Latitude Dependence of Meter-scale Surface Textures in Deuteronilus Mensae,

In this investigation, high resolution MOC images were examined to document the meterscale surface textures on lobate aprons and valley floors within Dueteronilus Mensae. The spatial distribution of four meter-scale surface textures exhibits a latitude dependence with knobby terrain more prevalent at lower latitudes and pitted terrain more common at higher latitudes within the study region. This suite of surface textures is interpreted to be due to degradation of the upper-most surface mantle dominantly through ice sublimation. This study agrees with several recent works that propose a latitude dependent near-surface ground-ice cemented dust mantle that has undergone degradation coinciding with the last major obliquity excursion a few hundred thousand years ago. Introduction and Methods: The presence of near-surface ice on Mars has been postulated since the Mariner 9 and Viking missions when landforms that resembled terrestrial glacial or periglacial features were identified in images of the martian surface, particularly in the middle latitudes. 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 at mid-latitudes (25° bands centered at ~40° N and ~45° S) [1]. 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 iceenabled flow of material, presumably mass wasted debris from valley walls and escarpments [1-2]. In this investigation, we examined high resolution (1.4-12 m/pix) Mars Orbiter Camera (MOC) images of these landforms to map the spatial distribution of meter-scale surface textures within one region of the fretted terrain: Deuteronilus Mensae (30-50°N, 330350°W). Four hundred nineteen MOC images acquired between May 1999 and September 2004 of lobate aprons or valley floors within Deuteronilus Mensae were examined. A classification scheme for several surface texture types was developed. Results: Here we report on four meter-scale surface texture classes (Figure 1). The classes, as listed below, represent a continuum with void spaces increasing in size. Knobs—Brain coral. Small, equidimensional knobs that are regularly spaced and resemble brain coral or corn on the cob kernals. Knobs can be rounded or flat on the topmost surface. Knobs—Bumpy. Larger knobs that are more variable in size and spacing than the brain coral variety. Larger intervening depressions between knobs. Pits—Wavy. Regularly spaced, linear pits. Pits—Irregular. Pits that have nonuniform shape. The largest depressions are in this category.