SPIRIT'S TRAVERSE AROUND HOME PLATE, COLUMBIA HILLS, MARS: METHODS AND SOME RESULTS

Compelling insights into the global geologic history of Mars were deduced by robotic observations from the MER Spirit rover. Results near Home Plate, a roughly circular 90 m diameter by 2 m high platform consisting of inward-dipping basaltic sand and lapilli tuffs, were particularly informative. Outcrops were visited, “sampled” chemically and mineralogically, examined at microscopic and macroscopic scales, lithologies were identified, and the results correlated among outcrops elsewhere along the overall traverse. Considerable effort went into the development of the methodology necessary for determining rover geographic position in order that correlations could be done between observations and with orbital remote sensing data.

Significant observations include the discovery of a widespread layer of lapilli tuff resulting from explosive basaltic volcanism, early sinter, and late hydration of drifted fines, as well as other fundamental insights into martian geologic complexity. The results correlated and mapped provide a working outcrop-level geologic context for understanding many of the more important in situ science questions at Home Plate. At least four bedrock units and three disconformities are identified across which there are significant changes in chemistry, alteration, lithology, and emplacement environment. High silica/sulfates have been detected mostly in the lowest stratigraphic unit exposed along the axis of breached antiformal arrangement of bedding that surrounds Home Plate.

Based on the field results, we can outline a local geologic history of (1) early thermal springs and explosive volcanism, followed by (2) extensive erosion and alteration in the presence of water, (3) a more regional phase of more effusive volcanism, and (4) a relatively dry period dominated by aeolian erosion interrupted by brief moist phases. The similarity of this sequence to known secular trends in global geologic history may mean that this is our first glimpse of a lithostratigraphic sequence common to many areas of Mars.