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Planetary News: Cassini-Huygens (2005)Cassini's RADAR Images of Titan: Saturn's Moon is Beginning to Make Some SenseBy Emily LakdawallaFebruary 18, 2005 RADAR images from Cassini's fourth flyby of Saturn's giant moon Titan show some features that look familiar to the mission's planetary geologists: a beautifully preserved small impact crater, and a "multi-ringed impact structure, with little hills and dissected rings of ridges and little mountainous terrain within those rings," according to Larry Soderblom of the United States Geological Survey. Since arriving at Saturn Cassini's viewpoint on Titan has more often proved puzzling than illuminating, but geologists are reasonably confident that they know what they are looking at in at least some of these new images. These images were created using a technique called Synthetic Aperture Radar, or SAR. To create them, Cassini broadcast a radio signal at the surface, and listened for the echo. Brightness in SAR images can result from one of three effects: the slope of the surface (it's brighter if it tilts toward the spacecraft), the roughness of the surface (if it's bumpy on the same scale as the radio wavelength -- about 2.3 centimeters (1 inch) -- it looks bright), and the intrinsic radio scattering properties of the surface material. The first SAR images from Titan were extremely difficult to interpret because no obvious topographic features were visible, but "we can finally see topography," said Soderblom with evident relief.
That these craters are recognizable has appeared to give scientists a little more confidence in interpreting some of the other features in the images. "To the right of the impact basin you see things that look like bright dendritic channels," Soderblom continued. The channels could be bright because they may contain pebbles or cobbles a few centimeters or more across, as the Huygens landing site did. The large crater has similarities to multi-ringed structures on the Moon and other solar system bodies, as Cassini interdisciplinary scientist Jonathan Lunine explained. "It appears to have some fractures flowing into it on the southwest side, so it has been eroded. It has probably also been relaxed; it was large enough that [when it formed] it would have carved into relatively warm ice. So it would have flattened out over time. We can't really measure that with these SAR images; all we can do is look at the shape of the thing and the features inside the crater and try to deduce whether it has softened and flattened." The smaller crater is "just lovely, a beautiful example of an impact crater on Titan," Lunine said. The RADAR instrument on Cassini illuminated the view from above, bringing the crater into obvious relief. "The illumination is from the top down, so the rim on the north or upper side is shadowed, and the rim further down is exposed to the radar, so it's bright. You can also see the ejecta blanket, material that has been tossed out" of the crater. Craters are important because they are tools used by planetary geologists to figure out the age of planetary surfaces and the history of geologic events. Therefore, identifying them helps to begin to put Titan's history into a timeline. Lunine said, "This is a feature that would have persisted, even if large amounts of organic material were deposited on it, over billions of years, so we don't know if this is a feature that formed late in Titan's history, or early on. But the smaller crater looks relatively fresh and well-preserved, and it may indeed be a somewhat younger feature. Overall, based on the craters that we have seen, we would say that the age of the surface is less than a billion years, that the surface has been modified, so we're really only seeing things that have happened over the most recent quarter of the age of the solar system."
The first set of SAR images obtained by Cassini crossed a part of the far northern hemisphere that has not yet been seen close-up by the optical instruments, in part because it is winter in Titan's northern hemisphere and the areas of those first SAR images are presently in winter twilight. This swath, however, was at a much lower latitude, and crosses some areas that have actually been imaged at higher resolution. It will take some time to superpose the two different data sets, but Randy Kirk of the U. S. Geological Survey confirmed that "both of the craters are identifiable as bright spots in the camera images. Yesterday we took the camera image and we just slapped our image on to it, and it lined up with all sorts of things." In fact, the multi-ringed structure had already been spotted in an image of Titan captured last December as Cassini approached for the second Titan flyby. Another RADAR image covers an area just to the east of the multi-ringed structure and contains some interesting channel features. They are interesting, Soderblom explained, because they are very "primitive, or low-order," meaning that the channels do not branch much. Such primitive channels do not form from rainfall; instead, the fluid that shaped them probably came from underground. "They're very confined; they look like they are related to some subsurface source. It'd be intriguing to see if we could get some optical images that would resolve those," Soderblom said. The area is to the east of the hemisphere of Titan that has been viewed at high resolution by Cassini's cameras and spectrometers.
