Ok, time for some crazy, wild eyed scientific speculation, based on a tenuously plausable theory, which of course is the very best kind.

How about the idea that there is a cosmic, galactic panspermia, or super tough life form that can survive the rigors of space travel and colonize, or infect, any planet or asteroid.

From the article in Scientific American:

"But the problem, and the potential paradox, is that if evolved galactic panspermia is real it’ll be capable of living just about everywhere. There should be stuff on the Moon, Mars, Europa, Ganymede, Titan, Enceladus, even minor planets and cometary nuclei. Every icy nook and cranny in our solar system should be a veritable paradise for these ultra-tough lifeforms, honed by natural selection to make the most of appalling conditions. So if galactic panspermia exists why haven’t we noticed it yet?
There are all sorts of plausible reasons. The simplest is that we’ve not yet managed to look very hard in all these places. It’s also possible that we’ve just not put two and two together while studying the properties of terrestrial extremophilic organisms. But suppose we keep looking hard and find nothing – this would argue strongly against the possibility of galactic panspermia at all. And this would be interesting, because it would also serve to place a limit to the true extremes of life, a physical and chemical boundary condition. Perhaps the root cause turns out to be gravitational dynamics (interstellar transfer may be horrendously inefficient), or just the environmental limits of bio-chemistry and the molecular machines at the core of it all. In either case a null result might actually tell us something vitally important about the phenomena of life, and our own cosmic significance."

I doubt that this is possible, but then again, haven't we been surprised over and over again by nature?

Via American Digest