This page picks up the story of Cassini's exploration of Saturn from the time of the spacecraft's passage through the rings up to the present, including radar images of Titan and the successful atmospheric passage and landing of the Huygens probe onto Titan's surface. New images of several of Titan's moons (satellites) are displayed.
The schedule for flyby observations and data collection is long (68 individual passes through mid-2008). We show here two parts of a table of Encounters, in which Orbits 22 - 44 - all to Titan - have been omitted.
Titan - whose exploration is a main goal of the Cassini mission - is surrounded by a large cloud of neutral hydrogen atoms extending out to 80000+ km (50000 miles). This is a localized concentration within the larger cloud influenced by the magnetosphere displayed in an image earlier on page 19-19. X marks the axis of rotation of Titan; Z points to the Sun, and Y looks toward the dawn side of Titan. Titan is the dot at the juncture of axes. The red halo is the H cloud.
The Magnetosphere Imager instrument was turned on Titan. It discovered a glowing sheath of excited methane and carbon monoxide molecules that would appear to a visitor to Titan's outer atmosphere like a green-fluorescing glow:
Measurements of isotopic ratios for Nitrogen isotopes show values for Titan that have been provisionally interpreted as indicating significant loss of lighter isotopes; this translates into a conclusion that much of Titan's original atmosphere has been lost to space over time.
As Cassini was moving away from Saturn in its first orbital swing, it looked back from 789000 km (441000 miles) at the limb of Titan and using its UV sensor was able to image two haze layers - one hugging the surface and the other about 400 km (250 miles) above. Both appear bluish-gray in the image below. The tentative interpretation is that these are organics-holding gas clouds in which methane and nitrogen are being decomposed.
On the second day of its orbital operation, Cassini passed within 340000 km of Titan, obtaining first a general map of Titan's surface made from 16 narrow angle camera images. The wavelength used was at 932 nanometers, at which methane in the clouds acts as transparent. The surface as seen here may be largely solid, and shows several criss-crossing straight lines that could be tectonic faults.
Major features on Titan have now been given names, as shown in this map:
The first of 21 planned close flybys near Titan occurred in the evening of October 26, 2004. The spacecraft passed to within 1200 km (750 miles) of its surface. All instruments were turned on and successfully gathered data. Flybys that provided images are considered below. From a distance, Cassini obtained images of the full half disc of Titan, from different positions along its orbital path as Titan rotated:
JPL and the USGS have now assigned names to the major features and some of the smaller features. These two maps have been released (unfortunately, the persons responsible have made the print size for many features too small to be read). The USGS has also prepared a detailed table of surface feature nomenclature that is geologic rather than geographic.
This is a closer look at part of Titan's surface. At this distance resolution still allows only light and dark areas to be singled out. At first no firm evidence for the nature of these features, and whether they are solid or liquid surfaces, was gleaned from the observations. In this image is one area containing individual very bright spots. A closer look appears in this scene.
This next sequence of four panels represents observations at different times during the Cassini flyby. Close inspection shows the light spots to change size and location. The interpretation given is that these are methane clouds being convected upwards from the surface or lower atmosphere of Titan
Some hypotheses about the nature of the light-dark patterns have been made but in October of 2004 Cassini passed as close as 1200 km (730 miles) and obtained spectacular new views, revealing much about the surface features.
Near IR images show surfaces composed of light and dark areas, some elongated like broad streaks. This next set of released images consists of Titan features that prompted the Cassini scientists to venture educated "guesses" as their nature:
Feature a: Sharp bright/dark boundary; hints of impact craters.
b: Bright features of possible wind action.
c: Dark channels 1-2 km wide and tens of kilometers long.
d: Linear feature, possibly faults, in bright area west of Xanadu.
e: Bright areas possibly related to drainage activity.
f: Complicated boundary near Xanadu, perhaps indicative of individual terrains, possibly of platelike character.
The white bars above each image are 200 kilometers (124 miles) long. Imaging Titan through its thick atmosphere is a challenge, and the narrow, straight lines within the images are seams between individual images that have not been completely removed. North is to the top of each frame.