The apparently rough floors of the valleys and the bright plain that they empty in to could be akin to the Huygens landing site, Lunine added, although Soderblom cautioned that the image covers a much larger area than was visible to Huygens' cameras. Both Soderblom and Lunine are members of Huygens' camera team as well as being Cassini Interdisciplinary Scientists. But not all of the SAR images are so easy to interpret. "What are all the tiger scratches?" asks Soderblom. He is referring to linear sets of dark features visible across a large part of the RADAR swath to the west of the multi-ringed structure.
The "tiger scratches" or "cat scratches," as they are being called, are curious because they are dark linear features with no corresponding bright features parallel to them, which is what you would expect if they were fractures or faults in Titan's icy surface. "The first reaction was that they look like some kind of dune form," Soderblom says. "I think they are some sort of fine-grained mass of material in a stringer. It's conceivable that they could have formed from some sort of fluid flow across the surface, or they could be some fine-grained mix of hydrocarbons and ice flowing along. But were they laid down in an aeolian process in a nitrogen atmosphere or in fluid flow in a methane sea which has presumably since disappeared? I don't know. I think we don't have a clue what it really is. But I think the best bet is some kind of fluid flow in the atmosphere." Soderblom explained that the features could also actually lie underneath Titan's surface. Just as SAR images of deserts on Earth have revealed ancient river beds and trade routes lying just below a dusty, sandy surface, Cassini could be peering through a thin mantle of sandy material to see features that lie just underground.
Perhaps the most exciting fact about these images is that the mission wasn't even sure that the Synthetic Aperture RADAR (SAR) technique would produce high-quality images from a flyby of this altitude (1577 kilometers or 980 miles at its closest point). "We never thought we'd get reasonable-quality data from that height, so this is good news," Soderblom says. "But from Titan-A we knew that the system was stronger and the signal was looking better" than they had originally expected. SAR passes have only been included on a fraction of the 45 Cassini flybys of Titan because planners thought that it would be necessary to fly quite close to Titan to get a good view. The excellent results from this flyby lend hope to the possibility that a SAR swath can be included on the Titan-8 flyby scheduled to occur on October 28 of this year at an altitude of 1450 kilometers (900 miles). The reason that Titan-8 is exciting is that "we could deflect one of the RADAR beams off track over to the Huygens landing site," Soderblom said. Overall, Lunine was struck by the "diversity of the terrain" that is visible in this one SAR swath. "You've got everything from impact craters, to dark plains with some brighter flows, to these rather mysterious linear black features that may be wind-driven, sand dunes, snow dunes, they may have been deposited in water; we've got a laundry list of possibilities. What these data illustrate is the extreme diversity of Titan's surface and the large number of processes that are at play on that surface, from winds, to the action of liquids, to the possibility of volcanism involving warm ice or liquid ammonia-water. This is a world with a complex history that the Cassini orbiter and the Huygens probe together are just beginning to elucidate." As Cassini and Huygens build up their storehouse of data, interpretations will come more easily to the science team, Soderblom promised, but that doesn't necessarily mean that the interpretations will be the right ones. "The third [SAR pass] is going to be easier, and the fourth one's going to be easier than that, and then we're going to convince ourselves that we know what we're looking at when we absolutely have no clue." Project scientist Dennis Matson agrees with this cautionary remark. "We don't know it all," he said. "We see these examples of places in the solar system where we have paradigms of what we expect, and then we have discoveries which show us what's really there. Exploration of the outer solar system has a wealth of surprises, which we have yet to discover." |
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