Hi Chris

I think you've been scooped on this one as I've already posted on it, as have some of the others. It's still a cool news item, though. If you manage to read the Nature reference it mentions the need for methane to sustain the Mars greenhouse - implying methanogenic life and/or odd mantle chemistry brewing up methane and related hydrocarbons.

Paul Davies, in this probability analysis says that Mars is more likely to have been the planet of origin for life on Earth than Earth itself, simply because it was probably bio-friendly before Earth and it's easier to carry life from Mars to Earth than visa versa. So going back to Mars wouldn't be an invasion, but a home-coming.

Adam