Gravity and topography signatures of shallow water bodies in the subsurface of Europa and Ganymede

Kurzfassung

The Jupiter moons Ganymede and Europa are prime targets for icy moons exploration by ESA’s JUICE and NASA’s Europa Clipper missions [1,2]. Future measurements by JUICE and Europa Clipper will provide key information about the ice shell structure and the depth of the subsurface oceans of these moons. While the ocean itself is the largest water body beneath the surface, liquid brine reservoirs may be present locally within the ice shell, in the shallow subsurface. These reservoirs may represent niches for habitability that may provide ideal targets for exploration because of their location close to the surface. Evidence for the presence of shallow water reservoirs within the ice shell of Europa has been presented in a recent study that performed detailed geomorphological-structural investigations of Menec Fossae [3]. The observed tectonic activity in this region on Europa could be related to a shallow water pocket located close to the surface that would explain the observed overall topography of this area in addition to the presence of specific geological features such as chaos terrain and double ridges. On Ganymede, possible past cryovolcanic activity was suggested in a few isolated spots on the surface, the so-called “scalloped depressions” (“paterae”), which have been interpreted as possible caldera-like features [4] and could be potentially sourced from shallow water bodies. While the low-resolution data currently available prevents a precise characterization, age estimates, and composition of these regions, future measurements by JUICE will reveal the origin and formation mechanism of Ganymede’s paterae. In this work, we perform numerical modeling of the outer ice shell of Ganymede and Europa to test the expected gravity and topography signatures of shallow water bodies. We vary the size and location beneath the surface of such reservoirs. Moreover, since the composition and physical state (i.e., liquid state or solidified state) of such reservoirs is poorly constrained but affects the density in these regions, we test different values for density anomalies. In our models, we assume that these reservoirs are located within the conductive part of the ice shell, close to the surface. In order to quantify the effect of large-scale dynamics on the gravity and topography signal induced by shallow density anomalies, we test scenarios in which the entire ice shell is purely conductive (no additional density anomalies) and cases where the deeper ice shell is convective (additional density anomalies due to solid-state convection). Our models will provide scenarios that can be tested with current data, where resolution permits, and help to interpret future measurements. This will help us to locally constrain the structure of the ice shell and determine the presence of shallow water bodies in the subsurface of Ganymede and Europa.