Effect of Enceladus's rapid synchronous spin on interpretation of Cassini gravity

Abstract

Enceladus's degree 2 gravity, determined by Cassini, is nominally nonhydrostatic to 3σ (J₂/C₂₂ = 3.38-3.63, as opposed to 10/3). Iess et al. (2014) interpret this in terms of a hydrostatic interior (core) and isostatic (not hydrostatic) floating ice shell. Enceladus's rapid (1.37 d) synchronous spin and tide distorts its shape substantially, though, enough that the predicted hydrostatic J₂/C₂₂ is not 10/3 but closer to 3.25. This leads to the following revision to the internal picture of Enceladus, compared with Iess et al.: (1) the satellite's core is somewhat smaller and slightly denser (190 km radius and 2450 kg/m³); (2) the compensation depth (shell thickness) of the global (degree 2) ice shell is ≈ 50 km, rather close to the base of the modeled ice + water layer; and (3) the compensation depth (shell thickness) beneath the South Polar Terrain (from J₃) remains shallower (thinner) at ≈ 30 km, independent of but influenced by the degree 2 solution.