ANCIENT FLUVIAL ACTIVITY IN THE MARTIAN HIGHLANDS

Geologic and geomorphic analyses are being conducted for three regions of the cratered highlands adjacent to the Hellas and Isidis impact basins on Mars: the Reull Vallis region (27.5° - 47.5°S, 245 - 270°W) in Promethei Terra, part of the Terra Tyrrhena region (13° - 30°S, 265° - 280°W), and the Libya Montes (5°N - 5°S, 267.5° - 280°W). Evaluating degradational histories, documenting the timing of resurfacing and erosional events, and determining fluvial histories and hydrology of these regions are of particular interest. High resolution Mars Orbiter Camera (MOC) and Viking Orbiter images are being used to construct detailed maps of fluvial features and to characterize the geology of the terrains in which they occur. Mars Orbiter Laser Altimeter (MOLA) data are being used in conjunction with geographic information system (GIS) software to conduct detailed topographic analyses of drainage basins and valley network systems.

Geologic materials in each study area are diverse in their origins and ages, and record long histories of degradation and modification by fluvial processes. The study areas preserve different amounts of Noachian-aged highland materials and intercrater plains, as well as several Hesperian-aged sedimentary deposits that embay (or bury) highland materials. Some sedimentary deposits occur within low-lying regions of the highlands and are distinct from more extensive plains in that the highland deposits display small but well-integrated valley networks. The intercrater plains of Terra Tyrrhena contain a large (~400 km long) network of well-integrated valleys, as well as numerous smaller valleys. The floors of many large craters are covered with smooth deposits; narrow, parallel gullies on the interior crater walls suggest the craters were infilled with sedimentary deposits. Some craters may have contained standing bodies of water. Analyses of these fluvial systems will provide a better understanding of the longevity of the processes that formed them, which could give insights into the history of Mars' climate. Identification of long-lived local and regional hydrologic systems will provide an estimate of the potential these systems may have had to support life, and thus have an impact on selection of future landing sites.