Could we come from space?

Hold on there with the tinfoil hats - I’m not about to start talking about how I was kidnapped and probed back in '01*.

Seriously, who travels thousands of lightyears just to stick things up people's bums? Who does that?!

What I’m talking about is (slightly) less crazy: Naturally occurring space travel. Most of humankind's great inventions have been done first by mother nature, so why should space travel be any different? 

Well… actually, because Earth’s gravity is strong, and it’s atmosphere deep. Only the most violent natural events – massive asteroid strikes – could throw something hard enough to overcome those and reach space. Still… Earth’s history is incredibly long, and filled with such impacts. Could it have happened? 

Maybe. 

First a little background: Theories suggesting that Earth life might have naturally migrated to other worlds, or extra terrestrial life might have naturally migrated here, fall under the umbrella term ‘panspermia’. There're a lot of such theories and, yes, many of them belong firmly in the box marked ‘any advocate of this probably wears a tinfoil hat and talks about how he was kidnapped and probed in ‘01’ . 

But others are considered sound science. The least controversial comes from the observation that asteroids and comets, which occasionally hit planets, carry in them some surprisingly (chemically) evolved compounds - exactly the kinds of chemistry we'd expect to see immediately prior to the start of life. Some of these chemical compounds can even form cell-like structures when exposed to water. So it seems quite un-tin-foil-hatish to suggest that asteroids and comets could have brought the chemical ingredients of life to Earth, and to other planets. 

Which, in a roundabout way, gave us this picture....

Another variant of the idea is litho-panspermia, the idea I alluded to earlier: Major asteroid impacts on early Earth and other planets could have splatted large amounts of rocky debris into space, with some of it eventually wandering far enough to fall on other worlds as meteorites. Simulations have shown that the interiors of rocks blown into space this way can stay fairly un-damaged by the forces of the blast - enough for microbes living in pores of the rock to survive the launch and go into hibernation without dying. Then, when one of those rocks eventually drops onto another planet…. 

We know that rocks can travel between planets this way: We’ve found meteorites that were definitely once rocks on the surface of Mars, or the Moon. They’ve told us things about the history of Mars and the Moon that even space probes couldn’t. Some carry evidence of ancient habitable conditions on Mars. 

The journey can be done the other way too: The surface of the Moon is estimated to contain around 200 kg of Earth rocks per square kilometer, blasted free from Earth during various asteroid impacts. The surface of Mars is also thought to hold massive numbers of Earth rocks, scattered across it's surface.

If you can spot one I'll buy you a beer. All the scientists will buy you a beer, for the rest of your life - that's how important it would be. well? Go on, there's a lifetime supply of beer to be had...

Tests and simulations suggest that a micro-organism in hibernation could survive the ‘launch’, and the landing. So it does seem possible that a piece of stone from Earth, thrown onto a course that took it to a nearby world, could transport viable microbes (if the trip was relatively quick – space radiation can damage even a microbe inside a rock several meters across). 

Today there’s a fairly obvious problem with this idea: There are no habitable planets for a wandering space rock from Earth to land on. But 3.5 billion years ago the story was different: A whole host of worlds that are dead today are thought to have hosted habitable environments – we know this from studying rocks from those worlds that made the reverse trip - to Earth! To add fuel to our tin foil hat wearing fire, the rate of asteroid strikes was much higher billions of years ago, allowing many more ‘launch opportunities’ than we see today. 

No–one can say if Earth life has ever made that trip, or if alien life from one of those worlds might have ever landed here. But, if we ever do find microbial life on nearby worlds, we should be cautious before we get too excited – it might not be truly alien, just a long lost cousin… 

*It wasn’t aliens, I just hung around with the wrong crowd. I don’t like to talk about it. 

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