Did Life Come From Comets?

DB: No one knows what "the right" amount is. We just know that Earth was lucky to get about the right amount. But it probably is chance that the Earth got what it got. If it had a lot more or a lot less, it would certainly be a different planet

If it had a lot more, if it had so much water -- if it had twice as much water -- there wouldn't be land sticking out above the oceans. The oceans are about 4 kilometers deep. So all the Earth's cycles involving land and atmosphere and ocean wouldn't be functional. And if it had less -- we don't know how much less, but my guess is we'd be better off with less than more -- If we had less water and organics, biology would have had to operate somewhat differently. We don't know what the magic portion is.

There also are some somewhat eerie things: If in fact the amino acids in meteorites were formed in asteroids that were heated internally -- sort of like little pressure cooker-kind of environments to synthesize these things -- that means the asteroids had to be heated. Now, our asteroids were probably heated by short-lived radioactive decay of aluminum 26. If you go to other planetary systems, it's unknown whether they would have this short-lived radioactivity that would produce heating inside of asteroid-like bodies early in the history of planetary systems. We're probably unusual in that regard, but we don't really know because we can't measure it around other planetary systems.

The asteroids are quite far from the sun and their normal temperature (as heated by the sun) is colder than Mars. It's about 150 Kelvin, which is not enough to produce melting of ice internally unless you have a very large body.

IK: And what would be the importance of having the internal melt? It sounds like you're saying there was some prep-work done on these amino acids before they got here.

DB: There are a couple of things. One is that meteorite samples we have from the most primitive asteroids are sort of like concrete: They've been altered by liquid water inside. That liquid water has changed a lot of the silicate minerals in them, bonded them together, made them stronger than would have been originally, which helps them survive in space and also helps them survive coming through the Earth's atmosphere.

But also, when you have a warm environment, for perhaps millions of years, you have rock and organics and liquid water -- not oceans, but in places between grains -- this environment may have ... cooked up organic chemicals that were important for Earth life. The interesting thing is that there's this one meteorite, Tagish Lake, that doesn't have amino acids in it so somehow it formed in a place where the process did not occur to produce amino acids.

IK: Is there any debate that some of the ingredients for life were delivered by comets and asteroids? Is that fairly universally held at this point?

DB: Well, we don't know about comets. Comets we know contain organic compounds, but the best study materials are meteorites because they land on Earth and we can study them and they're full of organic compounds. Whether that's enough for life is unknown.

The real question, I think, about delivery of organics from space is "Do you need them or not?" because if Earth can produce all the organic materials it needs for life on its own, then it doesn't need an outside source. If it can't, if there are not terrestrial processes that can make prebiotic chemicals necessary to develop life, then the only place it can get it is from space. It really is a critical concern.

IK: The verdict is still out on that?

DB: As far as I know, yeah.

IK: If you could get your hands on any bit of sample from anywhere in the solar system, what would be the most useful information to have to be able to distill this theory into some more discernible facts?

DB: Right now, the best clue we have that connects the Earth's oceans to extraterrestrial bodies is this deuterium-to-hydrogen ratio. So measuring this (would be nice), plus organic contents, from bodies in different places: from comets that formed in the Kuiper Belt out past Neptune where Pluto is, and there's a whole bunch of bodies that are orbiting the same orbit as Jupiter called Trojan asteroids -- it'd be very interesting to have samples of those things -- also, just samples of any kind of asteroid that has a known origin.

It's amazing how primitive the state is that we're in right now. We have lots of meteorites, but we don't know where they come from and the only primitive body that we have samples from is Comet Wild-2 that we sampled with Stardust. It's the only single case. Everything else is only wishful thinking and guesswork.

Article posted January 23, 2009.

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