma loading type interaction occurring in the vicinity of the moon (Khurana et al 2008). The mass loading rates inferred from the observations are rather small (<7 g s⁻¹) but the existence cannot be explained from surface sputtering alone. An increase in ion-cyclotron waves close to the position of Dione also indicates that the satellite is acting as a source of a source of newly ionized plasma. This finding was underpinned by observations of molecular oxygen ions on a follow-on close Dione flyby in April 2010 (Tokar et al 2012) confirming the presence of a very tenuous atmosphere or exosphere.

From improved resolution and global mapping coverage capabilities of Cassini the amount of past endogenic activity on the Dione surface has been one of the surprises. From Voyager it was clear that the surface of Dione was covered by a near global network of tectonic troughs as well as a smooth plain criss-crossed by both ridges and troughs (Jaumann et al 2009). Cassini confirmed these observations as well as an extension of the smooth plain. In addition the troughs have been revealed to be morphologically fresh (Wagner et al 2006). Dark material on Dione's surface spectrally matches that seen elsewhere in the Saturn system and Clark et al (2008) reveal that bombardment by fine particles have impacted the trailing side of the surface, with an external origin for the source being postulated. E ring particles also coat the surface, brightening its albedo.

The origin of wispy streaks on the Dione surface observed by Voyager could not be resolved due to the resolution of the Voyager images (Morrison et al 1984). The streaks have lengths of tens to hundreds of kilometers and sometimes cut through plains and craters. Cassini observations, see figure 9, have revealed that the wispy streaks are in fact bright canyon ice walls (some of them several hundred meters high), probably caused by subsidence cracking. The walls are bright since the darker material falls off them, exposing the brighter water ice beneath (Wagner et al 2006). The existence of such fracture cliffs suggest that Dione experienced tectonic activity in its past. The density of Dione is almost 1.5 times that of liquid water, suggesting a dense core such as silicate rock and the rest being made up of ice (Morrison et al 1984).

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Dione is in an orbital resonance with two of the satellites orbiting nearby, that of Mimas and Enceladus. As a result the three moons speed up as they approach each other and slow down as they move apart, a process which assists in keeping them locked into their orbital positions, for example, Dione causes Enceladus to remain locked at a period which is exactly one half of the Dione orbit (Meyer and Wisdom 2008).

The Cassini spacecraft had five flybys past Rhea, two of them targeted and three of them untargeted. As Tethys and Dione are, Rhea is tidally locked in phase with Saturn and it also has a high geometric albedo which suggests a composition mainly of water ice (which at the temperature range of Rhea behaves like rock). Following on from the Voyager era, Rhea was thought to be the least endogenically evolved of the medium sized satellites (Jaumann et al 2009). Wispy albedo markings were observed in the Voyager images with lengths of tens to hundreds of kilometres sometimes cutting through plains and craters. The Cassini images observed during the 2006 flyby revealed that these wispy areas (as on Dione) are subsidence fractures which form canyons up to several hundred metres high, confirming that Rhea may have been tectonically active in its past (Schenk and Moore 2007, Wagner et al 2007). The wispy markings are associated with an extensive fault system (similar to Dione) although Rhea does seem less endogenically evolved than the other satellites. Rhea's surface spectra confirm it is mainly dominated by water ice although as with other satellites in the vicinity there is also coating of the surface by E ring particles.

During the 2010 Cassini flyby of Rhea, a very tenuous atmosphere (or exosphere) composed of oxygen and carbon dioxide was detected (Teolis et al 2010). The exosphere appears to be sustained by chemical decomposition of the surface water ice under irradiation from Saturn's magnetospheric plasma. This discovery of a tenuous atmosphere with oxygen and carbon dioxide makes Rhea unique in the Saturn system.

