Europa and Ganymede
The exploration of Europa is of great interest because it may be hospitable to certain life forms. Several lines of evidence suggest that a subsurface ocean exists beneath an icy shell, but there is debate about the thickness of the shell, with important implications for Europa's astrobiological potential. As in the case of Mercury, it may be possible to determine whether an outer shell is decoupled from the interior and to evaluate the shell thickness by measuring the obliquity and the amplitude of small variations in the spin rate.
Europa reached closest approach in September-November 2011, offering a rare opportunity to measure spin rate variations by tracking radar speckles. Simulations indicate that the signature of a rigid shell thinner than 50 km would be detectable. Observations of both Europa and Ganymede have gone very well, as well as follow-up observations in 2013 and 2014. Additional observations at different geometries are required to refine the preliminary estimates. Because opportunities for these measurements are rare, the next observations are not scheduled until 2019 and 2020. Unfortunately, all of the 2019 and all of the 2020 observations were cancelled due to a lengthy failure of the Goldstone klystrons.