Correlating Meter-scale Morphology and Kilometer-scale Topography: Evidence for a Degraded Mid-latitude Surface Layer on Mars

Introduction: We have been investigating several independent data sets from recent Mars missions that provide evidence for a meters-thick, latitude-dependent surface layer [1, 2, 3]. Based on apparent physical properties observed in Mars Orbiter Camera (MOC) images and Mars Laser Altimeter (MOLA) data, and the distinct latitude dependence of these observations, we interpreted this to be an ice-rich layer, and interpretation supported by recent Mars Odyssey results [4, 5, 6]. Our current focus is on the latitude zones 30-60°N and S where the surface layer is discontinuous. The relative paucity of impact craters supports an interpretation that the ice-rich layer is recent. Our goal in this analysis is to directly compare the distribution of MOC images exhibiting the discontinu-ous layer [2] with MOLA observations of roughness on several scales [1, 3] and to assess the degree of correspondence. The MOC images provide high spatial resolution observations of morphology, but each image covers a very small portion of the surface. The MOLA observations are global, but relating the various roughness measurements with specific surface features can be difficult. By combining these observations we can a) provide a means of relating the MOLA roughness measurements to specific surface morphology and b) scale the MOC observations across the regions between observations. The ultimate goal is to provide insight into the regional and global properties of the ice-rich surface layer. Data Processing and Integration: A database of MOC observations has been created identifying a particular surface morphology characterized by the following features: 1) a smooth, intact surface is present, 2) the smooth material is broken up or dissected somewhere in the scene, 3) the pitted or hummocky terrain resulting from dissection is distinct from dunes, yardangs, or other eolian features. Over 15,000 MOC images have been systematically analyzed and classified according to the presence or absence of these features. The images were examined orbit by orbit without knowledge of geographic location to minimize bias in the recognition of the terrain. Over 1700 images exhibit this morphology. The resulting distribution of dissected terrain shows two latitude bands between 30-60° N and S with almost identical means of ≈41° [2]. For this analysis we required a more refined understanding of the spatial distribution and therefore calculated the mean latitude, equatorward minimum latitude, and standard deviation of the north and south observations separately in 30° longitude bins. Each 30° bin was then shifted 5° and the