Object: Oval moon Methone
Location: Saturn’s rings
Out among Saturn’s menagerie of moons, a shiny white egg rests in a nest of ice crystals.
Named Methone, this small, oval moon was seen in close-up for the first time last year by NASA’s Cassini spacecraft. Methone is utterly unlike the other small balls of ice and rock that dot the solar system, which are deeply scarred by impacts. Instead it is smooth, with not a hill or pockmark in sight. Now astronomers may have a clue as to why: Methone is made of lightweight fluff.
Objects in our solar system have been battered by asteroids and comets for billions of years, but planets and big moons have ways of smoothing themselves out. For one thing, their strong gravity pulls them into a spherical shape. Some worlds have enough internal heat to create lava flows and other volcanic events that can fill in craters, and a few boast rain or strong winds to erode away evidence of the impacts.
Small moons, though, are geologically inactive and airless, so are unable to erase the damage. “When we look at objects less than 200 kilometres in radius, they are all like potatoes. They have lumps, grooves, craters,” says Cassini team member Peter Thomas, at Cornell University in Ithaca, New York. That makes Methone’s smooth exterior a puzzle.