Constraints on Mimas’ interior from Cassini ISS libration measurements

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Science  17 Oct 2014:
Vol. 346, Issue 6207, pp. 322-324
DOI: 10.1126/science.1255299

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Like our Moon, the majority of the solar system’s satellites are locked in a 1:1 spin-orbit resonance; on average, these satellites show the same face toward the planet at a constant rotation rate equal to the satellite’s orbital rate. In addition to the uniform rotational motion, physical librations (oscillations about an equilibrium) also occur. The librations may contain signatures of the satellite’s internal properties. Using stereophotogrammetry on Cassini Image Science Subsystem (ISS) images, we measured longitudinal physical forced librations of Saturn’s moon Mimas. Our measurements confirm all the libration amplitudes calculated from the orbital dynamics, with one exception. This amplitude depends mainly on Mimas’ internal structure and has an observed value of twice the predicted one, assuming hydrostatic equilibrium. After considering various possible interior models of Mimas, we argue that the satellite has either a large nonhydrostatic interior, or a hydrostatic one with an internal ocean beneath a thick icy shell.

What's inside Saturn's tiniest moon?

The icy body Mimas is the smallest of Saturn's main moons, only slightly wider than Switzerland. Like our own Moon, Mimas is tidally locked in its orbit and shows nearly the same face to Saturn at all times. The rotational and orbital periods continually overtake each other slightly, however, so that the moon would appear to rock back and forth as viewed from Saturn. Tajeddine et al. measured these movements with the Cassini spacecraft to see what they reveal about the moon's interior. Surprisingly, the data are consistent with models for a subsurface ocean or an elongated core.

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