Our knowledge of Iapetus has greatly improved from the Voyager era following three flybys by the Cassini spacecraft (one targeted and two untargeted). Iapetus has a density only 1.2 times that of liquid water and it is suggested that it (like Rhea) is constituted of three quarters ice and one quarter rock (Castillo-Rogez et al 2007). Iapetus orbits at 59Rs from Saturn and is therefore less affected by Saturn's tidal forces than the other moons but despite the distance, Iapetus is still tidally locked by Saturn, always presenting the same face to its parent planet. Iapetus is also in resonance with Saturn's largest moon, Titan, which orbits at 20 Rs implying that the two moons speed up and slow down as they pass each other in a complex set of variations. Since Iapetus is less than one third in diameter compared to Titan, the influence on Iapetus is much stronger. Iapetus has been described as the yin and yang of the Saturn moons since its leading hemisphere has a reflectivity (or albedo) as dark as coal whereas its trailing hemisphere is much brighter. The other very unusual feature on Iapetus is the equatorial ridge (Porco et al 2005a) which extends more than half way around its circumference, see figure 10.

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The yin and yang nature of the Iapetus surface (see figure 11) has been known about since the time of Cassini when he first discovered it and was confirmed by Voyager observations (Morrison et al 1984) with the possibility been mooted that the satellite may be sweeping up particles from the much more distant (and also dark) moon Phoebe. Such a mechanism would continually renew the dark surface (compatible with the lack of many fresh bright craters within the dark terrain). Another theory is that there may be ice volcanism distributing material such as such as hydrocarbons which will then darken after chemical reactions arise from the effects of solar radiation.

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The September 2007 Cassini flyby of Iapetus confirmed that a third process, suggested soon after Cassini arrival at the Saturn system and the first Iapetus flyby in December 2004, thermal segregation, is probably the most responsible for Iapetus' dark hemisphere (Spencer and Denk 2010). The very slow rotation of Iapetus (79.3 d) is a critical element for this process, since this results in an extremely long daily temperature cycle enabling the dark material to absorb heat an warm up (which it is able to do much better than bright icy material is). Such heating will result in volatile or icy species within the original dark material to sublime out and migrate towards the colder regions on the moon's surface. has a very slow rotation, longer than 79 d. Such a slow rotation means that the daily temperature cycle is very long, so that the dark material can absorb heat from the Sun and warm up. (The dark material absorbs more heat than the bright icy material.) This heating will cause any volatile, or icy, species within the dark material to sublime out, and retreat to colder regions on Iapetus. As a result of this thermal segregation the dark material becomes darker and the bright, cold, neighbouring regions become brighter. For this process of have initiated an influx of a small amount of exogenic dust from some unknown source would have been required to trigger the process.

Our understanding of the two moons Hyperion and Phoebe, with irregular orbits around Saturn has improved during the Cassini era based on observations from five flybys of Hyperion (one targeted and four untargeted) and one targeted Phoebe flyby, the first satellites flyby of the mission which occurred just before Saturn Orbit Insertion.

Hyperion is the largest of Saturn's irregular shaped moons and is thought to be the remnant of a violent collision that shattered a larger moon into different pieces (Matthews 1992). Its large orbital distance from Saturn and orbital resonance with Titan has stopped it from becoming tidally locked with Saturn and it rotates roughly every 13 d with the orbital period around Saturn of 21 d. We have known about its strange shape, heavily cratered surface and chaotic spin from the Voyager era (Morrison et al 1984) with Cassini revealing a curious sponge-like appearance (see figure 12) and the presence of complex compounds (Cruikshank et al 2007). The density of the moon is just over half of that of water which could be a result of the water ice containing gaps or porosity as well as some lighter materials such a frozen methane or carbon dioxide also being constituents (Thomas et al 2007). This is compatible with the suggestion that Hyperion accreted from numerous smaller ice and rock bodies without having sufficient gravity to compact the constituents. The sponge like appearance arises since its craters are deep without any clear ejecta rays, although landslides have clearly occurred within some of the larger craters. It has been theorized that the high-porosity and low density of the Hyperion surface would crater as a result of compression from impactors instead of the usual excavation route. An abundance of water ice is implied from the brightness of the majority of the crater walls whereas the floors of the craters are mostly dark perhaps since the temperatures here could cause sublimation of volatiles allowing darker material to accumulate (Jaumann et al 2009).