A synthetic aperture radar (SAR) instrument has produced even higher resolution images of a small part of the titanian surface. Dark areas are smooth reflectors; light are rougher, as is the case for terrestrial images (see Section 8) Definitive identification of what these radar low "albedo" patterns consist of is being subjected to further data acquisition and more interpretation - final proof may demand actual onsite sampling. The dark areas may be low terrain occupied by "lakes" of methane liquid. Strong evidence for liquid methane collected in a unit, as a lake, is seen in this image:
The "lake", off upper center left, is very dark, as expected for the signature of a liquid such as methane. It measures 234 x 72 km (145 x 45 miles) wide. Other, more patchy dark areas may be smaller "pools" of methane and ethane. The region is located in that part of Titan in which clouds (whitish) are most common. Two radar observations on July 21, 2006 produced images that support the conclusion that many of the dark patches are methane/ethane lakes which form in lower areas from "rain" out of the atmosphere. Interpretation of data indicates this rain is widespread, fairly steady over the year, and composed of liquid methane and nitrogen, turning surface material into a thin layer of mud. The upper strip covers an area 420 x 150 km (260 x 93 miles); the lower strip is 475 x 160 km (295 x 93 miles); resolution is 500 m (1650 ft). The patches act as smooth, but poorly reflecting surfaces.
This next image is more convincing as a lake since it also shows thin shorelines:
This image shows that these low reflectance flat areas - most likely interpretation: lakes - are widespread (the coloring is artificial; added during processing):
Two of the largest lakes on Titan are now being called "seas". One, on the right below, is larger than either Lake Superior in the U.S. or the Black Sea east of Constantinople:
This image shows another smooth "sea" and adjacent land, with low mountains:
There is evidence that these lakes are actively changing - as Saturn's orbit changes relative to the Sun, its weather system is affected, so that the lake boundaries expand or contract owing to condensation or evaporation of the liquid, presumably related to variation in methane rain.
The lake hypothesis has come into question, at least for some dark patches. An April 30, 2006 pass over a dark area, heretofore considered a "lake" candidate, revealed it to be a solid surface with hundreds of parallel rises - perhaps some type of dunes:
However, strong support for the hydrocarbon lake hypothesis comes from observations a year apart of a region near the southern pole. In this set of images (top without notation), new or darker patches have appeared after a stormlike period on Titan, suggesting precipitation of the hydrocarbons took place, so that the resulting lakes may be ephemeral.
Linear features are evident in the next two images. In the upper one, in addition to the thin parallel markings (again, possibly dunes), there is a straight black line which appears to be a fault.
The composition of the sand particles making up the dunes is still speculative. It may be droplets of frozen methane or other materials. The presence of the dunes indicates circulating surface winds; some are calculated to move as slowly as 1 km/hr but may be sufficiently active to produce these dunes.
This next image highlights a rough, mottley surface and several drainage channels (liquid methane?)
This next radar image shows light areas that seem to rise above the surrounding dark, lower surfaces. One interpretation holds these areas to be hills caused by some tectonic activity; a possible fault line runs through the image center:
No strong evidence of widespread higher terrain (yielding notable relief) has yet been confirmed. However, radar pulses have been used to obtain a rather coarse indication of general elevation variations along this first pass flight line, showing relief on the order of 250 meters:
The second close flyby of Titan took place on December 13, 2004. Much the same area was looked at, but better views of larger parts of Titan were acquired, such as this view of the upper half of the satellite:
Clouds in the atmosphere had been noted during the first pass. The Titan scientists speculated that other clouds would be found in the second pass. They were vetted by these results:
A UV detector in Cassini's sensor system was able to distinguish up to 15 separable bands in its atmosphere (which was also shown to extend further out than previously known). This image shows most of these layers. Below it is another diagram of the Titan atmosphere, this time compared with that of Earth.
