The Cassini Imaging Team has published its first findings about Titan in the journal Nature. The complexity of Titan’s surface and the extent to which it is continually modified draws the most attention. Where are the craters that should have pocked its surface over the past billion years? Thanks to Cassini/Huygens, some answers are beginning to emerge. Working with the last eight months of imagery from the orbiter, the team reports that thirty percent of the satellite’s surface has now been mapped with resolutions high enough to pick out features as small as one to ten kilometers.

From a press release from the Cassini Imaging Central Laboratory for Operations (CICLOPS):

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.)

That means large craters have been erased by tectonic activity, erosion and the flow of surrounding material. The tectonism shows up in the linear boundaries found between Titan’s light and dark areas as seen from Cassini’s close passes of the moon.

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.”

Image: This map of Titan’s surface brightness was assembled from images taken by the Cassini spacecraft over the past year, both as it approached the Saturn system and during three closer flybys in July, October and December 2004. The map reveals complex patterns of bright and dark material on Titan’s surface. The large scale features, including Xanadu Regio — the large, bright feature that extends from approximately 80 degrees to 130 degrees west near the equator — have been observed from Earth over the past several years. Credit: NASA/JPL/Space Science Institute.

Intriguingly, cloud activity over Titan’s south pole indicates to some scientists that this is where a methane rain cycle, with accompanying channel carving, runoff and evaporation, may be most active. All of this means we may have the opportunity to observe surface changes on Titan within short time frames, something that would have seemed unlikely to many planetary scientists on any of the outer planets’ moons until 1979, when Voyager 1 revealed volcanic activity on Io, and ten years later, when Voyager 2 imaged the strange ‘ice volcanoes’ of Triton.

And another indication of change on Titan, also from CICLOPS:

In Titan’s hazy stratosphere, it looks as though modelers may have to go back to the drawing board. Voyager images of Titan detected a faint detached haze layer above Titan’s main stratospheric haze, at altitudes of 300-350 kilometers (190 to 220 miles). Cassini ultraviolet images, which are sensitive to scattering of sunlight by small particles, detect a similar detached haze layer, but at an altitude of 500 kilometers (310 miles) instead.

“The change we see in the detached haze over the 25 years since Voyager suggests that either the photochemical process that produce the hydrocarbon haze particles, or the atmospheric circulation that distributes them around the planet, may change with the seasons,” said imaging team member Dr. Bob West of the Jet Propulsion Laboratory, who designed all the Titan atmosphere imaging sequences for the Cassini mission. “It will be a challenge for models to be able to predict how and where these detached hazes occur,” he said.

The Nature article is Porco, Baker, Barbara et al., “Imaging of Titan from the Cassini spacecraft,” Nature 434, 159 – 168 (10 March 2005).