DISTINGUISHING IMPACT AND VOLCANIC DEPOSITS ON MARS WITH DATA FROM SPIRIT AND OPPORTUNITY

The rovers Spirit and Opportunity have enabled the first in-situ comparative studies of martian impact and volcanic deposits. Spirit’s landing site lies within the 160-km Gusev Crater. The dominant rock type there is “Adirondack-class” olivine-rich basalt, which forms plains deposits that cover much of the Gusev floor and is interpreted as volcanic in origin. Opportunity’s landing site is dominated by flat-lying sedimentary rocks rich in sulfate salts and hematite-rich diagenetic concretions. Both sites have small impact craters; ejecta deposits from these craters have been extensively studied.

At the Spirit site the Columbia Hills predate the Adirondack basalts and contain very ancient moderately to poorly sorted clastic deposits. Based on texture alone, these could be interpreted as either pyroclastic volcanic deposits or distal impact ejecta from a distant crater. In a few instances, observed geologic relationships provide distinctive clues; an example is the inward-dipping strata of Home Plate that point toward a pyroclastic origin. In other cases, the best information comes from geochemical clues, like Ni concentrations higher than expected for most volcanic materials but consistent with an impact origin.

Opportunity has spent the past several years exploring the rim of the Noachian age ~22 km Endeavour Crater. Most Endeavour rim materials are impact ejecta. These include what appear to be coarse-grained lithic breccias and, more commonly, breccias with angular clasts embedded in a fine-grained matrix. In the few instances where stratigraphic relationships between breccia units can be distinguished, they mimic those at comparable-sized impact craters on Earth.

Recently, Opportunity has documented local exposures of fine-grained rocks of basaltic composition on the rim of Endeavour Crater. The textures and compositions of these rocks are consistent with an extrusive volcanic origin. However, their location on the rim of a crater of Endeavour’s size also permits the interpretation that they are fine-grained impact melt.