Blast Beds at the Rover Sites on Mars

Introduction: Ancient, variably salty, friable, basaltic sedimentary beds with relatively uniform grain sizes and prominent, remarkably shallow crossbedding have now been imaged at the Martian surface by all three Mars rovers, Spirit (MER1), Opportunity (MER2), and Curiosity (MSL). Where they have been found, these beds overlie other, older rocks. Owing to alleged analogies with terrestrial sediments deposited by wind, water, and volcanism, these beds have been variably interpreted as aeolian, fluvial, lacustrine, or volcanic, despite their highly uniform appearance and composition from landing sites to landing site. They are not usually interpreted as having possibly resulted from meteorite impact, despite the impact-dominated early history and heavily cratered surface of Mars [1]. These beds were first encountered by the Opportunity rover in Meridiani Planum, where they cover essentially the entire plain, except where still more ancient impact breccias and altered glasses were recently encountered around the edge of Endeavor Crater. Although these beds were inititially interpreted to have formed in an evaporitic, semi-marine environment, published work later attributed them to windblown salts and sands eroded from a vanished dry lake or sea, as well as to water flows in vanished streams. The uniformly-shaped and sized, generally unclumped tiny spherules that these beds contain, left behind by wind erosion as an extremely widespread lag deposit, were interpreted as concretions formed in groundwater, despite multiple features inconsistent with typical concretions and distinctive other features inconsistent with the presence of groundwater [2][3].