The breakup of Martian boulders

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 meters remain on the older fans, presumably because they have since disintegrated. How long did that reduction in size of at least 4.75 meters take place? The youngest fan must have formed sometime before meltwater from the last Martian glaciers ceased flowing (0.4 million years ago). The second- and third-youngest fans, unlike the oldest, lack cratering; they must have formed sometime after the last bombardment (1.25 million years ago). The boulders’ implied breakdown rate of at least 3.5 m/Myr is far higher than estimates derived from crater morphology and suggests the action of a substance known to accelerate weathering and erosion: liquid water. (T. de Haas, E. Hauber, M. G. Kleinhans, Geophys. Res. Lett., in press .)