Impactor missions to Europa and ganymede: Seismic approach for estimating icy crust thicknesses¹

Jupiter’s satellite Europa is believed to harbor a global ocean beneath its ice-covered surface. The thickness of the ice, despite its significance to the habitability of this moon, is currently unknown. Estimates range from as thin as hundreds of meters to as thick as tens of kilometers. A subsurface ocean is also expected at Ganymede. In this paper, we propose a mission to measure the thickness of the icy crust via seismic analysis, enabled by an artificial impactor striking the surface of the icy satellite where a seismometer would be placed in advance. We envision the impact analysis to be a complementary experiment, rather than a primary objective, of a mission to the icy moons of Jupiter. Two possibilities for the impactor are considered: (1) an orbiter probe impacting a satellite at the end of its life, and (2) a spent rocket stage impacting a satellite upon Jupiter arrival. The end-of-life impact of an orbiter would represent a low-energy impact case, whereas the beginning-of-mission impact of the spent rocket stage would represent a high-energy impact case. Our preliminary analysis shows that the majority of the impact energy would be converted into shockwave. For example, the estimated energy of shockwave for the high-impact-energy case was estimated to be 8.13×10¹⁰ J (an equivalent of 19.4 t of TNT), which is likely sufficient to determine the thickness of the icy crust via seismic analyses.