If MOXIE works...
For a trip to get rocks back to earth, maybe they could launch a mission in two parts. The first part would use standard rockets and a lander with a super-MOXIE unit. It might even leave the MOXIE unit at one place and go around collecting samples. Meanwhile, the second part of the mission, a sort of "truck", a slow robot with an ion drive, heads for Mars orbit. Once enough fuel has been generated by the MOXIE unit, part of the lander could fuel up and takes off into a stable Mars orbit with the rocks. The "truck" could rendezvous with the lander's ascent stage, get the rocks, and then head back to Earth orbit where a manned mission could bring the cargo down. Ion drives are low-power, but can run for years, and rocks wouldn't mind a slow trip. It might even be possible to have the ascent stage of the lander carry extra MOXIE-derived fuel fuel for use by the truck on its return trip.
Biting the hand that feeds IT
