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 (Margot et al. 2007, 2012), 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.
Jupiter reached its closest approach to Earth in September-November 2011, offering a rare opportunity to measure the spin states of Europa and Ganymede by tracking radar speckles. Observations went well in 2011 and 2013, and were marginally successful in 2014. A log of all attempted observations is available.
Additional observations at different geometries are required to refine the preliminary estimates. Unfortunately, opportunities for these measurements are rare. The signal-to-noise ratio was too low in 2015-2018, preculding measurements. The next opportunities occurred in 2019 and 2020. All of the 2019 and all of the 2020 observations were cancelled due to a lengthy failure of the Goldstone klystrons. The speckle trajectory in 2021 and 2022 precludes measurements. We are planning additional observations at the next opportunity, in 2023.