Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Who laid that? (Image: NASA/JPL-Caltech/Space Science Institute)

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.

Clutch of lunar eggs

In fact, this 5-kilometre-wide moon is one of a clutch of space eggs, all orbiting Saturn in the same region between the larger moons Mimas and Enceladus. None of its siblings have been imaged as closely as Methone, but from a distance two – Pallene and Aegaeon – appear to be fairly smooth as well.

Each moon lies within its own ring arc, a fragmentary ring of Saturn. One idea was that ice crystals in the ring arc might be settling down on the surface of Methone to bury any craters or other topography. Something similar has probably happened to two other moons of Saturn, the flying-saucer-shaped Atlas and Pan. Material from Saturn’s rings seems to have piled up around each moon’s equator.

But Thomas points out that the ring arcs are much more tenuous than the main rings, so much so that it would take ten billion years for just 1 metre of obliterating snow to accumulate on Methone.

Thomas and his colleagues at Cornell have now attempted to crack the egg and look inside – by calculating Methone’s density.

They assumed that Saturn’s gravity pulls the moon into its elongated shape, just as Earth’s moon raises tides in our oceans. Then the team worked out how dense the moon would have to be for its own gravity to balance those tidal forces and create the egg-like shape.

It comes out at about 300 kilograms per cubic metre, as they reported in March at the 44th Lunar and Planetary Science Conference in The Woodlands, Texas. That’s less than a third the density of water, making Methone less dense than any other known moon or asteroid.

Fluffy fluid?

Material at Methone’s surface could be even more lightweight, says Thomas. It is possible that such fluffy stuff can flow, at least on timescales of thousands or millions of years, to erase any crater scars.

The team also suggests that electrostatic forces could help keep the egg soft-boiled. Electrons in Saturn’s radiation belt could be charging ice crystals on the surface, levitating them and making them more mobile. But so far these are just speculations.

“We are just beginning to try and figure out quantitatively how all this might smooth a surface,” says Thomas. Cassini might collect some indirect clues by analysing the ring arc material, but it is unlikely to come close to the moon again before the mission ends in 2017.

Meanwhile, planetary scientists are trying to work out how often crater-forming impacts should happen in this neighbourhood, which will tell us how fast Methone must be reforming its surface to cover up the damage and maintain its eggy appearance.