Speaking of the MER heatshields....
...what did JPL engineers learn from those images
returned by Opportunity of its smashed heatshield?
As far as Mars surface being perfectly sterilizing?
Hmmmm....We've got bacteria on Earth that can withstand
temperatures hot enough to parbroil any macroscopic creature;
cold enough to turn mammals into blood icicles; and we've
got bacteria that will withstand intense gamma-rays,
or highly acid fluid...
Bacteria easily mutate...and the tougher mutated strains tend to survive the harsher environments.