POLYGONAL FRACTURE NETWORKS, GALE CRATER, MARS

HiRISE images reveal distinct networks of fractures over much of the landing ellipse of the Mars Science Laboratory (MSL) in Gale Crater, and Curiosity landed with a few hundred meters of a particularly well-developed network. The geometry of the fractures, and the uniform size and shape of the intervening polygons suggest that they formed by contraction most probably near the ground surface. They are of direct interest to the primary goal of the MSL, which is to shed light on the past and present habitability of Mars, because the contractions fractures most likely reflect the former presence liquid water, a prerequisite for life, in at least in two ways. First, the loss of water can lead to large volumetric contraction in moist cohesive surface materials. Secondly, water provides sufficient cohesion for loose surface material to transmit stresses over length scales vastly exceeding the grain size, and to fracture either by forming ice in coarser, non-cohesive granular material, or by precipitating minerals that cement adjacent mineral grains. The mean size of Gale contraction fracture polygons at particular sites where they are most distinct ranges from 12 to 21 m, and the coefficient of variation ranges from 0.26 to 0.40. The polygons resemble those active today in Antarctica suggesting that they formed when seasonal temperatures fluctuated much more than they do today, perhaps during high obliquity periods. However, geomorphic and stratigraphic settings inferred from HiRISE images suggest that the fractures could have formed due to desiccation of meters-deep, moist, fine-grained sediments. The pervasive brecciation and arrays of inverted-fractures (ridge) patterns that are most common in the southern portion of the ellipse, south of the dune field at the base of Mt. Sharp, are consistent with a longer and more complex history involving impacts and other types of fractures, burial, alteration, and exhumation. We present the characteristics and geologic settings of the various fracture networks, and their significance for deciphering events in Gale Crater’s history.