How about an advanced rover with a deep drill for getting core samples.
Such a device would be quite large, but not nearly as large as life support and rockets to return crew to Earth. I figure five or ten tons should do it. It would have a stack of pipes and a mechanism for picking them up, one after the other, and inserting the end of each into the threaded end of the previous one. Perhaps 500 meters depth would be obtainable. It could then either analyze the cores on the spot, or cart them back to a base station where automated equipment could analyze them.