Geology of central Chryse Planitia and the Viking 1 landing site: Implications for the Mars Pathfinder mission

Abstract

1:500,000-scale geologic mapping in the central Chryse Planitia region of Mars was correlated with ?ground-truth? data gathered by the Viking 1 lander. Materials within the Chryse basin can be subdivided into plains and channel units that are typically separated from one another by gradational contacts. Hesperian Ridged plains materials, unit 1 (Hr1) are the oldest materials mapped. Typically, these materials contain numerous fresh impact craters and have sharply defined, mare-like wrinkle ridges similar to those appearing on the lunar maria. These materials grade into Hesperian Ridged plains materials, unit 2 (Hr2), which are characterized by buried and eroded impact craters and subdued wrinkle ridges. From analyses of crater age dates and their associated geologic contacts, channel materials appear to have formed at the same time as Hr2 materials, and it is likely both units represent fluvial sediments. Measurements of buried craters contained in Hr2 materials suggest that in places this unit may be ?50 m thick, but crater size-frequency distribution curves suggest that the areal average may be closer to ?170 m. Based on these observations, our interpretation is that Hr2 materials were deposited into a standing body of water during channel formation. This interpretation implies that many of the rocks visible in the Viking 1 lander images were emplaced by fluvial processes. Possibly, finer-grained sediments remained in suspension and were subsequently transported out of Chryse Planitia and into the northern plains during draining of the ponded water. East-west trending surface undulations, visible in lander views toward the south, may represent aeolian dunes, lava flow fronts, or sediment waves formed at the bottom of the standing body of water. Broad physiographic units seen at the surface are not clearly visible in Viking orbiter images; however, they can be projected at the resolution of the orbiter images. These units show that concentrations of drift materials are oriented in a northwesterly direction, contrary to the strongest prevailing wind direction which is toward the northeast. These materials were probably deposited on Ridged plains materials, unit 2, during a period of time when aeolian processes were more active in the region. Both Earth-based radar and Viking thermal data suggest that the Mars Pathfinder landing site will be similar geologically to the Viking 1 site. If this is true, then the Mars Pathfinder mission provides the opportunity for building directly on results of the Viking program. Some of the outstanding questions that Mars Pathfinder may be able to address include determining the aeolian modification history of the Chryse Planitia region, the degree and possibly the relative rate of sediment induration, the fraction of rocks and boulders emplaced by impact processes, the possibility that some materials are the result of in situ weathering, and whether materials were emplaced by fluvial processes and the associated depositional environment.