The meteorite bombardments and volcanic eruptions that scarred the surface of Mars ended billions of years ago. But weathering and erosion are still steadily reshaping the Martian surface. How quickly those processes occur is the topic of an investigation by Tjalling de Haas of Utrecht University in the Netherlands and his colleagues. To derive his estimate, de Haas exploited a system of four overlapping alluvial fans of widely different ages that spill from a crater in southern Mars. The fans, which consist of rocks of various sizes, formed in the same way as the terrestrial fan in the photo did: Water once carried debris down a slope whose abrupt reduction in gradient caused the debris to spread outward. Images taken by the HiRISE instrument aboard NASA’s Mars Reconnaissance Orbiter reveal boulders as large as 5 meters on the youngest fan, but no boulders larger than the instrument’s resolution of 0.25...

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