‘Death Star’ moon harbours more secrets

It’s a moon that looks like the planet-killing Death Star from Star Wars, but while it might not be hiding a captured princess, an important discovery may lie beneath its surface.

Mimas is the nearest of Saturn’s major moons to the yellow planet, completing an orbit in 22 hours.

Its highly cratered surface – which includes the 130km wide ‘Herschel’ crater, reminiscent of the laser dish from the Star Wars space station – has puzzled scientists for years.

A small grey moon with a large impact crater in its northern hemisphere
Mimas. Credit: JPL/ASA/JPL-Caltech/Space Science Institute

That’s because one of its fellow moons – Enceladus – is known to billow geysers of water. While Mimas’ more eccentric orbit (it doesn’t zip around its host planet in a uniform circle) and closer proximity to Saturn suggests it should be more likely to do the same, it instead appears inactive, with crater impacts largely preserved in its frozen outer shell.

Now, French scientists have published another unlikely characteristic of the moon. Beneath its icy surface is likely to be a subsurface ocean.

This ocean, located 20-30km beneath Mimas’ crust, is relatively “young” and still evolving according to the study published in the journal Nature – about 25 million years old.

The researchers from the Paris Observatory in France used data from NASA’s Cassini probe and mathematical models of the moon’s orbit to determine very specific features of its journey around Saturn could only be explained by a subsurface global ocean.

That puts Mimas into a family of moons with this water layer, including Europa and Ganymede (orbiting Jupiter) and fellow Saturnian satellites Enceladus and Titan.

In publishing the findings, the Paris Observatory also hopes Mimas will become another candidate to explore life-supporting conditions in the solar system.

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