Titan's Impact Craters and Associated Fluvial Features

The Cassini spacecraft has detected remarkably few impact craters on the surface of Titan. By early 2010, with surface radar coverage reaching 33%, seven certain impact craters were discovered, with another 52 nearly certain and probable ones. The paucity of craters implies that the surface of Titan is very dynamic and relatively young. Dynamical models of the internal structure of Titan suggest the possibility of a subsurface ocean of ammonia-water liquid beneath its icy shell. If a large subsurface ocean does exist, it should have measurable effects on Titan's surface and the morphology of its craters. Using a combination of available Cassini radar-SAR, ISS, and VIMS data, we construct geomorphologic maps of Titan's "certain" impact craters with associated features we interpret as fluvial in origin. The best example, Menrva, a 445 km wide double-ring impact basin, hosts a complex network of channels. On the western, more degraded side of the crater, channels cut through the outer rim. To the east of Menrva, a curious network of channels start near the rim crest and appear to have flowed away into a large catchment basin; the complex is termed Elivagar Flumina. Channels surrounding Menrva display a low order - a classification of stream segments based on the number of tributaries upstream - measuring one or two, occasionally up to three. This matches observations of two other confirmed impact craters with associated fluvial features. A halo of low-order channels encircles Selk, an 80 km diameter crater with a small central peak. Also, Ksa, a 30 km diameter crater with a bright central peak and radial ejecta, has a feature that appears to be a first order channel. These differ radically from the tree-shaped dendritic channels common on Titan, which are generally attributed to heavy rainfall. For example, the Xanadu region, as observed on the T13 swath, exhibits a very complex and dendritic network of channels, where the order of channels reaches six to seven. The extensive area covered by dendritic systems indicates an origin from rainfall, rather than seepage of subsurface liquids, which has a low stream order. Thus, we argue that the association of channels with Titan's largest craters may not be pluvial in origin, and instead may be the result of seepage or even record a flood initiated by a large impact.