What is the Herschel crater?

Mimas, Saturn’s closest moon, looks like the Death Star with its massive impact crater

Image credit: NASA/JPL/SSI

Of Saturn’s major moons, Mimas is the closest to the planet at 185,520 kilometres away. The moon is believed to have created the Cassini Division, a 4,800-kilometre gap between Saturn’s A and B rings. Mimas has an average diameter of 396 kilometres, with an ovoid shape. This is due to its low surface gravity – about one 25th that of Earth’s moon – as well as the strong gravitational pull from Saturn. The same side of Mimas always faces Saturn, and it has an asynchronous rotation (meaning that it takes the same amount of time to both orbit and rotate on its axis) of 22.5 hours.

Mimas has a very low density, about 1.17 times that of water, so astronomers believe that it probably comprises a small rocky core with an outer layer of ice. It appears to be solidly frozen at about 64 Kelvin. The moon’s main geological features are chasms and impact craters.

Mimas is best known for its massive Herschel crater, however. This crater has a diameter of 130 kilometres, about a third of the moon’s own diameter. Its walls are about five kilometres high, and it has areas that are 10 kilometres deep. If a crater of the same scale were found on Earth, it would be wider than the entire country of Canada.

The Herschel crater mystery

Mimas’s most distinguishing feature is also something of a mystery. Astronomers cannot figure out why the force necessary to create such a wide, deep crater didn’t destroy the moon completely. The massive impact appears to have left fissures on the opposite side of the moon, although these may also be the result of cracking in its icy surface. If Mimas had been destroyed, its remaining pieces might have become other Saturnian moons or even formed another ring around the planet. It is not known exactly what caused the crater, which has an unusual, hexagonal shape. It could have been a massive meteor, or rubble that broke away during the formation of Saturn’s moons.

This article was originally published in How It Works issue 47

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