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Pebble beach

·1 min

It might look like a moon and orbit like a moon, but the latest observations of Phobos suggest that it’s really just an overgrown rubble pile. High-resolution photos, movies, and gravitational readings from the European Space Agency’s (ESA’s) Mars Express orbiter suggest that the larger of Mars’s two satellites resembles a certain class of asteroids that are loose agglomerations of pebbles rather than solid objects. Soon, ESA scientists say, they’ll also be able to tell whether Phobos was once a captured asteroid that was shattered by a collision or whether its present, highly irregular shape coalesced from debris lofted by impacts on the martian surface.