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The surface of Hyperion has a low reflectivity, as does Phoebe and Iapetus (on its dark side). This has been theorized to be for a number of possible reasons including; frozen carbon dioxide mixing with frozen water and other material for example hydrocarbons, or perhaps hydrogen could be being removed by solar radiation from methane in the atmosphere of Titan causing carbon dust to sublimate onto Hyperion, and a third possibility concerns the dark material from Phoebe which has been proposed to color both Hyperion and Iapetus (Jaumann et al 2009).

Phoebe which is roughly spherical and has a diameter of about 220 km. It has an irregular, elliptical and inclined orbit which is retrograde, the only moon at Saturn which orbits in the opposite direction to the other satellites. Its surface is extremely dark, reflecting only a few percent of the sunlight it receives. The combination of its darkness and retrograde and irregular orbit suggest that Phoebe is probably a captured object from outside of the Saturn system, most likely the outer solar system where dark material is more usual. It could potentially be a captured Kuiper belt object which are believed to be primordial (dating from the formation of the solar system, (Jewitt et al 2007)). The surface of Phoebe is consistent with a substantial amount of fine grained dust, generated by particle infall (Clark et al 2008) and also shows signs of a violent collisional history, see figure 13).

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The Saturn system contains numerous tiny moons, including seven that orbit close to Saturn's main rings. Some of their key characteristics can be found in Thomas (2010).

Pan, the innermost of Saturn's known moons, is a small, ravioli-shaped moon approximately 34 km across. Pan orbits inside the 325 km-wide Encke gap in Saturn's A ring. Pan's gravity creates beautiful wavy edges (wakes) on either side of the Encke gap. The images (see figure 14, left) show exquisite detail along an equatorial ridge of material circling Pan (Charnoz et al 2007). Impact craters are visible on both the main body of Pan and on its dramatic, uneven ridge. A landslide from the ridge to the main body is clearly visible in the images.

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Daphnis is a tiny potato-shaped moon approximately 8 km across. Daphnis orbits inside the 42 km-wide Keeler gap in the outer portion of Saturn's A ring. The little moon's gravity raises waves in the edges of the gap in both the horizontal and vertical directions (Weiss et al 2009). A narrow ridge around its equator and a fairly smooth mantle of material on its surface is likely an accumulation of fine ring particles. A few craters are obvious at this resolution. An additional ridge further north runs parallel to the equatorial band.

Atlas is a small saucer-shaped moon about 42 km across and just 18 km pole-to-pole. It orbits just outside Saturn's outer A ring edge. Atlas consists of a central body with an extended ridge of small, icy grains from the rings (Charnoz et al 2007). The ridge has grown as far as it can grow. Saturn's gravity would pull off any additional ring particles that try to accrete on the edge of the ridge. The ridge is smooth and crater-free. Subtle ridges and grooves wind across its core and may hold clues to Atlas' history and evolution, see figure 14, right.

Prometheus is a pockmarked moon about 136  ×  79  ×  59 km in size. It orbits interior to the F ring and is a shepherd moon for the inner edge of that ring, contributing to the confinement of the F ring (Beurle et al 2010). Prometheus radically alters the appearance of the F ring, sometimes carving deep channels into the dusty ring material. It has several ridges and valleys as well as a number of impact craters.

Pandora is a potato-shaped, cratered moon about 104  ×  82  ×  63 km in size. It orbits just beyond Saturn's F ring (Beurle et al 2010). Pandora appears to be covered by fine particles captured from the F ring, and these deposits vary in thickness with location on the satellite. A network of thin, parallel grooves is conspicuous far from Pandora's broad equator, but they appear to vanish under a thick layer of smooth material that blankets the terrain.

