a typical Pu RTG are not
sufficient to melt through salt of some thickness threshold that an
engineer familiar with the problem could calculate. My semi-educated
guess is anything over 0.5 meter will be a problem. Therefore, a
mechanical system, such as a rotary drill(s) will likely be
necessary. The best idea I've heard is to first probe the crust's
surface for clues to composition and life. This can be easily done
with drill-equipped landers or penetrator probes. Of these, the
penetrator probes are the most cost effective. Please refer to the
JPL Galileo picture gallery.
Due to the hard radiation problem discussed, any surface machinery would have to be heavily shielded. Perhaps it would be possible to search for an area recently impacted by an asteroid? This would then allow:
1) a possible fracture in the ice, to facilitate submersion, and
2) perhaps a few large chunks of ice or stone that any surface machinery could employ for shielding.
It may be a sidestep on the march to the sea, but taking extra time and effort to protect against the radiation experienced by surface machinery would likely pay off, particularly considering that this is a mission which would require at least 2 years time to complete.
-- JH Byrne
