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.
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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.
Reference:
Tajeddine et al., “Constraints on Mimas’ interior from
Cassini ISS libration measurements”, Science 17 October 2014: Vol. 346 no. 6207
pp. 322-324