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Tonight I watched the latest installment of The History Channel’s “The Universe.” The week’s episode focused on Spaceship Earth (which re airs Sunday night). This episode addressed many different aspects of the Earth’s formation, how it gained a moon, and how the Earth+Moon system was able to support the formation and evolution of life. Along the way, the touched on some of my favorite elements of Earth science, specifically: how comets have carried water to Earth, the sharing of rocks (and possibly life) between planets, and global warming.

For some strange reason, the ideas of moving life around the solar system on rocks / comets / other random objects and alien life have been coming up a lot with me this week. First blogged on the probabilities of finding radio signals from aliens (hat tip to Fraser on that one), then Fraser and I talked with Swoopy about the new National Research Academies Report on things we need to think about in trying to find life on off of this world. And now… The Universe is jumping in and bringing up alien life as well.

And when talking about alien life in the solar system, I always thing about panspermia.

One theory off life states that life didn’t originate on Earth but was rather carried here from another world. Research done by J. Melosh (and talked about in Life as We Do Not Know It), shows that at most maybe 1 or 2 rocks could have made it out of our solar system in the past 4.6 billion years or so. So… assuming that other solar systems are equally likely to fling rocks around the galaxy, it is close to impossible that another solar system managed to fling a rock filled with life out such that it arrived here and landed on Earth at just the moment the Earth started to be able to support life. This basically throws the idea of galactic panspermia originating life on Earth out the window in my (admittedly non-expert in this area) mind. What isn’t eliminated is the possibility of life originating at some other place in our solar system.

Early in the Solar System’s life, toward the end of its first billion years and beginning of its second billion years, there was a period of heavy bombardment when vast numbers – tens of thousands – of asteroids capable of creating craters and carnage hit the Earth. These rocks from space not only donated their mass to planets like Earth and Mars, but in some cases they hit planets with so much energy that they caused pieces of the planet’s to fly into space. During this period, the exchange of bacteria and microbes would have been possible between worlds.

Evidence of former oceans on Mars (in the form of hematite and subsurface ices) and its greater and safer distance from the Sun, may have made it a safer initial place for life to evolve. It’s distance kept it a little cooler when the Sun was younger and hotter, and the distance would have also helped life cope with the young Sun’s extremely powerful coronal mass ejections.

I’m not convinced this is the way things happened. I personally fall into the “Deep sea trenches look like a nice place to start life” camp, but as a scientist I have to admit that I don’t have enough information to say life originating on Mars is a scientifically invalid idea. Its quite possible that panspermia worked and life was actually carried from there to here, as life flew through the air (and the vacuum of space).