Modelling the interior of Enceladus : a combined view from gravity, topography, and libration

Abstract

The year 2017 saw Cassini plunge to its demise into Saturn's atmosphere, putting an end to a twenty-year odyssey that challenged our beliefs on the possible presence of water and life elsewhere in the Universe. The Cassini-Huygens mission left a rich legacy of observations of Saturn and its ever-evolving world of rings and moons. The intense geological activity discovered throughout Enceladus's south-polar terrain is a puzzle in itself: where are the geysers' water supplies? how abundant is water? has it always remained unfrozen, and if so, how? The purpose of this thesis is to address some of these questions using Cassini's measurements of Enceladus's gravity field, surface topography, and diurnal libration. The latter, a subtle modulation of the moon's spin rate over its eccentric orbit around Saturn, induces slight offsets in images taken by Cassini's cameras. To correctly interpret the observations, we develop new, mutually consistent isostasy and libration models, correct to the second order in the flattenings associated with the moon's triaxial shape. We then show that the liquid water reservoir must be a global ocean as opposed to a regional sea, and obtain the best post-Cassini constraints on the thickness of the icy crust, the ocean, and the core. To alleviate the mathematical burden, we present a systematic method to handle equations in near-spherical geometries based on series expansions in powers of the flattenings, and prove their convergence. By sharing our code TenGSHui, we hope to entice the community into more reproducible and intuitive modelling, and reconnect global geodynamics with its most fundamental principles.(SC - Sciences) -- UCL, 201