  During its mission in the Saturn system, Cassini performed five close flybys
of Dione. During three of them, radio tracking data were collected during the
closest approach, allowing estimation of the full degree-2 gravity field by
precise spacecraft orbit determination.
  The gravity field of Dione is dominated by $J_{2}$ and $C_{22}$, for which
our best estimates are $J_{2} \times 10^6 = 1496 \pm 11$ and $C_{22} \times
10^6 = 364.8 \pm 1.8$ (unnormalized coefficients, 1-$\sigma$ uncertainty).
Their ratio is $J_{2}/C_{22} = 4.102 \pm 0.044$, showing a significative
departure (about 17-$\sigma$) from the theoretical value of $10/3$, predicted
for a relaxed body in slow, synchronous rotation around a planet. Therefore, it
is not possible to retrieve the moment of inertia directly from the measured
gravitational field.
  The interior structure of Dione is investigated by a combined analysis of its
gravity and topography, which exhibits an even larger deviation from
hydrostatic equilibrium, suggesting some degree of compensation. The gravity of
Dione is far from the expectation for an undifferentiated hydrostatic body, so
we built a series of three-layer models, and considered both Airy and Pratt
compensation mechanisms. The interpretation is non-unique, but Dione's excess
topography may suggest some degree of Airy-type isostasy, meaning that the
outer ice shell is underlain by a higher density, lower viscosity layer, such
as a subsurface liquid water ocean. The data permit a broad range of
possibilities, but the best fitting models tend towards large shell thicknesses
and small ocean thicknesses.
