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Meet Mimas, the bullseye moon

Mimas showing Herschel Crater

NASA's Cassini spacecraft took this image of Mimas—the most heavily cratered body in the Solar System—from a distance of 103,000 kilometres. The huge Herschel Crater is prominent on the right, spanning a third of the moon's diameter.

  • Mimas, innermost of Saturn’s major moons
  • Most heavily cratered body in the Solar System
  • Herschel Crater is one-third the moon’s width

SATURN’S MOON Mimas (pronounced MY-muss or MEE-muss) looks somewhat like a bullseye when viewed from a certain angle. The feature responsible for this is the huge, 130-kilometre-wide Herschel Crater, which is a third of the diameter of the tiny moon.

If the object that struck Mimas and formed the crater had been larger or moving faster, the moon would probably have been shattered into pieces. Those pieces might have collapsed back to form a new moon or could have scattered to become another ring around Saturn.

Mimas back-dropped by Saturn

An amazing Cassini view of Mimas against the hazy limb of Saturn.

Mimas is the innermost of Saturn’s major moons, averaging 396 kilometres in diameter…not quite big enough to hold a perfectly round shape. It orbits at a distance of 185,520 kilometres from Saturn in a time of 22 hours and 37 minutes. It is also ‘tidally locked’, which means that one side always faces in toward Saturn.

Along with another of Saturn’s moons, Rhea, Mimas has been called ‘the most heavily cratered body in the Solar System‘. It’s close-in orbit means that it probably receives several times the rate of collisions with meteoroids as do the other moons of Saturn.

That it isn’t even more heavily cratered is probably because, being closer to Saturn, it was warmer (and consequently ‘softer’) for a longer time, so early features have softened or eroded away.

However, with so many impacts the youngest craters have tended to obliterate the older ones and, like Rhea, it is cratered about as much as it can get.

The craters in the southern polar region are generally 20 kilometres in diameter or less—this suggests that some melting or other resurfacing processes occurred there later than on the rest of the moon, removing any traces of larger craters. (Interestingly, the south polar region of another of Saturn’s moons, Enceladus, is the source of that moon’s geysers.)

The bullseye crater

The walls of Herschel Crater are approximately five kilometres high, parts of the floor are approximately 10 kilometres deep, and the central peaks are almost six kilometres above the floor of the crater.

A comparable crater on Earth would be 4,000 kilometres in diameter.

Shock waves from the Herschel impact might have caused the fractures—also called chasmata—that appear on the opposite side of Mimas.

Mimas showing Herschel Crater

A mosaic of Cassini images put together to give us a birds-eye view of Herschel Crater.

See the amazing full-size, high-resolution image here (will open in a new window or tab).

Mimas’ low density (1.17 times that of liquid water) indicates that it is composed mostly of water ice with only a small amount of rock. It seems to be solidly frozen at a temperature of -209 degrees Celsius.

This is puzzling because Mimas is closer to Saturn than Enceladus, and its orbit is much more eccentric (out of round) than Enceladus’ orbit. Thus, Mimas should have much more tidal heating than Enceladus. (Tidal heating occurs when the gravity of another body, in this case Saturn, pulls and compresses a moon’s solid body, creating heat.)

Yet, Enceladus has geysers of water, while Mimas has one of the most heavily cratered surfaces in the Solar System. This suggests that Mimas’ frozen surface has persisted for a very long time.

The paradox has led astronomers to use the ‘Mimas test’, by which a hypothesis that explains the partially thawed water of Enceladus must also explain the entirely frozen water of Mimas.

Mimas apparently sweeps out the 4,800-kilometre-wide gap—called the Cassini Division—between Saturn’s two widest rings, the A and B rings. Observations from NASA’s Cassini spacecraft have revealed that there is still some ring material in the Cassini Division, although it is sparse enough that the area appears empty from a distance.

Mimas and Saturn and its rings

Tiny Mimas is dwarfed by Saturn and its rings.

Moons that stick together

Mimas is in orbital ‘resonance’ with two nearby moons, Dione and Enceladus. That is, these moons speed up slightly as they approach each other and slow down as they draw away, causing their orbits to vary slightly in a long series of complex changes, which help keep them locked in their positions.

The gravity of Mimas strongly affects the tiny 3km-diameter moon Methone, the 4km-diameter moon Pallene, and the 2km-diameter moon Anthe, all of which orbit between Mimas and the next major moon going out from Saturn, Enceladus.

The vastly more massive Mimas causes Methone’s orbit to vary by as much as 20 kilometres. The affect is larger for tiny Anthe, and slightly smaller for Pallene.

English astronomer William Herschel discovered Mimas in 1789. His son, John Herschel, suggested that the names of the moons of Saturn be associated with Greek mythical brothers and sisters of Kronus (known to the Romans as Saturn).

The name Mimas comes from the god (or Titan) Mimas in Greek mythology, who was slain by one of the gods of Olympus in the war between the Olympians and the Titans. Different accounts have Mimas dispatched by Hercules, by Ares (the god of war), or by Zeus himself using a thunderbolt. Legend has it that the island of Prochyte near Sicily rests on his body.

For years, ground-based astronomers could only see Mimas as little more than a dot, until the Voyagers 1 and 2 spacecraft flew past and imaged it in 1980. The Cassini spacecraft has made several close approaches and provided detailed images of Mimas since it achieved orbit around Saturn in 2004.

Adapted from information issued by NASA / JPL / Space Science Institute.

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