Mimas is the smallest and innermost member of Saturn’s mid-sized icy moons, a group which includes Enceladus, Tethys, Dione, Rhea and Iapetus. With a diameter of 396 km, Mimas is also one of the smallest objects known in the Solar System whose self-gravitation is sufficiently strong to keep it rounded in shape. Mimas whizzes around Saturn about once every 23 hours. As it goes around Saturn, Mimas wobbles back and forth. This type of motion is called libration.
By carefully analysing images of Mimas taken by NASA’s Cassini spacecraft using a technique known as stereophotogrammetry, one particular component of Mimas’ libration was found to have an amplitude roughly twice as large as predicted. Since this component of libration depends on the interior structure of Mimas, its large value suggests that Mimas has a ‘weird’ interior. A few interior models of Mimas have been proposed to explain the large libration.
An image of Saturn’s moon Mimas taken by the Cassini spacecraft on 13 February 2010. Credit: NASA/JPL/Space Science Institute.
The 140 km wide Herschel impact crater on Mimas makes it resemble the Death Star from the Star Wars franchise. One interior model suggests the presence of a large mass buried beneath the Herschel impact crater, making Mimas more massive on one side. However, this model is inconsistent because the presence of such a large buried mass would have permanently reoriented Mimas such that the Herschel impact crater would face more towards Saturn, which is not the case.
Mimas is basically comprised of a shell of icy material overlying a denser rocky core. A more plausible interior model to explain the large libration involves Mimas having an elongated rocky core. However, such an elongated rocky core is expected to have an effect on the global shape of Mimas. If the icy shell is fully relaxed over the elongated rocky core, then the overall shape of Mimas should appear more elongated, which is not the case. Nevertheless, a low gravity object like Mimas can maintain large internal porosities which can create space for an oddly shaped core without affecting its overall shape.
A more exciting interior model of Mimas suggests that this small icy moon of Saturn might have an internal global ocean of liquid water located 24 to 31 km beneath its battered icy surface. For a small object like Mimas, it is difficult to keep an internal ocean from freezing. Heat generated from the decay of radioactive isotopes in the rocky core of Mimas would easily escape through the icy shell and cause the internal ocean to quickly freeze.
However, such an internal ocean on Mimas can still be kept liquid through heat generated from tidal heating. This is because Mimas’ orbit around Saturn is somewhat eccentric and the eccentricity may even have once been higher. As a result of its eccentric orbit, Mimas is sometimes closer to Saturn and sometimes further away. This causes Mimas to feel a difference in the gravitational pull from Saturn, which has the effect of alternately squeezing and stretching Mimas. Such a flexing motion creates friction in the interior of Mimas and friction generates heat. A tidally heated internal ocean of liquid water on Mimas is not inconceivable. Its neighbour, Enceladus, is known to have an internal ocean of liquid water sustained by tidal heating.
Tajeddine et al., “Constraints on Mimas’ interior from Cassini ISS libration measurements”, Science 17 October 2014: Vol. 346 no. 6207 pp. 322-324