Gravity signals on Europa from silicate shell density variations

Kurzfassung

Future missions to Jupiter's moon Europa will attempt to measure the gravity field of this planetary body. Here, we study the detectability of silicate shell density variations in the gravity field. The first step in the gravity processing will be to remove the gravity signal of the ice shell. The detection of the ice shell signal, however, can be technologically challenging depending on its thickness and compensation state since the predicted anomalies are only a few mGals or even smaller. For long-wavelength topography below degree 40, the signal of the silicate shell will likely dominate the gravity field. Assuming that the anomalies of the silicate shell are only caused by the ocean floor topography, thus neglecting possible density anomalies in the mantle, we should be able to detect the ocean floor signal even if its topographic variations are only a few hundred meters. When studying the gravity signal of isolated midsize topographic features like volcanoes, we find a good chance of detecting objects with a size of 75–200 km with measurement accuracy of 1 mGal. Owing to the large number of unknown parameters for the gravity inversion, the reconstruction of a global ice-water/silicate interface shape is uncertain, in particular, as possible contributions to the gravity field from a low-degree convecting mantle cannot be distinguished. The comparison between the standard measurement technique of Doppler tracking (detecting the gravity anomalies) and a microgradiometer (measuring gravity gradients) shows that the latter will not improve the detectability of the ocean floor structures.