GREELEY HAVEN, MARS: FIELD GEOLOGY ON MARS FROM MER OPPORTUNITY OBSERVATIONS IN AND ABOUT ITS WINTER HAVEN AT ENDEAVOUR CRATER

Mars Exploration Rover Opportunity “wintered over” for 151 sols (Martian days) at a location on the north end of a ridge of ancient cratered highland material. The location was informally named “Greeley Haven” in honor of planetary geologist Ron Greeley. Remote sensing observations from this location also included a 360 degree color panorama, informally named the “Greeley Pan”, as well as observations of temporal changes in both the atmosphere and aeolian fines associated with local and distal crater floor mobile materials. Opportunity crossed a major stratigraphic unconformity on its 2681^th sol on Mars when it arrived at the southern end of an isolated elongate ridge remaining from the eroded rim of the 20-km diameter, crater Endeavour, a remnant from ancient cratered highlands now nearly inundated by the sulfate sandstones of Meridiani Plaum (“Burns Formation”). Outcrop measurements during the traverse north along the ridge of ancient cratered highland material (”Cape York”) have characterized rocks from a much early period in Martian geologic history than preserved within the surrounding plains. The contact between Cape York and the outcrops at Greeley Haven consist of altered, coarsely clastic impactite lithologies (informally named the “Shoemaker Formation”) identifying two or more generations of fundamentally basaltic substrate incorporated into local and regional impactites. These materials consist of mixed altered nanophase oxides of possible glassy original origin and enclosed larger clasts, some basaltic, and occur as agglomeratic-appearing outcrops and blocks. The proto-lithologies are consistent with the location on the rim of a 20-km diameter impact crater. And the oxidized, soft, and otherwise altered-appearing character of the outcrops support an interpretation of an environment of vigorous weathering during and following emplacement. Field reconnaissance style geologic mapping leading up to and following the stay at “Greeley Haven” have revolutionized current ideas about early environments on Mars. The presence, for example, of gypsum veins in the units in contact with the unconformity between the Shoemaker and Burns Formations, are evidence for the continued presence of significant water in the substrate long after the initial emplacement of the Meridiani sandstone