MORPHOLOGICAL ANALYSES OF MESAS AND KNOBS IN THE NORTHWEST FRETTED TERRAIN OF MARS: CONSTRAINTS ON THE PRESENCE AND DISTRIBUTION OF ICE-FACILITATED MASS-WASTING

The fretted terrain on Mars (found 30°N to 50°N and 0°E to 80°E) is a transitional zone between highly cratered uplands and younger, less cratered lowlands, which exhibits flat-topped, steep-walled mesas and knobs (Sharp, 1973). These mesas and knobs are separated by younger flat-lying lowlands and lineated lobate debris aprons thought to result from ice-facilitated mass-wasting process from either ground or atmospheric sources. Although the origin of lobate debris aprons is still unclear, formational processes may be constrained from the morphologicial analyses of the adjacent mesas and knobs. The distribution and morphology of mesas and knobs within 38ºN to 51ºN and 12ºE to 37ºE were analyzed using recent data from Mars Orbiter Laser Altimeter (MOLA), High Resolution Stereo Camera (HRSC), Thermal Emission Imaging System (THEMIS), and Mars Orbiter Camera (MOC) with GIS tools. Mesa and knob areas were delineated from their base up and calculated as a function of distance from the dichotomy boundary escarpment. Results indicate no systematic decrease in mesa area northward away from the escarpment to a maximum distance of 500 km. Mesa and knob spatial densities are greatest in two bands at the 15 km and 150 km distances and are the result of local clustering of small knobs <20 km² in these areas. Maximum mesa relief also decreases from nearly 2 km to 100-200 m with increasing distance from the escarpment. Where mesa walls are high (0.5 km to 1 km), wall erosion typically includes two to tens of small alcoves several kilometers in length with floors exhibiting alcove-wall-parallel lineations. These results differ from a pattern of decreasing mesa size from the highland escarpment, as suggested for the majority of the fretted terrain. Ice-assisted mass-wasting processes may be less active in mesas near or above 50ºN due to low mesa relief or latitudinal variations in ice placement. Determining the type and the presence of ice-assisted mass-wasting processes in the fretted terrain may provide valuable insight on the past and present distribution of ice on Mars.