This diagram gives more details for Titan's atmosphere:
Both methane (CH4) and ethane (C26) are present in the atmosphere - with ethane (derived from methane by photochemical processes) being probably more abundant. Prior to Cassini's arrival, speculation placed ethane as widespread over most of Titan's sphere. Actual measurements proved different:
The Visual and Infrared Mapping Spectrometer (VIMS) on NASA's Cassini spacecraft recorded these infrared images of Titan's northern hemisphere. Image (A) was taken on Dec. 13, 2004; image (B) on Aug. 22, 2005; image (C) on Aug. 21, 2005; and image (D) on Sept. 7, 2005. The images show the reflection of sunlight on Titan's atmosphere at 2.8 microns, which is a longer wavelength than human eyes can detect. The image appears in false color so that the highest reflection is displayed as a reddish hue. The vast cloud can be seen in all images as a reddish band just north of 50 degrees latitude. The top of the image in panel D also shows a strong reflection off the limb of the planet (also reddish) which is caused by the lighting angle and does not indicate the presence of clouds. (Credit: NASA/JPL/University of Arizona).
Information helpful in the location and mapping of hydrocarbon concentrations is also provided by the Infrared spectrometer on Cassini. Here is an image of Titan's surface made into a false color rendition, with blue = 1.6 µm, green = 2.0 µm, and red = 5.0 µm. The reddish areas in the lower part are clouds of hydrocarbons. 
The predicted widespread oceans of methane (with dissolved ethane) have yet to be observed on Titan. While liquid methane/ethane seems present in the polar regions, it appears to be limited to lakes in other latitudes (Titan's rotation pole is strongly tilted). Nevertheless, the amounts of methane and ethane both on and above the titanian surface are estimated to be huge, comprising more quantities of these hydrocarbons than is present in all the petroleum reserves on Earth.
Now to the spectacular short-term activity of the Huygens probe: Near the end of 2004 (Christmas Eve, December 24), the Cassini Orbiter
released the probe to journey in its own orbit over 22 days and finally on January 14, 2005 to descend through the atmosphere of Titan. Named for a Dutch astronomer, who discovered this large satellite, the probe
used a wide parachute deployed at 175 km (109 miles) to slow its fall. Then, after jettisoning that one, it deploy a smaller parachute for the final
drop onto the surface; a schematic of the sequence, which took up to 2.5 hours, is shown below, along with an artist's conception of Huygens as deployed after separation and the final curved probe components that will pass through the atmosphere
The development of the Huygens probe was the responsibility of the European Space Agency (ESA). A helpful overview of the Huygens probe program is found in this Wikpedia summary. This site describes the six onboard instruments; these are also listed here: 1. The Huygens Atmospheric Structure Instrument (HASI): This is "a suite of sensors that will measure the physical and electrical properties of Titan's atmosphere," says NASA. Scientists are interested in determining the density of Titan's atmosphere, the thermal properties of the atmosphere, and if the probe is able to land on Titan's surface and transmit back information, whether its surface is liquid and, if so, whether there is wave movement.
2. Doppler Wind Experiment (DWE): The DWE, by using an oscillator, will detect the Doppler shift as the probe parachutes down. The shift might possibly affect communications from the probe back up to the mother ship. DWE will allow the communications suite to make adjustments and thereby provide a very stable communication frequency. In lay terms, the DWE will let the probe send the data it records to Cassini. 3. Descent Imager/Spectral Radiometer (DISR): This instrument will detect radiation in the atmosphere as well as determine light properties from the Sun as these properties are diffused by Titan's cloudy atmosphere. Scientists can calculate mass from such data.
4. Gas Chromatograph Mass Spectrometer (GCMS): This is a gas chemical analyzer designed to identify and measure chemicals in Titan's atmosphere. One of the critical measurements it can perform is the composition (solid or liquid) of Titan's surface in the event of a safe landing.
5. Aerosol Collector and Pyrolyser (ACP): ACP will be able to chemically analyze the aerosols in Titan's atmosphere. It will sample the aerosols during descent and "prepare the collected matter (by evaporation, pyrolysis and gas products transfer) for analysis" by the Huygens Gas Chromatograph Mass Spectrometer (GCMS), which it conveniently has transported for more than a billion miles.
