Time for new celestial targets for radar sounders?
With the success of the ground-penetrating radar sounder,
MARSIS, on Mars Express, and that of the ground-penetrating radar
sounder on the Kaguya lunar-orbiter showing promise, maybe it's time to
look further afield?
Europa?
Mercury?
Some comet nuclei?
Some NEO asteroids?
The radar on the Cassini spacecraft can penetrate deep into ice;
maybe a scan of Enceladus and what's beneath its geysers should be
tried out during the extended mission?