The determination of the gravity field by Cassini is challenging because of the small mass and short duration of the gravitational interaction, even with data from three encounters. E19 data have been successfully integrated into the multiarc analysis, providing a stable and consistent gravity field. This required inclusion of the effect of atmospheric drag due to Enceladus' plumes. This presentation will deal only with the interpretation of these data. The dominant features of the non-central gravity are large values for the harmonic coefficients J2 and C22 and a much smaller but statistically significant negative J3. The value of J2/C22=3.55×0.05 is moderately in excess of the value of 10/3 that applies to a synchronously rotating body with no lateral variation in material properties. Given the obvious latitudinal variation of Enceladus' physical characteristics, primarily expressed by the activity centered on the South Pole, it is plausible that the deviation from 10/3 arises primarily because of a positive anomaly in J2 rather than any anomaly in C22. However, applying Radau-Darwin to the value of C22/q (where q is the usual dimensionless measure of the centrifugal effect on gravity) implies that the moment of inertia is about 0.34MR^2. The high heat output and indirect inference for liquid water suggests a fully differentiated Enceladus. For the known mean density and any plausible mantle density, this would require an unreasonably low core density of 2.5 g/cc or less. A more realistic interpretation is that both J2 and C22 are modestly non-hydrostatic, but that J2 is affected more because of a negative mass anomaly in the Southern hemisphere, consistent with the observed negative J3. One non-unique way to reconcile the observed gravity with a realistic MOI of 0.32 to 0.33MR^2 is to assume that the rocky core of Enceladus has retained some memory of a previous faster rotational state. Even if the ice shell is perfectly relaxed, this reconciles the data for a core shape corresponding to a rotation that is only a few to 10% larger then the current value, small compared to the total change expected for the expansion of the assumed synchronous orbit over the age of the Saturnian system. Other possibilities will be discussed. We assume that the only source of additional gravity beyond that from rotation and tides, past and present, is a mass anomaly centered at the South pole. A negative point mass anomaly then predicts a contribution to J2 that is of the same magnitude but opposite sign to J3. The inferred J2 anomaly is of the right sign but about a factor of two larger than this point mass prediction. This can be understood by assuming that that the mass anomaly is extended in latitude away from the pole, consistent with the observed extent of the South pole topographic depression. A physically plausible explanation that explains both the J2 anomaly and J3 is to invoke this observed depression of nearly 1km as a negative mass anomaly. A compensating ocean of around 10km in thickness at the base of the ice (depth ~50km) is not essential to explain the gravity but is consistent with the gravity and plausible based on other geophysical considerations. More precise topography and gravity in a future mission could strengthen the case for an ocean but ambiguity will always be present because of the uncertain nature and shape of the core.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- 5417 PLANETARY SCIENCES: SOLID SURFACE PLANETS Gravitational fields;
- 6282 PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS Enceladus