What happened to Titan's craters? NASA's Cassini mission should have seen hundreds of impact craters on Saturn's giant moon, but so far it has only spotted a handful. The latest clues in the mystery of the missing craters suggest a conspiracy between volcanoes, rain and settling soot - perhaps aided by an eggshell-thin crust.Cassini has aimed its radar at Titan five times, mapping five narrow strips of terrain. In a paper published in Nature, the radar team analyse the second strip in detail. If Titan were like other dead moons in the outer Solar System, this strip would be scarred by perhaps 100 craters bigger than 20 kilometres across, created by cosmic impacts. But only two appear, meaning the others must have been destroyed.One is a ring 80 kilometres across called Sinlap. The other, named Menrva, is an impact basin 450 kilometres in diameter and there is a clue to the mystery on its rim - gaps in its ramparts and fluid drainage patterns nearby.

Radar team member Steven Wall at the Jet Propulsion Laboratory in Pasadena, California, US, thinks that the gaps have been cut by flowing fluids, probably the liquid methane that is thought to rain down occasionally on Titan. Methane streams might have completely washed away smaller craters.But other processes are probably at work too. "We see so much evidence for surface modification, it is likely that craters are being buried or obliterated," says Wall. On other passes, Cassini has seen bright lobes of material that are thought to be volcanic flows. Many craters might have been filled in by such cryovolcanism - floods of liquid water from Titan's interior.

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