FIRST LOOK AT THE THERMOPHYSICAL PROPERTIES OF THE DISSECTED MANTLE

Introduction: Previous studies have identified a latitude dependent surface layer that appears to be continuous poleward of 60°, absent equatorward of 30°, and discontinuous in the 30-60° N and S latitude bands [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, the surface layer has been interpreted to be ice-rich where continuous, an interpretation supported by recent Mars Odyssey results [4, 5, 6]. The discontinuous zone is interpreted to be the a degraded equivalent of the continuous layer [2, 3], and as such represents the maximum extent of a recent excursion in the stability of near-surface ground ice towards the equator. Here I report on preliminary analysis of THEMIS brightness temperature data [7] of observations in the mid-latitude zones to assess if the degraded mantle has properties distinct from the underlying substrate. The MOC images provide high spatial resolution observations of morphology, but each image covers a very small portion of the surface. In additional to the ability to identify composition, the THEMIS observations provide unique insights into the near-surface thermal properties [7] and it is this property examined here. The goal here is to examine nighttime THEMIS images of surfaces that exhibit distinct properties of dissected mantle identified in MOC images [2] to provide insight into the regional and global properties of the ice-rich surface layer. Data Processing and Integration: To focus this first look, previous observations of dissected mantle were used to select THEMIS images. 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 [2]. 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. Three regions were selected for this preliminary analysis: Reull Vallis for the presence of viscous flow features in combination with the dissected mantle [Milliken et al., 2003], a region of knobby terrain near 185°W 34°S, and the Nili Syrtis regions north of Syrtis Major (287°W, 29°N). For each region, co-located MOC daytime and THEMIS nighttime images were examined for the correspondence between the observed MOC morphology and the nighttime temperature. Nighttime temperature is used as a proxy for thermal inertia, based on the principle that materials that are relatively warm at night have a higher thermal inertia than materials that are relatively cool. There are many possible causes of such temperature differences, including particle size, relative rock abundance in the pixel, cementation, and relative albedo. Full determination of the true properties requires detailed modeling which will be the focus of future investigations. The objective in this first-look is qualitative examination and assessment of any correlation between MOC morphology and THEMIS nighttime temperature. Nili Syrtis: This regions marks the transition from the highlands of Arabia and Syrtis Major to the northern lowlands (Figure 1). The terrain is rugged, with abundant steep slopes. Here the dissected mantle is sparsely represented, primarily preserved on northfacing slopes. It is apparently thin, and good examples of substrate exposed beneath the thin mantle are observed in MOC images. This represents an extreme equatorward example of the postulated ice-rich surface layer.