| The Cassini spacecraft flew by the small Saturnian moon Enceladus in three close flybys
(April 28, 2010, November 30, 2010 and May 2, 2012, to carry out measurements of the
satellite’s gravity field [1]. One of the main motivations was the search for a hemispherical
asymmetry in the gravity field, the gravitational counterpart of the striking North-South
asymmetry shown by optical imaging and other Cassini instruments in the geological features
of the moon.
The estimation of Enceladus’ gravity field by Cassini was especially complex because of
the small surface gravity (0.11 m/s2), the short duration of the gravitational interaction (only
a few minutes) and the small, nearly impulsive, neutral particles drag occurring when the
spacecraft crossed the south polar plume during the first and the third flyby. Including
the non-gravitational acceleration due to the plume in the dynamical model was
crucial to obtain a reliable solution for the gravity field. In order to maximize the
sensitivity to the hemispherical asymmetry, controlled by the spherical harmonic
coefficient J3, the closest approaches occurred at the low altitudes (respectively
100, 48 and 70 km), and at high latitudes in both hemispheres (89°S, 62°N, and
72°S).
Enceladus’ gravity field is dominated by large quadrupole terms not far from those
expected for a body in a relaxed shape. Although the deviations from the hydrostaticity are
weak (J2/C22=3.55±0.05), the straightforward application of the Radau-Darwin
approximation yields a value of the moment of inertia factor (MOIF=C/MR2) that is
incompatible (0.34) with the differentiated interior structure suggested by cryovolcanism and
the large heat flow. The other remarkable feature of the gravity field is the small but still
statistically significant value of J3 (106 x J3 = -115.3±22.9). A differentiated interior
structure (corresponding to a smaller MOIF) may be reconciled with the gravity
measurement by assuming that the rocky core has retained some memory of a faster
rotation rate (about 10% above current). J3, whose value is uncontaminated by
tides and rotation, provides a way to separate the non-hydrostatic contribution to
J2 and C22, from which we infer a MOIF of about 0.336, now compatible with a
differentiated structure. Similar conclusions are obtained from the analysis of the
admittance.
The interpretation of J3 and the associated, negative gravity anomaly (about 2.5 mGal) is
non-unique. In a proposed explanation, the anomaly originates in the core and is not directly
related to the presence of liquid masses beneath the surface. Our interpretation seeks the
source of the anomaly in the observed 1 km depression in the southern polar region. This
mass deficiency generates indeed a negative anomaly, but its magnitude is far smaller (about
20%) than expected from an uncompensated topography. An obvious source of compensation
is a reservoir of liquid water at depth, in contact with the rocky core. This interpretation is
consistent with the observed cryovolcanism and the presence of silicate grains in the
plumes. The estimated gravity field is more consistent with a reservoir that extends in
latitude about halfway to the equator, but our data cannot rule out a thin, global
ocean. |