DECAMETER-SCALE SURFACE TEXTURES IN DEUTERONILUS MENSAE, MARS

Along portions of the Martian highland-lowland boundary, a complex physiographic province called ′fretted terrain′ is characterized by rectilinear valleys on the heavily cratered uplands and isolated mesas and knobs on the sparsely cratered lowland plains. Prior studies based on Viking images interpreted certain landforms within the fretted terrain (e.g. lobate aprons and lineated valley floor material) as evidence for ice-enabled flow of material, presumably mass wasted debris from valley walls and escarpments. In this investigation, we examined high resolution Mars Orbiter Camera (MOC) images of these landforms to map the spatial distribution of decameter-scale surface textures within one region of the fretted terrain: Deuteronilus Mensae (30-50°N, 330-350°W). An initial classification scheme has been developed consisting of 8 surface texture types. The location of these texture classes as well as other middle latitude landforms (e.g. circular features, etc.) are documented to look for regional differences that may offer insight into the formation history of fretted terrain landforms.

Preliminary results suggest that there is a geographical correlation for certain mid-latitude surface textures. Specifically, equidimensional knobby terrain is more prevalent at lower latitudes while pitted textures are more common at higher latitudes. The latitudinal relationship suggests that devolatilization has played a role in pit formation. Circular mesas present on both valley floors and apron surfaces also suggest removal of material by erosional processes. Although lineations on valley floors typically parallel the valley wall, examples of hypothesized mass movement (e.g. deflection of material around obstacles or deformation of circular features) are rare and debatable. Consistent with our observations to date is an alternative hypothesis for the generation of lineated valley floor material and lobate aprons: the erosional expression of a layered substrate whose slopes and surface characteristics reflect the composition and physical properties of the layers. Results of this study will be important for constraining the climatic and substrate conditions required for formation of these landforms, and for evaluating the proposed present and past location of near-surface mid-latitude volatiles.