6. Surface-Science Package (SSP): The SSP contains a number of sensors designed to determine the physical properties of Titan's surface at the point of impact, especially and most importanly whether the surface is solid or liquid. This is no small feat and needs to be done immediately should the environment the probe finds itself in upon landing be so corrosive as to degrade Huygens ability to continue conducting experiments and/or transmit data up to Cassini. SSP employs an acoustic sounder, which as the probe descends will be "listening" for solid or liquid surfaces. If the surface is liquid, then other sensors mentioned above will kick in and measure Titan's density, temperature and light-reflecting properties, thermal conductivity, heat capacity, and electrical permittivity.
The area where Huygens was planned to set down in January 2005 is shown in the box (enlarged on the left) in this image: Much of the surface below the atmosphere was postulated to be liquid making up some kind of ocean (nitrogen plus organics), but with an underlying solid bottom which may rise in places above the ocean level. A high area was targeted and the probe
briefly survived the landing (landing in the ocean would prevent signal transmission). Huygens' instruments are designed to determine atmospheric composition and something of the nature of the landing spot, and to search for organic molecules, identifying chemical types where possible. The highpoint of the Cassini mission was reached on January 14, 2005 when Huygens commenced its descent into Titan's atmosphere and successfully landed on a supportable surface, where it was able to take pictures of its surroundings, gather other data, and transmit these to the Cassini mothership for 70 minutes (after which Cassini was out of range; Huygens batteries lost their charge before the next pass of Cassini). Some 350 images were received during the active phases of Huygens mission, which exceeded the expectations of ESA and NASA. During Huygens' descent, the wind measuring instrument recorded stronger winds than expected causing problems with that instrument that prevented acquisition of some quantitative data. The atmosphere proved richer in nitrogen than expected. An atmospheric band higher in methane (CH4) and extending downward closer to the surface (15-20 km) was found. There is evidence that this methane can condense into droplets which could fall to the surface as "rain". Subsequent to the Huygens' pass through the atmosphere, some reliable data on wind speeds were obtained using the VLBA radar telescope system. At a height of 120 km (75 miles), winds were determined to be up to 435 km/hr (270 mph). The imaging devices on Huygens are working extremely well. Here is a view of part of Titan made as Huygens descended showing the approximate point where the probe safely landed.
This Cassini image includes the Huygens landing site. Lighter areas are features above the lower levels of the terrain.
The image below is a mosaic of individual images obtained as the Huygens probe passed through the atmosphere at an altitude of about 10 km (6 miles)
The main imaging instrument is DISR (Descent Imager/Spectral Radiometer). The most striking image made during descent contained evidence of lighter-toned landlike terrain, which contains numerous gullies typical of fluid erosion; a darker flat area may be some kind of liquid surface (if so, most likely, condensed methane (CH4) that could be of lake size).
The probe landed safely and broadcast for at least 70 minutes (one report has said 3 hours). The surface was firm enough to support the probe and keep it upright. This material is said to have the consistency of a mud or wet sand seemingly with a harder, thin crust. A 360° panoramic view of the surroundings appears in this mosaic:
Looking out from the lander, one sees a surface that resembles some of the pictures obtained at martian landing sites. Here are small rocks with rounded corners that are believed to be water ice but may also include blocks of methane ice
The color of these rocks is probably due to organic molecules - including ones that give both the Titan surface and its atmosphere its distinctive orange tones.
Returning to the mothership Cassini exploration of Titan, drainage channels (methane fluid from ice volcanoes?) are noted in various parts of Titan. Here is a striking example:
The next image is an enlargement from this scene, rotated 90° clockwise, in color:
The image below shows part of the surface in more detail. One interpretation considers the thick lighter-toned "channel" to be filled with water ice. The dark, stubby channels may contain liquid methane.
