[gloriousduke]

> way too cold or way too hot

This isn't as big of a concern as it once might have been. See extremophiles.

> It's possible that we'll evidence of past life on Mars, but not current.

If life takes hold anywhere and has any amount of (geologic) time to spread, I'm guessing only an extremely powerful gamma ray burst or other high energy event could completely exterminate it. It might be impossible to sterilize the earth at this point without completely destroying it, and even then there'd probably be microbes in the resulting meteors, etc (until our sun dies at least).