Habitability and exchange processes on the Jovian moons: implications for planetary protection measures

The discovery of subsurface oceans in the icy moons of the giant planets has revealed the potential presence of deep habitats, despite very cold surface conditions. The exploration of Jupiter's icy moons, Europa, Ganymede and Callisto, by NASA's Europa Clipper and ESA's JUpiter ICy moon Explorer (JUICE) missions will provide unprecedented data that will further characterize these icy worlds and determine the potential habitability of their subsurface oceans. The existence of these subsurface oceanic environments and their possible (past and current) communication with the surface requires the implementation of specific and stringent planetary protection (PP) procedures for any future missions to those worlds, particularly for Europa and Enceladus. In this paper, after presenting the main PP considerations for the exploration of icy worlds, we review the known characteristics of Jupiter's icy moons, in terms of the habitability of their subsurface reservoirs and exchange processes through their ice shell. We mostly focus on Europa, where direct communication with the subsurface water reservoirs (subsurface ocean and near-surface liquid pockets) at present is possibly active, and on Ganymede, where efficient exchange processes with the subsurface water reservoirs may have occurred in the past (more than 1 Gyr). Based on the known characteristics of Europa and Ganymede, we then summarize the actions taken by Europa Clipper and JUICE to satisfy the PP requirements. This article is part of the theme issue 'Planetary Protection for sustainable space exploration'.