Cassini’s causing quite a stir with the images and data it’s returning from a certain moon of Saturn these days. But the buzz around Enceladus, and the confirmation of a watery ocean beneath the crust of ice on its surface, has taken our attention away from some of Saturn’s other notable satellites. This is despite the fact it’s the contrast between Mimas and Enceladus that make both particularly interesting.
Mimas is a somewhat oblate (squashed spheroid) moon that is an average of 397 kilometres (247 miles) in diameter. It’s also a pretty low-density object of around 1.17 times the density of liquid water, which suggests that most of this moon is made of water ice, with a small proportion of rock, frozen solid at a temperature of around -209 degrees Celsius (-344 degrees Fahrenheit). This caused an interesting problem for scientists, because not only is Mimas closer to Saturn than Cassini’s current favourite target, Enceladus, but it has a more-eccentric orbit that should subject the moon to the greater tidal heating of Saturn’s powerful gravitational field.
Geologically speaking Mimas is dead, while Enceladus is still spouting great geysers of water ice into its atmosphere, along with a range of organic compounds. This paradox of Saturn’s moons has been explained, in part at least, by the higher density of Enceladus, which likely has a much greater rock content. It’s thought that Enceladus’ liquid interior and Mimas’ solid body are explained by the same theory.
Mimas’ permanently frozen state has persisted for billions of years, probably since it was formed. The evidence for this is the dense population of impact structures on its surface, rivalled by few other objects in the Solar System. A sister moon, Rhea, has a comparable number of craters to Mimas, but has no single feature quite as impressive as the enormous Herschel crater. With a diameter of around 140 kilometres (87 miles), walls five kilometres (three miles) high and a floor ten kilometres (six miles) deep, this is by far the biggest crater on this tiny satellite.
Proportionally, it’s one of the biggest impact structures in the Solar System, too. From rim to rim, it’s spread across one third of the face of Mimas: if Earth had an equivalent crater, it would stretch around 4,000 kilometres (2,500 miles) wide and have walls over 200 kilometres (124 miles) high. It’s a wonder that the impactor that created this crater didn’t smash Mimas into fragments, because its legacy literally runs deeper than the 4.1 billion-year- old crater it left behind.
The enormous energy of the impact must have travelled as a shockwave through Mimas to the other side, where massive stresses on the surface cracked open into chasmata, like the Ossa Chasma. The impact likely played a role in the strange temperature pattern of Mimas, which bears an uncanny resemblance to the classic videogame character Pac-Man. Herschel’s other legacy to pop culture is also pure coincidence: from certain angles, in images taken both by Cassini and Voyager 1, it looks distinctly like the Death Star from Star Wars Episode IV. Voyager 1 discovered Herschel three years after the film was made, so Mimas couldn’t have inspired the fictional super weapon.
The moon is also responsible for clearing most of a huge region in Saturn’s orbit, creating a 4,800-kilometre- (2,980-mile-) wide gap between two of Saturn’s widest rings, A and B. This is known today as the Cassini Division.