I usually avoid these bar brawls, but geez, sometimes the back-and-forth here makes the Current Events board look like something from Masterpiece Theater. And that's saying something.

I see nothing wrong with having a legitimate and reasonable concern about back-contamination, although IMO, contaminating Mars from Earth is about fifty times more likely. The proper way to address these concerns is by landing enough probes to check for as many possibilities as we can before sample returns. Reasonable precautions for sample return are also not ridiculous. I personally think a sample reentry capsule for direct Earth reentry can be designed with enough margin to ensure an intact capsule at the other end no matter what the failure mode. I can go into general details if anyone's interested.

Setting up a lab on the ISS is overkill, and one still has to deal with the failure mode of the return capsule doing a direct reentry if the orbit insertion burn or skip entry maneuver fails. And bringing a capsule back to Earth orbit takes a LOT of extra fuel and weight. Should the ISS be contaminated, it would require setting up a propulsion module that would kick the ISS into deep space, and that ain't no small module. We have already seen that a LOT of stuff comes back intact from an Earth orbit reentry. There were live animal specimens still kicking in one of Columbia's experiments, and bacteria would easily survive.

Heck, I'm wondering if we're going to end up contaminating Jupiter with Galileo... ;)