THEMIS Observations, Discoveries and Predictions for the MER A Landing Site in Gusev Crater

THEMIS has "followed the water" and discovered the youngest water flow into the MER A landing site in Gusev crater. This flow has a rumpled looking texture and is interpreted to be viscous material emanating from the mouth of Ma'adim Vallis. The flow can be traced for over 150 km across the floor of Gusev. This flow is emplaced on top of the smooth plains material and covers the whole western half of the landing ellipse. The flow does not show up in THEMIS IR indicating that it is mantled with at least a few cm of dust. This flow is either a debris flow (15-40% water volume content) or hyperconcentrated flow (40-80% water volume content) and not a lava flow based on its morphology, geologic setting, and lack of nearby volcanic sources. Debris flow deposits can be differentiated from hyperconcentrated flows on the basis of particle sorting, sedimentary structures, and inferred rheological properties. Access to this deposit will allow sampling of the most recent water related sediment in the basin. A very interesting relationship has been found to exist for many craters in the region of Gusev. These craters contain stacks of layered sedimentary deposits. It should be noted that these craters lack a large inflowing channel system. A similar layered morphology is seen on the floor of Gusev, especially the SE portion of the crater, where a 190 m thick deposit is being eroded. We propose that this material in Gusev is the remnant of a formerly more extensive regional unit related to the layered deposits seen in the many nearby craters. This observation suggests that the region was formerly buried by several hundred meters of material that is now being exhumed. This also implies that Ma'adim Vallis was a superposed channel that cut down from above and across Gusev. We also offer another scenario for Gusev in that it received periodic outwash deposits and may have contained shallow ephemeral playas with short lifetimes and not deep long lived lakes as suggested by some researchers. If indeed a lake ever formed here we feel that it is very possible that the lake sediments may have been primarily eroded away and or buried. Thereby making it difficult for MER A to sample and identify any putative lacustrine sediments with the Athena payload. It may be possible that impact craters have dredged up these putative deposits. Other large craters 30 to 86 km diameter located around Gusev are likely to have contributed material to the floor of Gusev. Additionally, Apollinaris Patera located only 300 km away is also very likely to have dumped ash into Gusev. Predictions: MER A should find a landscape similar to VL 1 and MPF in terms of thermal inertia, albedo, dustiness and topography. However, rock abundance will be half that of previous sites. In terms of landforms it will depend on where MER A lands in the ellipse. A hummocky debris flow covers the western half of the ellipse and relatively smooth cratered plains cover the eastern half of the ellipse. The smooth plains unit appears to be composed of many thin layers and the rover could possibly encounter layered mini mesas and buttes. The smooth plains material also has dark aeolian streaks superimposed upon it. These dark streaks are depositional in some locations because they are draped on bright aeolian bedforms. These bright aeolian bedforms then take on a dark appearance in these locations. The bright aeolian bedforms may be granule ripples and/or the remnant of an eroded layer. These features are scattered throughout the ellipse. Finally, small impact craters and ejecta may be observed.