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 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 mission - is surrounded by a large cloud of neutral hydrogen atoms extending out to 80000+ km. This is a localized concentration within the larger cloud within the magnetosphere displayed in an image earlier on this page. 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.
During the approach Cassini's magnetometer encountered several magnetic spikes that are equivalent to magnetopause bow shocks.
One of the discoveries, made by the UV spectrometer, is that there is a small but detectable concentration of oxygen over much of Saturn's environment beyond the rings. One tentative hypothesis: Ionic bombardment of the E ring caused a sequence of reactions in the ice particles that led to free molecules of oxygen that dispersed in the pattern shown here that covered much of December 2003. Readings taken in 2004 showed an increase in oxygen for several months, but the amounts may now be abating.
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.
Computer processing provided an improved overall rendition; resolution was in the 1 to 2 km range
JPL and the USGS have now assigned names to the major features and some of the smaller features. These two maps have been released (unformately, 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. To date no firm evidence for the nature of these features, and whether they are solid or liquid surfaces, has been 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
The image below is the best resolution achieved during the flyby.
Only generalizations about the nature of the light-dark patterns can be made but in October of 2004 Cassini will pass as close as 1200 km and spectacular new views, revealing much about the surface features, can be expected.
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. From a far distance as it approached or during its first major swing around Saturn, Cassini's Magnetosphere Imager has measured the distribution of trapped radiation in the Main Magnetic Belt around the planet, as shown below This belt begins (from the surface) out at 70000 km (48000 miles) and reaches to about 783000 km (489 miles). Cassini also detected a previously unknown inner belt near the surface that is about 6000 km (4000 miles) thick. This same 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: The first of 42 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. Those that provided images are considered below. From a distance, Cassini obtained several images of the full half disc of Titan, from different times during its location. Three of these appear here: 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. Visible-Near IR images show surfaces composed of light and dark areas, some elongated like broad streaks. So far no compositional differences have been identified from sensor data still being evaluated. Keep in mind that these images comprise only a few percent of the titanian surface; future passes should eventually image most of Titan's surface. This next set of released images consists of Titan features that the Cassini scientists have ventured 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.
The area where Huygens was planned to set down in January 2005 is shown here 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 "albedo" patterns consist of must await later data and more interpretation. (So far, only 1% of Titan's surface has thus been imaged.) The dark areas may be low terrain occupied by "lakes" of methane liquid. The best evidence yet 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 methane. It measures 234 x 72 km (145 x 45 miles) wide. Other, more patchy dark areas may be smaller "pools" of methane. The region is located in that part of Titan in which clouds (whitish) are most common. Very few patterns that could correspond to impact craters were observed. This suggests a continuous resurfacing of the solid part of Titan's outer shell, thus strongly implying that Titan is "geologically" active. Precipitation of methane and organic molecules - postulated pre-Cassini for Titan - was not verified in this first pass but the light-dark IR images may be revealing differential coating of the surface. Another radar image contains a light-toned pattern that may be evidence of a lobe of flowing material. At the very cold temperatures (averaging -178°C [-289°F]) on its surface, this would have to be some exotic form of ice, perhaps made of methane. Compositional data so far have not been specific enough to characterize the nature of such features. A radar image taken on October 28, 2005 shows a series of parallel ridges of considerable width (the long dimension of the image below is 340 km [212 miles]). These have not been conclusively identified - ideas range from folds in the surface material owing to lateral movements to super ice particle dunes. 
This next image shows a rough, mottley surface and several drainage channels (liquid methane?)
This 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:
No strong evidence of widespread higher terrain (yielding notable relief) has yet 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:
As Cassini continues its multiple orbits about Saturn, many calculated to produce a close-pass as indicated in the above table, image processing at JPL produces new, improved quality images. Here are two released at Thanksgiving time in 2004 that improve the views to Titan and Tethys, two satellite quite dissimilar in appearance:
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:
Here is another showing a different part of Titan; the dark patch on the left ties into the same dark patch in the upper area in the above image:
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 band 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.
Near the end of 2004 (Christmas Eve, December 24), the Cassini Orbiter
released the Huygens probe to journey in its own orbit over 22 days and finally on January 14,2005 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 mi) 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 (shown as being tested before integration).
Much of the surface below the atmosphere is 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 carried a descent imager, spectral radiometer,
a Doppler wind measuring device, an aerosol collector, a Gas Chromatograph,
a Mass Spectrometer, and a surface science package. These 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 occurred when the Huygens probe successfully separated from Cassini on Christmas Eve, 2004. Then, on January 14, 2005 it 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 will lose 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 bit problems with that instrument prevented 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 worked extremely well. Here is a view of part of Titan made as Huygens descended showing the approximate point where the probe safely landed.
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 or small sea size.
Drainage channels (methane fluid from ice volcanoes?) are noted in various parts of Titan. Here is a striking example:
The next two images are enlargements from this scene, rotated 90° clockwise, in black and white and in color:
The image below shows part of a 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.
Some of the Cassini radar images above 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:
The image below shows a dark plain, with the whitish spots, referred to as "islands", presumed to be material covering topographic features that rise above the general surface.
This image may include 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 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 be 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.
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:
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. This is the general view as it approached:
A closer look at part of that surface once again imaged areas of stark albedo contrast, whose compositional natures are yet to be identified:
During the April 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. First, look at its general setting, then an enlargement:
Unlike the Earth, in which almost all volcanoes are made from silicate magman, 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 latter, as the methane 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 (a methane ocean has now been ruled out although some dark areas may be liquid methane).
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:
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.
Cassini passed by the moon Enceledus on February 16, 2005. 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 three images:
The last three images in a panel show other views of the fractures and "fading" craters on Enceladus:
This image of Enceladus has an enlarged section within it which shows ice boulders:
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. It presence, not observed directly, is inferred from perturbations of the weak magnetic field around Enceladus. 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 the most recent Enceladus pass. This moon is geologically active. Ice particles are being expelled from at least one part of the moon's surface out to distances of 300 km (200 miles). This is strikingly shown in this colorized image, with the several colors representing different densities within the ice plume:
Still another pass by Enceladus on July 14, 2005 produced the closest views yet. Read the captions for more information:
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 with limited detail on page 19-18. Cassini has now obtained a better view, from about 130000 km away. 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. 
Hyperion was passed within a few hundred kilometers on September 26, 2005 with this high resolution result, confirming the presence of numerous craters:
Two days earlier, on the 24th, 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.
One of the smaller inner moons of Saturn is Janus, was imaged for the first time during Cassini's orbital transits around the parent planet:
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:
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:
Saturn's second largest moon, Rhea, was image full face and in detailed small area scenes on November 25, 2005.
As a bonus, the F-Ring shepherd moon Pandora was imaged in November. It is just 84 km (52 miles) in long dimension.
Enough imagery of many of Saturn's moons has now been obtained to produce global image mosaics of each. Three of the most interesting are:
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 this marvelous exploration of Saturn.
Primary Author: Nicholas M.
Short, Sr. email: nmshort@ptd.net