Surface properties of ancient cratered terrain in the northern hemisphere of Mars

Abstract

The hilly and cratered material of Scott and Carr [1978], interpreted to be the oldest extensively exposed surface of Mars, has been examined where it is covered by high‐resolution (16–62 km2) infrared data from the Viking IRTM. This allows spatial variations in observed properties to be associated directly with geologic features and an assessment to be made of the contribution to the remotely determined properties from the ancient rock unit. The measured nighttime temperatures at 20 and 11 μm indicate that from 0 to 20% of the surface consists of high thermal inertia ‘blocks’ (assumed to be I = 30 × 10−3 cal cm−2 s½ K−1) surrounded by lower thermal inertia ‘soil’ (I = 1.5 to 7.5). Block abundances are typically ∼9% within the Arabia low thermal inertia region (I = 1.5 to 3.0) identified from global mapping and increase away from Arabia, particularly to the south. Geologic features crossed by the IRTM data generally show no distinguishable difference from the regional properties with the exceptions of dark patch material and craters near 27°N, 338°W. The dark patch material, typically within craters, has block abundances (5–20%) and soil thermal inertias (5.0–7.0 for an albedo of 0.15) distinctly different from the Arabia material; nonunit emissivity for this material would make the block abundances upper limits and the soil thermal inertias lower limits. The effective grain diameter of the dark patch material (160 to 360 μm) is indicative of sand‐sized particles capable of saltation. Two craters near 27°N, 338°W have slightly higher soil thermal inertias (∼3.5, corresponding to ∼70 μm) and lower block abundances (0–5%) than the surrounding Arabia material, but no prominent dark patches are associated with them. Imaging data from within and around the Arabia region indicate that subdued surface morphology is not always associated with low thermal inertias. The extensive mantling of northern hemisphere locations of ancient cratered terrain by fine‐grained material does not allow thermal measurements to be directly related to rock unit properties, but less mantling may be present in southern hemisphere locations of this material.