Enceladus's internal ocean and ice shell constrained from Cassini gravity, shape, and libration data

Abstract

The intense plume activity at the South Pole of Enceladus together with the recent detection of libration hints at an internal water ocean underneath the outer ice shell. However, the interpretation of gravity, shape, and libration data leads to contradicting results regarding the depth of ocean/ice interface and the total volume of the ocean. Here we develop an interior structure model consisting of a rocky core, an internal ocean, and an ice shell, which satisfies simultaneously the gravity, shape, and libration data. We show that the data can be reconciled by considering isostatic compensation including the effect of a few hundred meter thick elastic lithosphere. Our model predicts that the core radius is 180–185 km, the ocean density is at least 1030 kg/m3, and the ice shell is 18–22 km thick on average. The ice thicknesses are reduced at poles decreasing to less than 5 km in the south polar region.

Internal Energy Dissipation in Enceladus's Subsurface Ocean From Tides and Libration and the Role of Inertial Waves

“…Current models, however, have so far been unable to account for the observed ∼10 GW of endogenic heat flow emanating from the south polar region (Howett et al, ; Spencer et al, ) without invoking the presence of a highly porous (“fluffy”) solid inner core (Choblet et al, ; Roberts, ) or a convecting icy crust (Běhounková et al, , ). As suggested by Barr and McKinnon (), this latter hypothesis appears unlikely in view of the relatively small thickness of the crust (∼20 km) deduced from geodetic observations (Beuthe et al, ; Čadek et al, ).…”

The impact of a pressurized regional sea or global ocean on stresses on Enceladus

“…The northern sea angle is therefore somewhat smaller than the southern sea angle, according to equation . Čadek et al [] concluded that the ice is about twice as thick the north pole as it is at the south pole but this does not translate into a simple ratio for the thicknesses of the two indentations. It should be noted that variations in ice thickness in Čadek et al [] assume constant density ice whereas Schenk and McKinnon [] argued for the importance of Pratt isostasy.…”

“…These various models can be reconciled if the ice is thinner at the south pole even though the extent of the ocean is global. Such an interior structure model is consistent, for example, with Enceladus's spin state [ McKinnon , ] and joint inversions of gravity, shape, and libration [ Čadek et al ., ; Thomas et al ., ]. The latter studies imply ice thicknesses of only 2 to 4 km at the south pole, where a thicker ice, 30 to 60 km thick, was deduced by McKinnon [].…”

“…Based on the quadrupole gravity field of Enceladus, Iess et al [] proposed that a regional sea would extend to ~50°S. Interestingly, Čadek et al [] infer a second region of thin ice centered near the north pole. In their model, the ice at the north pole is not thinner than ~10 km, about twice the minimum thickness they infer at the south pole.…”

Climatologie comparée des planètes : un bref état des connaissances

“…En orbite autour de Saturne, Encelade est l'une de ces « lunes glacées » des planètes géantes gazeuses (voir aussi, autour de Jupiter, Ganymède et Europe), qui abriteraient un profond océan global sous une épaisse banquise permanente de plusieurs kilomètres d'épaisseur, elle aussi globale (Čadek et al, 2016 ;Nimmo et Pappalardo, 2016). La surface d'Encelade, composée de glace d'eau dont l'albédo est proche de celui de la neige fraîche, est soumise à une température moyenne de -198°C (Grundy et al, 1999 ;Verbiscer et al, 2007 ;Howett et al, 2010).…”