GALE CRATER AND IMPACT PROCESSES: THE MARS SCIENCE LABORATORY PERSPECTIVE

Curiosity is beginning its exploration of the ~150 km diam. Gale crater; we report here results on impact processes observed up to Sol 269. Setting - Gale crater was formed predominantly in the highlands crust, except where it straddles the dichotomy boundary, into the northern lowlands. Crater counts over a ~500,000 km² region around Gale yield a formation age of 3.74 +0.021/-0.024 Gy, close to the Late Noachian-Early Hesperian boundary; and consistent with counts over the northern side of the Gale ejecta with an age at 3.55 +0.06/-0.09 Gy. Post-impact modification of Gale includes the deposition (and later erosion) of strata forming Mount Sharp, and the deposition of extensive alluvial materials of the Peace Vallis Fan and the bedrock of Bradbury Rise. Hydrothermalism - Mineralogical or chemical evidence of hydrothermal processes expected to be associated with the formation of Gale are not evident from available data on YKB sediments. Although the YKB sediments are thought to be derived from the crater rim, the clays are probably diagenetic in origin and formed in situ. The YKB sediments also show no evidence of the enrichment of the mobile element lithium expected as a signature of hydrothermal processes. Other craters in Gale – Craters observed on Bradbury Rise are heavily degraded with subdued rims, shallow (d/D <= 0.1) sand to gravel filled depressions, lacking ejecta blankets/blocks or other diagnostic features (shatter cones, impact breccias, etc.). YKB craters exhibit the same morphology, lacking even upturned rims, consistent with the more erodible 'mudstone' target material. Orbital views show this subdued morphology is wide-spread with only a few potential examples of ejecta blocks around possible secondaries from unknown origins. Only a handful of very fresh craters with continuous ejecta exist within the Gale crater basin, but all are beyond MSL's nominal traverse path. Impact materials - Possible examples of shocked rocks and/or impact melts, and spherules observed in the Rocknest eolian deposits are present in the regolith and in some outcrop, but an impact origin has not been conclusively confirmed. While the crater’s primary impact features are not reflected by Curiosity’s observations so far, the effects of small recent craters are evident on the surface and in the regolith.