Curiosity's field site in Gale Crater, Mars, in context

NASA's Mars rover, Curiosity, is anticipated to land in Gale Crater in August 2012. Gale is a 155 km-diameter impact crater adjacent to the ancient crustal "north-south dichotomy boundary." It contains a mound of layered rock (of yet-unknown proportions of clastic sediment, tephra, and chemical precipitates) ∼5 km-high that was eroded by fluvial, eolian, and mass-movement processes. The stratigraphy includes erosional unconformities representing periods when new impact craters formed and streams cut canyons into layered rock. The majority of known impact sites on Earth are craters that were filled and buried in sediment; examples occur under the Chesapeake Bay and beneath the Chicago O'Hare Airport. The upper crust of Mars, with its relative lack of tectonism, is almost entirely a layered, cratered volume of filled, buried, and complexly-interbedded craters and fluvial systems. Some of these have been exhumed or partly exhumed; some, like Gale, were once filled with extensive rock layers that were eroded to form mounds or mesas. Landforms all across Arabia Terra show that similar materials were also deposited between craters. Gale is of the family of Mars craters that were filled and buried (or nearly so). The highest elevation on the Gale mound exceeds the crater's north rim by ∼2 km and is within 500 m of the highest point on the south rim. Many similar craters occur in Arabia Terra; these are instructive as some contain mounds, others have mesas or buttes or other erosional expressions. Craters within 10s to a few 100s of km of each other typically contain very different materials, as exhibited by varied erosional expression, bedding style, and layer thickness. This suggests that the depositional environments, sources, and physical properties of the deposited material differed from place to place and time to time, even in neighboring settings. The Curiosity site in Gale has the potential to illuminate processes that acted locally and globally on early Mars. In addition, Gale occurs southwest of a region of volcanic flows and small edifices that have the youngest crater retention ages (< 100 Ma; doi:10.1016/j.icarus.2009.06.032) for high strength igneous rock on Mars. Nearby terrain includes yardang-forming materials in which were buried ancient streams, some of them now inverted. Gale is down-slope from Herschel and the Terra Cimmeria highlands; some of its secondary craters superpose neighboring craters Lasswitz and Wien. The field site on the floor of Gale is at an elevation (-4.5 km) lower than almost anywhere outside Hellas and the northern plains. Because water runs downhill, the low elevation and sedimentary record make Gale attractive to those seeking evidence of habitable ancient Mars environments. With a record of fluvial erosion in the lower part of the mound, and a lack of fluvial features higher on the mound, the strata in Gale might also record the transition of Mars itself from early, wet conditions to the hyper-arid setting of today.