The origin of these channels is still being debated. But there is now definite evidence that the titanian atmosphere is quite active, both in winds and cloud formation. Furthermore, there is some indication that methane in the atmosphere continually condenses and falls as methane rain drops; this is counterbalanced by some methane evaporation returning that substance to the atmospheric envelope.
Now, to later Cassini observations of Titan: Some Cassini radar images of Titan show large areas of dark reflectance (i.e., smoother surfaces which scatter little of the incoming beam back to the radar instrument). These areas may be a frozen, or possibly liquid, surface underlying methane "seas" or "lakes". This image was acquired on Sept. 7, 2005: it shows sculptured terrain on the left and a smooth surface on the right; the boundary is being interpreted as a shoreline:
This interpretation must be tempered with the discovery that at least one other dark area contains dunes and thus is not liquid (see above).
The image below shows a medium-dark plain, with the whitish spots, referred to as "islands", presumed to be material covering topographic features that rise above the general surface.
A pass in April of 2006 produced some quality radar images that includes a closer look at Xanadu. This is a region, the size of Australia, that appears bright in telescope views. It is a continent-like rise in elevation that includes, as seen in the image below, streams (probably carved by liquid methane that rains down on Xanadu) and darker hills with relief of 300 meters (about 1000 ft) or more.
Combining the Cassini and Huygens results so far, a favored model (there are alternatives) of the general composition and structure of Titan has been proposed. Titan likely has a rocky core (silicates?). Outward is a mantle likely to consist of water ice. This ice may extend to the solid surface. But methane concentrations can lead to features composed of that substance - either areas of frozen methane and/or possibly large low areas, lakelike, of liquid methane.
The Cassini mothership passed near Titan again on February 15, 2005. Its radar system explored a new area beneath the cloud cover. A huge crater (the size of the state of Iowa) was imaged as was a much smaller crater. Their persistence gives further testimony to the Huygens data that the surface has some strength, firm enough to support and preserve rim topography:
So far, very few patterns that could correspond to impact craters have been observed. One radar example that could be impact, or less likely a volcanic caldera, is shown below:
The low frequency of craters suggests a continuous resurfacing of the solid part of Titan's outer shell, thus strongly implying that Titan is "geologically" active.
The titanian surface also displays numerous thin, dark linear features that are analogous to a series of faults that are filled with subsurface material.
Cassini got its third close look at Titan in early April, 2005. During the pass, Cassini detected in the northern hemisphere what might be called a "hot spot", shown below in the color mode as a bright yellow. "Hot" is quite relative in that by Earth standards it is still well below freezing under terrestrial conditions. It is however notably warmer than its surroundings. The cause is still speculative - something internal bringing about melting of surficial ice; heat residual from a recent impact; process unknown? If it persists in future flybys then it is not a transient. Measurements will be taken to get more data pertinent to identifying its nature.
In another area, an edifice that rises above the surface has been tentatively interpreted as a volcano. Look at its general setting, and the enlargements:
the first Cassini passby Titan.">The structure in the upper right enlargement (about 30 km in long dimension) appears differently at various IR wavelengths;
Here is a generalized "geologic" map of this feature:
One interpretation holds the feature to be some type of eruptive structure. Unlike the Earth, in which almost all volcanoes are made from silicate magma, and Io, in which a silicate magma is enriched in sulphur and its compounds, the volcanoes on Titan are made of ice (most probably frozen water but methane ice has not been ruled out) making it a cryovolcano (known on Earth). If the effluent is water, then that would be possible if the water contained considerable dissolved ammonia; together these constituent lower the freezing point significantly. If the effluent is methane, as it spills on the surface, it may be driven in some kind of eruption by part of the methane converting to a gaseous form. This may be released as plumes that deposit as a surface mound that builds up over time. Thus, the term "volcano" is expanded to include relatively very cold material that is mobilized to extrude in a manner similar to conventional silicate lava volcanoes. This, and similar "methane ice volcanoes" on Titan may be the main source of methane in Titan's atmosphere.
