EVIDENCE SUPPORTING AN IGNIMBRITE ORIGIN FOR THE MEDUSAE FOSSAE FORMATION, MARS

A Thermal Imaging System (THEMIS) image of a portion of the eastern exposures of the Medusae Fossae Formation (MFF) on Mars reveals important new evidence that supports an ignimbrite origin for these enigmatic materials. Many alternative hypotheses have been proposed for the origin of the MFF materials, found along the equator of Mars between longitudes 135 and 230 E. THEMIS data provide new information to help test the proposed hypotheses. The daytime thermal infrared image I01665006 shows a cliff within a component of MFF (at 7.0 N, 218.5 E); near the top of the cliff is a horizontal layer that is measurably cooler than its surroundings. Cooler daytime temperatures are indicative of a higher competency than that of the surrounding cliff materials. The competent layer corresponds to an erosional embayment into the topmost portion of the cliff, indicating that the competent layer is much more resistant to erosion than the uppermost material. Both of these attributes are consistent with exposure of a welded zone in an ignimbrite, with enhanced erosion of the friable vapor-phase altered materials above the welded zone. The competent layer is consistent with MOC images of multiple dark horizontal bands visible on erosional scarps of other exposures of MFF materials. All of these features are more difficult to reconcile with the other proposed hypotheses of origin. This is the first physical properties evidence that directly supports the ignimbrite hypothesis, proposed in the 1980s based on Viking image analysis. Evidence for similar competent layers in other portions of MFF will be sought in THEMIS images from other MFF exposures.