Janus–Epimetheus: Janus is a co-orbital moon, sharing its orbit with Epimetheus. Janus has a mean diameter of 180 km while Epimetheus has a mean diameter of 120 km. Janus and Epimetheus orbit too close together to pass one another. Instead, approximately every four years they essentially swap orbits in an intricate cosmic dance as the inner moon overtakes the outer moon, and then switches positions with it, moving to a slower, higher orbit (Lissauer 1985). The two moons never approach closer than 10 000 km to one another. Cassini observed two such swaps, one in 2006 and another in 2010. Janus and Epimetheus are both heavily cratered, containing several craters larger than 30 km, and both may be quite old. Janus and Epimetheus both have a very low density and are highly porous. They may have formed from the disruption of a single parent body quite early in the history of the Saturn system and may be rubble piles of icy shards.

Eight small moons orbit farther from Saturn than the inner ringmoons (Thomas et al 2010, 2013). Three of these moons, Aegeaon, Methone and Anthe, form a family of moons that orbit between Mimas and Enceladus (Thomas et al 2013). The other four share orbits with Tethys (Telesto and Calypso) and Dione (Polydeuces and Helene) (Izidoro et al 2010). Cassini discovered five of these tiny moons (Porco et al 2005b).

Ageaeon is the smallest known moon in orbit around Saturn and was first seen by the Cassini cameras in 2008 (Hedman et al 2010). It has a very elongated shape, is less than a kilometer in size, and is relatively dark. It orbits within a bright arc of Saturn's G ring and is probably the source of particles for this ring. Aegeaon orbits between Janus/Epimetheus and Mimas.

Methone, Anthe and Pallene all orbit Saturn between Mimas and Enceladus and may once have been part of these much larger moons (Thomas et al 2013). They orbit close enough together to be part of a dynamical family. Resonances with the much larger moon Mimas strongly perturb the orbits of Methone and Anthe, with the greatest effect on Anthe. Pallene's orbit is perturbed by a resonance with Enceladus. Micrometeoroid bombardment kicks dust off these three moons (Hedman et al 2008). Partial rings, or ring arcs orbit with Methone and Anthe. Pallene's dust forms a complete diffuse ring, the Pallene ring, about 2500 km in radial extent around this moon's orbital path. Methone is a smooth, egg-shaped moon with an average diameter of only 3 km, no visible craters and a very low density. Cassini discovered it in 2004 (Porco et al 2005). Anthe is a tiny moon, only about 2 km in size, discovered by Cassini on 30 May 2007. Once its orbital elements were calculated it was visible in Cassini images all the way back to 2004. Pallene is about 4 km in size and was discovered by Cassini in 2004 (Porco et al 2005).

Telesto and Calypso both share an orbit with Tethys and are known as Tethys Trojans (Izidoro et al 2010). Telesto, the leading Trojan, orbits 60° in front of Tethys and Calypso, the trailing Trojan, orbits 60° behind Tethys (Thomas et al 2010). These stable Lagrange points are created by the gravitational interaction between Saturn and Tethys. Both moons display very smooth surfaces composed of loose material that may help soften and absorb craters (Porco et al 2007). Telesto is 25 km in size, irregularly shaped, with very few small craters as seen in Cassini images in 2005. Calypso is about 30 km in size and irregularly shaped as seen in Cassini images taken in 2010. Calypso is one of the brightest moons in the solar system, and perhaps because its surface is being blasted by E ring particles from Enceladus. Telesto and Calypso were first discovered in 1980 from ground-based observations (Reitsema 1981, Veillet 1981).

Polydeuces and Helene both co-orbit with Dione (Izidoro et al 2010). Helene orbits 60° in front of Dione and Polydeuces orbits 60° behind Dione at the stable Lagrange points described for Telesto and Calypso (Thomas et al 2010). However, Polydeuces wanders farther from its Lagrange point than any of the other Trojan moons, moving about 30° away from this point in either direction over about two years. Polydeuces is about 3 km in size and was discovered from Cassini images in 2004 (Porco et al 2007). Cassini was too far away to clearly resolve Polydeuces. Helene is about 35 km in size and was discovered in 1980 in ground-based observations (Lecacheux et al 1980). It has a smooth, sculpted surface (Porco et al 2007).