Pit crater formation: Laboratory simulations and applications to Mars
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Pits craters are circular to ovoid collapse features observed on Earth and most solid bodies in the Solar System. To better understand the origins and evolution of pit craters and chains, laboratory simulations (using layered sand and sand mixed with flour deposited on two horizontal wooden that can be incrementally separated) were conducted. As the opening between plates increased, isolated, sub-circular pit craters evolved into elongated, coalesced craters or troughs, with adjacent parallel fractures (trending perpendicular to the direction of extension) becoming more evident and the width of the pits remaining unchanged. A relationship was found between the spacing between the adjacent parallel fractures and the total thickness of material: the ratio between these parameters ranges from 1.13 to 1.88 in the laboratory simulations. Laboratory pit-crater chains were compared to measurements of Martian pit craters observed in high-resolution (<1 m/pixel) images; qualitative and quantitative comparisons suggest that the measured Martian pit craters also formed via collapse over a zone of extension. Applying laboratory simulation results to measurements of Martian pit craters allowed estimation of affected material thicknesses on Mars between 20 km (near Alba Mons). Such variation in thickness is likely caused by more or less accumulation of lava flows around these major volcanoes. Structural mapping of a portion of Arsia Mons' northeast flank, containing abundant troughs, pit-crater chains and some sinuous rilles, was completed. Within the map area, structural mapping established that pit craters are older than other features such as chaotic terrains, rilles, or sinkholes, and (for the most part) younger than troughs. Both pit chains and troughs generally trend in the same direction (NE to NNE) and could have originated through a common volcanic rifting event (or events) affecting Arsia Mons. Results from the work presented here support the concept that extensional fracturing and dilational faulting are common mechanisms for Martian pit crater formation.