Unless fossils are somehow deadly
There won't be anything living on Mars to threaten Earth.
Plus by the time we can recover a sample of the Red Planet, they
should have some kind of facility in Earth orbit to keep the rest of
us safe. Only the poor souls on the orbiting station will have their
own little Andromeda Strain to deal with.
If you are using the threat of alien contamination to support your
endless tirade about dangerous space rocks, don't waste your time
with me at least on that one. I doubt there is anything living now
on Mars. Fossils maybe, but that's it.