Titan is about the same size and density as Jupiter's largest moon, Ganymede. Unlike Ganymede, though, it probably has not undergone tidal heating - a well-known internal engine for modification of planetary surfaces. For these reasons, Titan was expected to have a surface at least as old as Ganymede's and pocked with at least as many large craters. Over the past billion years, Titan should have accumulated as many as a hundred craters, 20 kilometers (12 miles) wide and larger, across its entire surface.
Yet, that is not what is seen in the images of this world Cassini has obtained so far.
Dr. Elizabeth Turtle, imaging team associate in the Lunar and Planetary Lab at the University of Arizona in Tucson (and co-author on the paper in Nature) said, "We've only just begun exploring the surface of Titan, but what's struck me the most so far is the variety of the surface patterns that we're seeing. The surface is very complex, and shows evidence for so many different modification processes."
Images collected over the last eight months during a distant flyby of the south polar region and three close encounters of Titan's equatorial region have covered 30 percent of its surface with spatial resolutions high enough to pick out features as small as 1 to 10 kilometers (0.6 to 6 miles). At this scale, what has been discovered are geologically young terrains with signs of tectonic resurfacing, erosion by liquid hydrocarbons, streaking of the surface materials by winds and only a few large circular features thought to be impact craters formed in the ice 'bedrock'. (The largest of these - a 300-kilometer (190-mile) wide, double-annulus structure to the northeast of the large region called Xanadu - was recently imaged by Cassini's Radar instrument, providing independent confirmation of an impact origin.)
Any large craters that were once there - and there should have been hundreds of them if Ganymede is any guide - appear to have been eliminated or obscured by a combination faulting, viscous relaxation (in which features subside over time due to flow of surrounding material), erosion, and burial. Titan's surface appears to be as complex as planet Earth's, though the rates at which the various forces modify its surface may be much slower than on our planet.
Tectonism (brittle fracturing and faulting) has clearly played a role in shaping Titan's surface. Linear boundaries between bright and dark areas are pervasive on Titan at the global and regional scales seen from orbit, as well as the smaller scales seen by Huygens.
Dr. Alfred McEwen, imaging team member from the University of Arizona, said, "The only known planetary process that creates large-scale linear boundaries is tectonism, in which internal processes cause portions of the crust to fracture and sometimes move - either up, down, or sideways. Erosion by fluids may then serve to accentuate the tectonic fabric by depositing dark materials in low areas and enlarging fractures. This interplay between internal forces and fluid erosion is very Earth-like."
Cassini images collected from orbit have also shown dark, curvilinear patterns in various regions on Titan, but mostly concentrated near the south pole. Some in the polar region extend up to 1,500 kilometers (930 miles) long. Images collected by the Huygens probe during its descent down to the Titan surface in January showed clear evidence for small channels a few kilometers long, probably cut by liquid methane. Cassini imaging scientists are suggesting that the curvilinear patterns seen in their images of Titan may also be channels, though there is no direct evidence for the presence of fluids. If these features are channels, it would make the ones seen near the south pole the Titanian equivalents of the Snake River.
Since most of the cloud activity observed on Titan with Cassini has occurred over the south pole, scientists believe this may be the place where the cycle of methane rain, channel carving, runoff, and evaporation is most active, an hypothesis that could explain the presence of the extensive channel-like features seen in this region.
With presently active geologic and erosional processes similar to those shaping the land areas of Earth, Titan offers scientists an intriguing place to explore and study in the years ahead.
"Throughout the Solar System, we find examples of solid bodies that show tremendous geologic variation across their surfaces. One hemisphere often can bear little resemblance to the other," said Dr. Carolyn Porco, Imaging Team leader. "On Titan, it's very likely to be this and more. Who knows, we may get lucky and have the chance to observe the surface change with time. It's a good thing we'll be coming back for more."
Cassini is scheduled to make 41 additional close flybys over Titan in the next three and one-quarter years.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.