Perhaps given a full blown xenobiology laboratory staffed with hundreds of the world's top scientists and every instrument money can buy, it would be easy to distinguish earth-borne bacteria from non-earth-borne bacteria. But since we've only got a car-sized rover with a handful of instruments and no scientists on site, we need to be more careful.
In any event, how exactly do you differentiate earth-borne bacteria from non-earth-borne bacteria? What's your rubric?
