Signatures of ice shell heterogeneities on Europa from gravity and topography

Many surface tectonic features on Europa have been hypothesized to form in response to liquid and brine reservoirs within the ice shell [e.g., Schmidt et al., 2011; Steinbrugge et al., 2020; Matteoni et al., 2023]. Because these reservoirs have higher densities than the surrounding ice, they cause stresses that deflect the surface, generating topographic and gravity anomalies. Although the existence of such signatures has been proposed [e.g., Schmidt et al., 2011; Michaut and Manga, 2017; Lesage et al., 2025], their expected characteristics have not been systematically quantified.

Here we investigate the topographic and gravitational signals produced by subsurface high-density deposits within Europa’s ice shell.Using both viscous and visco-elastic loading models [e.g., Richards and Hager, 1984; James et al., 2013; Maia et al., 2023], we predict the observable signatures generated by such reservoirs. We explore four key reservoir properties: (i) diameters of 10–100 km, (ii) thicknesses of 100–1000 m, (iii) density contrasts of 80–680 kg/m³ relative to water ice, and (iv) depths ranging from 1 km to 80% of the total ice-shell thickness. In addition, we perform the investigation for conductive and convective ice shells with thicknesses ranging from 10 to 40 km and test the effect on adding an elastic layer of 0.5 to 5 km thickness on the upper part of the shell.  A schematic view of the model is shown in the figure below. 

The second figure illustrates how each reservoir parameter influences the predicted topography and gravity for a 20-km-thick ice shell with a viscous, conductive viscosity profile. We also find that variations in ice shell thickness and viscosity structure produce only minor changes in the amplitudes and shapes of the modeled signatures. On the other hand, the addition of an elastic layer can strongly reduce the deformation of the shell, particularly for shallow reservoirs.

Overall, the models suggest that salty subsurface reservoirs can produce surface displacements of several hundred meters. For pure liquid water, surface displacements are in the range of tens of meters. These signals are potentially detectable by stereo topography and radar sounding from Europa Clipper and JUICE, as well as by GALA, the laser altimeter onboard JUICE. On the other hand, the associated gravity anomalies are on the order of a few milligals, and the expected horizontal scales of the features (~100 km) fall below the ∼500 km resolution limit of Europa Clipper’s global gravity field recovery [Mazarico et al., 2023]. Such small-amplitude signals will also be difficult to detect using line-of-sight acceleration from individual flybys, though detectability depends strongly on spacecraft altitude [e.g., James, 2016; Mazarico et al., 2023].