TITAN'S DYNAMIC SURFACE AND ATMOSPHERIC INTERACTIONS

Titan's surface reflects a complex interplay of active processes. As on Earth, geologic structures are heavily modified by erosional and depositional activity (e.g. channels and dunes), and surface processes are intimately connected to subsurface hydrology as well as atmospheric processes. Atmospheric models and observations demonstrate seasonal changes in Titan's weather patterns; however, cloud activity has been rare since 2010, despite predictions that clouds should increase well before now accompanying the onset of convection as summer approaches at northern latitudes.

The frequency and amount of precipitation from seasonal storms have important implications for Titan's methane cycle, atmospheric circulation, and rates of geologic modification. Surface changes have been observed following large cloud outbursts: in Arrakis Planitia at high southern latitudes in Fall 2004, during Titan's late southern summer; and at low southern latitudes in Concordia and Hetpet Regiones, Yalaing Terra, and Adiri in Fall 2010, just over a year after Titan's northern vernal equinox. Surface darkening attributed to wetting by methane rain persisted for up to a year in places, implying a shallow impermeable layer or saturated ground at the level of a methane table, consistent with surface observations including detection of methane by the Hugyens Probe after landing (~10° S). Variations in the rates at which the surface reverted to its original brightness suggest areas dried or drained (by overland flow or infiltration) at different rates. Comparison to estimated evaporation rates suggests liquid ponded up to several cm or few tens of cm deep. Curiously, several areas later brightened relative to their original appearance, best explained by evaporative cooling of the wetted surface resulting in accumulation of methane and/or ethane frost that subsequently sublimed away.

Long-term study of Titan's diverse landforms, geologic processes, and atmospheric interactions has revealed much about Titan's actively changing surface. Nevertheless, fundamental questions remain regarding rates of and balances between geologic processes, sources of materials (e.g. atmospheric sedimentation), processing in the surface environment (e.g. origins of dune-forming particles), and compositions of surface and crustal materials.