SIGNIFICANCE OF FLOOD DEPOSITS IN GALE CRATER, MARS

The Hummocky Plains Unit (HPU) is one of five rock units that deposited in the landing ellipse of the Curiosity rover in Gale crater, Mars, during the Noachian time. Its exposures are identified by dark gray color, smooth surfaces, hummocky morphology, and sparse impact craters on HiRISE images. Mast-mounted cameras (Mastcam) images show that the HPU underlies all other strata suggesting that it is the oldest unit in the study area. Mars Hand Lens Imager (MAHLI) and Mastcam images indicate that this unit consists of an unsorted conglomerate with sub-angular to rounded grains some up to 20 cm in size.

In areas where the HPU was not subjected to late erosion, it forms 300-500 m-long asymmetric ridges which are about 150 m apart. Ridges are up to 5 m high and cross-bedded indicating flow direction toward north. Supporting this conclusion is the elevations of the top surface of the HPU that decreases northward, suggesting that it formed a north-sloping (down flow) surface at the end of its deposition. These characteristics indicate that asymmetric ridges are giant gravel dunes. They are indicative of deposition by large floods rather than sedimentation by ordinary fluvial systems. Flood waters entered Gale crater through its southwestern rim and flowed northward.

Giant gravel dunes of Gale crater are identical to giant current ripples of Lake Missoula and Altai Mountains flood deposits of the Pleistocene Epoch. This suggests that Martian floods originated by glacial outbursts, as did their Earth’s counterparts. This scenario implies that Mars was cold and dry. But, high obliquity and high eccentricity under a modest atmospheric pressures caused above freezing (warm) temperatures over large areas of the Northern Lowlands and equatorial regions of Mars (including Gale crater) where liquid water accumulated. The Southern Highlands remained cold due to adiabatic cooling and melting of its glaciers produced flood waters that flowed to Gale crater. Alternatively, flood waters could be attributed to sustained heavy precipitation under a warm and wet Mars. Unless occurred very rapidly, this scenario should have also produced extensive chemical weathering at the source area. The presence of abundant unstable minerals such as olivine, pyroxene, and plagioclase in fine-grained sediments of Gale crater favors the cold and dry model.