As more passes have continued past Titan, some measurements seem to indicate that there have been small shifts (~35 meters) of specific surface features. If this proves to be substantiated, that would indicate surface mobility enhanced by some mode of subsurface fluidity (or plasticity). One suggested candidate would be a buried water layer (in effect, a subcrustal "ocean").
Now, to some of the close flybys of other saturnian Moons (see above table):
Iapetus was flown past in early January, 2005, producing these images of its cratered surface:
The first of these views includes a feature totally unexpected and different from anything yet seen on planets and moons. Note the straight thin topographic rise that extends across the face of Iapetus. It is a ridge some 1300 km (812 miles) long, and 20 km (12 miles) high and wide. This view shows the mountain-like aspect of the feature:
The nature of this feature is still puzzling. Two favored explanations are 1) compressional squeezing of the ice that covers Iapetus; and 2) upwelling of ice along a long tensional fracture. A clue may reside in the fact that the wall coincides almost exactly with the equator of this moon.
Analysis of spectral and visual data for Iapetus has now shown that its composition is largely ice, with much less rocky material than most jovian or saturnian satellites. Iapetus has a very dark surface over much of its sphere, in contrast to the whitish ice cover. The cause is believed to be a dust coating by material whose albedo is similar to that of asteroids, and thus represents ancient or primordial rock matter perhaps containing organics.
Another scheduled flyby of Iapetus took place on September 10, 2007, when the spacecraft passed as close as 1000 km. Here is a hemisphere view of the light side of Iapetus. Note the large crater:
These detailed close-ups of the Iapetus surface shed some light on continuing mysteries. Read the captions for these next three images to gain this new information:
The full nature of the dark material remains somewhat obscure. It resembles asteroidal dust but also looks light dark organics (much less likely) or possibly basalt (if so, that implies a thin ice cover over a rocky interior.
Cassini passed by the moon Enceledus on February 16, 2005. This earlier Cassini view indicates how small this satellite is compared with its mother planet:
Here is a "raw" view of its surface, rendered in color (note blue), chosen to be unstretched so as to display its very high brightness (largest albedo of any of the planets or their satellites).
The surface of Enceledus as observed during the Voyager flyby showed enlongated trenches and folds that appear typical of the styles observed in other icy satellites. The next image shows part of the view gained by Cassini of Enceledus' upper hemisphere.
This pair of images shows some of the ice folds and fracture patterns:
Another pass on March 9, 2005 produced these two Enceladus images:
(The black lines on the right side of each are artifacts caused by acquisition and processing anomalies.)
The three images in a panel below show other views of the fractures and "fading" craters on Enceladus:
A close pass (less than 2500 km) on August 12, 2008 showed details of some of these fractures:
This image of Enceladus shows ice boulders, some 100 meters or so in size::
In mid-March, 2005 information was released that noted some evidence for a thin atmosphere above Enceladus. Its composition appears to be ionized water vapor. Later observations showed that the highest concentrations of water, as associated with observed temperatures, were above Enceladus' south polar regions. The temperatures are probably related to warmer surfaces where "ice volcanoes" are located. An analysis of the data indicates the atmospheric "clouds" to consist of 65% water vapor, 20% molecular hydrogen, and most of the remainder CO2 with some nitrogen.
A surprise was revealed during several Enceladus passes. This moon is geologically active. Ice particles mixed with liquid water are being expelled from at least one part of this moon's surface out to distances of 300 km (200 miles). The result has been called a (mega)ice fountain. This is strikingly shown in the next four images, the first in natural color and the other three colorized to bring out subtle variations, with the several colors representing different densities within the ice plume:
The August 2008 flyby pinpointed individual fractures on Enceladus that were outlets for the ice fountains. In this view, the bluish tones indicate coarse ice next to their source cracks.
The exact mechanism by which the water and ice geysers are produced is still speculative. This diagram gives one favored hypothesis:
The geysers result from selective melting of part of the ice crust. Water is heated enough to build up pressures that force the water-ice particles up through fractures outward into jets that reach as high as 300 km (200 miles).
