Constraints on Mimas’ interior from Cassini ISS libration measurements

Abstract

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.

CassiniISS astrometry of the Saturnian satellites: Tethys, Dione, Rhea, Iapetus, and Phoebe 2004–2012

“…Some suggest that there is an internal global ocean under Enceladus' icy shell (Nimmo & Pappalardo 2006;Lainey et al 2012), others suggest a subsurface "sea" localized in its south polar region (Collins & Goodman 2007;Iess et al 2014). Meanwhile, the physical forced libration measurements (Tajeddine et al 2014) suggest that Mimas has a more complex Full Table 1 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/575/A73 interior than was previously believed and may have a subsurface ocean.…”

Classification of Satellite Resonances in the Solar System

“…As a sanity check, we compare this value with 0.069 predicted by Equation (18). The agreement is satisfactory although we note that according to Tajeddine et al (2014), the difference between a more accurate model calculation for ( ) -B A C and the value deduced from the physical libration hints that Mimas may have an ocean that comes within 30 km of its surface and reduces C by decoupling the interior from the external ice shell.…”

“…Probably this explains why astrometry has been unable to determine the obliquity, ò, although it has revealed an amplitude of  0 . 84 for Mimas's physical libration which implies ( )  -B A C 0.09 (Tajeddine et al 2014). As a sanity check, we compare this value with 0.069 predicted by Equation (18).…”

“…Thermal activity of Enceladus, Io, and Europa are outstanding examples of tidal heating in current resonances. It is less certain but plausible that Mimas has an internal ocean that reduces the apparent polar moment of inertia as deduced from the amplitude of its physical libration (Tajeddine et al 2014). This could be a consequence of its involvement with the MMRs mentioned in the preceding paragraph.…”

“…Tajeddine et al (2014) There is an alternative scenario for the unexpectedly large physical libration of Mimas that does not involve an internal ocean. Perhaps Mimas has a nonhydrostatic shape which enhances ( ) -B A C above its hydrostatic value (Tajeddine et al 2014). We estimate…”