The gravity field of Enceladus from the three gravity flybys

The Cassini spacecraft carried out gravity measurements of the small Saturnian moon Enceladus during three close flybys on April 28, 2010, November 30, 2010 and May 2, 2012 (designated E9, E12 and E19), at the low altitudes of 100, 48 and 70 km to maximize the accelerations exerted by the moon on the spacecraft. The goals of these observations were the determination of the gravitational quadrupole and the search for a North-South asymmetry in the gravity field, controlled primarily by the spherical harmonic coefficient C30. The estimation of Enceladus’ gravity field is especially complex because of the small surface gravity (0.11 m/s2), the short duration of the gravitational interaction and the small number of available flybys. In addition to the gravitational accelerations, the spacecraft was also subject to small but non-negligible drag when it flew through the plume emitted from the south pole of the satellite. This effect occurred during the two south polar flybys E9 and E19. The inclusion of these non-gravitational accelerations proved to be crucial to attain a stable solution for the gravity field. Our estimation relied entirely on precise range rate measurements enabled by a coherent, two-way, microwave link at X-band (7.2-8.4 GHz). Measurement accuracies of 10 micron/s at 60 s integration times were attained under favorable conditions, thanks also to an advanced tropospheric calibration system. The data were fitted using the MONTE orbit determination code, recently developed by JPL for deep space navigation. In addition to the satellite degree 2 gravity field and C30, the solution included the state vector of the spacecraft (one for each flyby) and corrections to the mass and the initial orbital elements of Enceladus. The effect of the drag in E9 and E19 was modeled either as an unknown, impulsive, vectorial delta-V at closest approach, or by using density profiles from models of the plume and solving for the aerodynamic coefficient of the spacecraft. Both approaches led to statistically identical gravity coefficients. The impulsive delta-V estimated in the orbital fit is almost parallel to the spacecraft velocity, as expected for an aerodynamic drag. The uncertainty of other known non-gravitational accelerations, such as Cassini RTG’s anisotropic thermal emission and solar radiation pressure, were also included when the covariance matrix is formed.  As expected, the C20 and C22 harmonic coefficients, associated to rotational and tidal deformations, dominate the gravity field. Their estimate is accurate to less than 1% (1-sigma). The other three quadrupole coefficients are consistent with a null value at a 2-sigma level. C22 is about one order of magnitude larger than C30, estimated to a relative accuracy of about 20%. The sign of C30 implies a negative mass anomaly at the south pole (consistent with the observed topography) and allows for the presence of a compensating positive anomaly at depth, such as a melt pocket. Although Cassini will fly by Enceladus two more times in 2015, E19 was the last opportunity to carry out gravity measurements. Our nearly final analysis of Cassini gravity data provides evidence for a large liquid reservoir at depth in the southern hemisphere of the satellite, as described elsewhere in this meeting.