GSA PLANETARY GEOLOGY DIVISION G.K. GILBERT LECTURE: DEGRADATION ACROSS THE SOLAR SYSTEM

Landform degradation, along with mass movement, reduces topographic relief by moving surface materials to a lower gravitational potential. In addition to the obvious role of gravity, abrasive mechanical erosion plays a role, often in combination with the lowering of cohesion, which allows disaggregation of the relief-forming material. Chemical weathering also contributes to disaggregation on worlds where solvents can react with constituents of “bedrock” exposed at the surface. The identification of specific landform types associated with mass movement and landform degradation provides information about local sediment particle size and abundance and transportation processes. Generally, landform degradation and mass movement can be classified in terms of the particle sizes of the transported material and the speed the material moved during transport. Most degradation on planets and satellites appears consistent with sliding or slumping, impact erosion, and regolith evolution. Some worlds, such as Mercury, Mars, Callisto, Hyperion, and Pluto, have an appearance that implies that some additional process is at work, most likely sublimation-driven, or seepage driven, landform modification and mass wasting. A variant on this process is thermally driven frost segregation as seen on all three icy Galilean satellites, Iapetus, Pluto, and perhaps elsewhere. Earth, Mars, Titan, and perhaps Pluto, are extraordinary among Solar System worlds in that aeolian and fluvial processes also operate to erode, transport, and deposit material. Glaciation also operates, or has operated, on Earth, Mars and Pluto. In this talk, I will review the common threads that appear to facilitate the formation of similar landscapes across a range of materials and conditions from Mercury to Pluto. By far, the most significant is the presence of “useful” amounts of a substance (and in some cases substances) that exists near, or at, a temperature (and pressure) phase change boundary on the surface of a world. By “useful” I mean in sufficient abundance that its phase change and movement can do work on the landscape. An obvious example is the role of H₂O on the Earth. Several examples will be given. Some of these examples will be illustrated by Landform Evolution Models, based on modeling originally developed by Alan Howard.