Rhea's Evolution: Undifferentiated? Hydrostatic?
Abstract
Measurements by the Cassini spacecraft have yielded Rhea's degree-two, gravity coefficients (Anderson and Schubert, GRL 34, 2007). Assuming that Rhea is in hydrostatic equilibrium, these authors have inferred that Rhea is undifferentiated.
On the contrary, we find that Rhea is probably at least partially differentiated and should not be in hydrostatic equilibrium. We have run coupled thermophysical-dynamical evolution models of Rhea using the approach of Castillo-Rogez et al. (Icarus 190, 2007). We find that even in the "coldest” possible model for Rhea, some differentiation occurs early in the satellite's history. This is accompanied by eccentricity damping that prevents significant, long-term tidal heating. Warmer models provide an environment conducive to geological activity, consistent with recent high-resolution mapping of the satellite (Schenk and Moore, LPS 38, 2007). Our models also yield the current orbital properties for Rhea and provide a rationale for non-hydrostatic anomalies. These could affect the gravity measurements due to Rhea's shape being frozen early in its history. Its dynamical properties subsequently evolved. Thus a fossil bulge is expected. A major uncertainty in interpreting the gravity data comes from potential large-scale topographic anomalies. The current uncertainty in the shape measurements is about 1.7 km. If Rhea has a large-scale topographic anomaly of that amplitude, that anomaly would swamp part of the gravity signal, and prevent meaningful interpretation. More measurements of Rhea's gravity field as well as refined shape data are needed to improve our understanding of Rhea's interior and thus to further constrain the orbital and thermal history of this satellite. This work was carried out at the Jet Propulsion Laboratory-California Institute of Technology, under contract to NASA.- Publication:
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AAS/Division for Planetary Sciences Meeting Abstracts #39
- Pub Date:
- October 2007
- Bibcode:
- 2007DPS....39.1107M