Planetary Science Seminar winter-2016

Measuring the tidal Love number h2 of Europa and Ganymede

The potential habitability of the subsurface ocean of Europa and Ganymede makes the icy moons of Jupiter a lucrative target for planetary exploration. While NASA will investigate Europa with a flyby tour over a period of several years, ESA decided to investigate Ganymede with a dedicated orbiter. One of the top priorities of both missions is the characterization of the structure of the icy shell. It is a key measurement for future exploration and provides invaluable information on the thermal behavior and dynamics of the moons. However in order to effectively constrain the interior structure a combination of multiple measurements will be necessary. Numerous publications already pointed out the importance to measure both tidal Love numbers h2 and k2 (e.g. Wahr et al. (2006), Wu et al. (2001)) as well as potentially l2 (Hussmann et al., 2011) for constraining the ice thickness, which arises from the ambiguity between the rheology and the thickness of the outer ice layer (Moore and Schubert (2000),Wahr et al. (2006)). While the tidal Love number k2 describes the tidal secondary potential induced by the mass redistribution as a consequence of the external forcing by Jupiter, the tidal Love numbers h2 and l2 express the corresponding radial and horizontal amplitudes of the tidal deformation. k2 can be measured by radio science experiments but the measurement of h2 requires an altimeter. On ESA's Jupiter Icy Moons Explorer (JUICE) this task will be performed by the Ganymede Laser Altimeter (GALA). The only selected instrument onboard the Europa mission with altimetric capability is the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON). While its main focus will be the direct detection of subsurface water reservoirs, we will show that it has the potential to also deliver altimetric measurements useful for the detection of solid body tides and therefore contribute a valuable contribution to the characterization of Europa's outer ice shell.