Structure of Polygonal Impact Craters at Meridiani Planum, Mars, and a Model Relating Target Structure to Crater Shape

Introduction: Among the classic field-and laboratory studies of crater structure, a handful have mentioned the influence of fracture networks upon crater shapes [1,2,3] and few later investigations have addressed the problem directly (see [4] for a recent study and thorough review). A polygonal simple crater is thought to result when the excavation flow exploits planes-of-weakness in the target, so that fracture sets approximately bisect crater corners. The straight walls of complex craters are thought to align with regional joints because crater walls collapse along pre-existing fractures. The ongoing MER mission at Meridiani Planum provides a unique chance to test ideas about the relationship between target structure and crater shape. Structure of Endurance Crater and fracture networks at Meridiani: Terrain meshes generated from MER NAVCAM and PANCAM stereo pairs were used to measure the strike and dip of bedding planes in the sedimentary rocks exposed in the walls of Endurance Crater. In this way, we have located the hinge of an overturned flap in the rim walls, and have identified faults with significant scissors-type displacement. Multi-spectral PANCAM images have been used to map and describe the deformation of lay-ering in the crater walls, as well as the inverted strati-graphy exposed in the rim. Especially noteworthy is that layers exhibit a festoon shape, suspended between corners of the Endurance polygon (Fig. 1), suggesting possible tear faults where the excavation flow exploited pre-existing fractures. In MOC imagery, the plains of Meridiani Planum appear dissected by fracture-like lineaments (Fig. 2): the "Anatolia" trough system that Opportunity has since visited en route to Endurance crater [5]. We have measured the orientations of these troughs in two MOC cPROTO images in the vicinity of Endurance (MOC cPROTOs R1104449 and R1602188). Both images separately exhibit bimodal distributions, with centers at ~35º and ~-12º from North. (The illumination angles are orthogonal.) More recently, Opportunity has encountered a system of fractures measuring up to 20 cm in width (Fig. 2c) and up to ten meters in length. Using the NAVCAM terrain meshes we have confirmed that a dominant mode of the widest fractures align with the dominant mode of the trough system, suggesting that the troughs correspond to fractures into which the plains sands have drained over time.