Deimos, the smaller moon of Mars, measures just 12 kilometres across. Its surface looks oddly smooth for such an ancient object. Europe's Hera spacecraft photographed the moon during a Mars flyby in March 2025. The pictures showed features never mapped in detail before. Researchers wanted to explain the missing craters. A new study combined those images with computer models of impacts.
The authors propose that a rock roughly 320 metres wide struck Deimos long ago. Because it arrived at a slanting angle, it carved a wide shallow basin rather than a deep hole. Dust thrown up by the blow later settled back across the moon. That soft blanket buried many older craters and left the smooth face we see. Scientists therefore picture Deimos as a loose pile of broken fragments. A Japanese mission will visit both Martian moons this autumn.