On page 19-18, a Voyager image of the moon Phoebe was shown; it had little detail. Compare that to these three close-up views of Phoebe acquired during the close flyby (2300 km) on June 12-13, 2004:
The Cassini team has proposed names for the larger craters, some of which are shown here:
Phoebe, according to initial interpretations, seems to be a rocky asteroid-like body with some ice at or just beneath its surface. One opinion holds it to be a captured asteroid from a body which strayed towards the Sun from the Kuiper Belt.
Another moon, Hyperion, was shown first on page 19-18. Cassini has now obtained better views. Hyperion was passed within a few hundred kilometers on September 26, 2005 with this high resolution result, confirming the presence of numerous craters. The image below indicates it is pockmarked with craters, has a low density of an estimated 0.6 grams/cc (consistent with interior voids), and is mostly ice. 
This next image again shows Hyperion on the right but also has a close-up of the spongelike surface, seen on the left:
During this transit, spectrometers on Cassini obtained compositional information, as plotted on this map superimposed on the cratered surface:
Red denotes dominantly carbon dioxide ice; blue is mostly water ice; magenta indicates a mix of CO2 and water; yellow is CO2 and organics. The organics appear to be simple hydrocarbons. These consituents are mixed with rock material. The nature of Hyperion's chemistry suggests it could be a captured asteroid.
On September 24th, 2006 Cassini came within 34000 km of Tethys and obtained informative higher resolution images. The extent of surface cratering was forecast but nevertheless the actual surface expression proved interesting. The top image shows a large crater with a central peak:
This is a typcal cratered surface on Tethys.
This close-up view, in near-true color, suggests ice plateaus and other markings implying an icy state.
The only planned pass by Mimas, with its huge crater Herschel ("Death's eye") (see page 19-18), was on August 2, 2005. Here is one resulting image, taken at 228000 km (142500 miles) when Cassini was still approaching Mimas, which better defines the extent of cratering:
As Cassini made its closest approach, it took this sharp image of craters in Mimas' southern hemisphere:
Dione (1126 km; 700 miles in diameter) has a bright band across its face that is bounded by cliffs. This feature appears to be folds in the ice that, in effect, constitute a compressional mountain range:
During the 16th orbit of Cassini, Dione was passed nearby in mid-October, 2005, for its only close encounter. This image shows it to be heavily cratered, as had been earlier indicated by the Voyager imagery.
Since then, JPL has processed a large number of individual close-up views of Dione's surface. Here are two representative examples:
Dione appears to have its own tiny moon, Helene, some 32 km (20 miles) in long dimension, shown here:
Saturn's second largest moon, Rhea, was imaged full face and in detailed small area scenes on November 25, 2005.
A 2006 pass provided this more detailed view of impact craters in the icy surface material of Rhea:
Even more detailed looks close-up show these Rhea surface features:
As a bonus, the F-Ring shepherd moon Pandora was imaged in November. It is just 84 km (52 miles) in long dimension.
A recent Cassini image encompassed two small moons - Janus and Epimetheus - seemingly in close proximity (Janus is actually 15000 km further away as the two moons seem superimposed owing to occupation of similar locations in their orbits):
This is an enlarged view of Janus:
Perhaps the strangest of the saturnian moons is Atlas. As it appears in the image below, one might think this is a giant UFO or spacecraft, as often depicted by artists conceptualizing "close encounters". The flange appears to be a ridge, something like that seen on Enceladus (above) but proportionately larger.
Enough imagery of many of Saturn's moons has now been obtained to produce global image mosaics of each. Three of the most interesting - Mimas, Phoebe, and Tethys - appear here:
In view of the success in emplacement of Cassini, it is anticipated that many more dramatic and informative images and other types of data will be forthcoming over the next 4 years. Stay tuned to the Internet site for the Tutorial, if you have a CD-ROM version, for periodic updates on the saga portraying this marvelous exploration of Saturn and its moons.
Primary Author: Nicholas M.
Short, Sr.