Did Life Come From Comets?

The scoop: The search for extraterrestrial life often leads to our own back yard as scientists try to understand how life managed to take root on Earth. Among the theories: Water and organic materials crashed into the planet inside comets and asteroids.

University of Washington astronomer Donald Brownlee, who heads the science team analyzing comet particles brought back to Earth aboard the Stardust spacecraft, gave Discovery News correspondent Irene Klotz his views on the issue (which left her with more questions at the end of the interview than she had when she started).

Irene Klotz: Since we last spoke, has there been anything that has updated or discard some of the theories about what came to Earth with the comets?

Donald Brownlee: I don't think so, but the whole issue of comet-delivery to Earth is just a wide-open question. You know the whole deuterium story?

IK: Let's run through that.

DB: The real evidence on this is the ratio of deuterium to hydrogen on Earth. There are three or four comets where the deuterium-to-hydrogen ratio has been measured, and it's higher than Earth's. And that's probably due to the fact that they formed under very cold conditions at the edge of the solar system.

You could probably use that to say that comets probably weren't a straightforward contributor to the water on Earth. They certainly contribute, because they're delivering water today, but they didn't contribute more than half, say, and probably much less than that if you just go by these isotope measurements. But it's actually much more complicated than that.

In icy bodies that formed in the solar system the deuterium-to-hydrogen ratio is expected to vary, based on where they formed. It'll be high out on the edge of the solar system where the comets formed, but it increases to sort of Earthly compositions when you get in somewhere around Jupiter.

So you'll hear things like "Oh, the oceans came from comets," and other people will say, "Well they couldn't have come from comets because of the deuterium-hydrogen (ratio)." ... My take on this is that it did come from comets or asteroid-like bodies, but it was probably dominated by things that didn't form out by Pluto, formed more in the range of where Jupiter is.

Some people make a big deal out of comets versus asteroids. I always think that's a little bit funny because comets contain water, but most of their mass is in rock, and asteroids are mostly rock, but they actually contain quite a bit of water. It really doesn't physically make too much difference whether it was a comet or an asteroid.

IK: What about the issue of the organic materials. Is that related to this discussion of the water and the deuterium-hydrogen ratios?

DB: The organic materials are absolutely fascinating because we know organic materials are carried to Earth in meteorites, and many primitive meteorites contain amino acids.

And then there was this fantastic event -- Tagish Lake -- that hit up in the (Canadian) Yukon 10 years ago; that was like a 100-ton object that blew up in the atmosphere. It (has) the highest carbon content in any meteorite ever found, and it contains basically no amino acids. So here's the most carbon-rich meteorite we've ever seen -- truly a primitive object -- and it has no amino acids.

One thing people have suggested is that the amino acids in bodies like asteroids -- which were warm and had liquid water inside them for a period of time -- may have played a role in synthesizing organic materials like amino acids. And so a comet, which normally doesn't have liquid water inside -- only frozen water -- may actually be poorer in interesting organic compounds than some of the asteroids.

IK: So, are you saying there were different types of objects that were impacting Earth that delivered this myriad of raw materials for all of what is on this planet to develop?

DB: Yeah. The way I look at it, the solar system was filled with planetesimals -- comet and asteroid-type materials -- that went from somewhere beyond Mars all the way out to Pluto. There were vast numbers of these things and these materials were definitely carriers of water and organics to Earth. They deliver this stuff to Earth right now. They certainly did much more spectacularly in the past. Some of these formed at the edge of the solar system where the temperature was below 50 Kelvin and other ones formed more toward the asteroid belt where the temperature was much higher, maybe 150 Kelvin. Nevertheless they had ice in them.

All those bodies that existed were either scattered out of the solar system, or they went into the sun, or they went into planets. The stuff we call asteroids or comets now are only a small fraction of what was there originally, but those are our samples. We see differences between comets and asteroids: Asteroids have been heated to room-temperature kind of things and comets weren't, otherwise they would have lost their ices.

A couple of major discoveries have happened over the years. One was the discovery of comets in the asteroid belt ... so the whole issue of what's a comet, what's an asteroid is more complex than it used to be.

There's also a theory that about the time the great craters on the moon were made, about 3.9 billion years ago. The solar system had an adjustment of the positions of the outer planets that ended up scattering Uranus and Neptune out to where they are from ... much closer to the sun, and this process disturbed a very large reservoir of comet-like bodies that scattered in and cratered the moon, producing the huge impact basins. That would have also, of course, delivered a lot of material to Earth.

IK: Would you say that it was a random assortment of materials from comets and asteroids that were delivered, and it was just a fluke that everything was in the proper ratios and at the right time to produce what we have on this planet today? Or is there any evidence that there is flexibility in what you get and when you get it for life to take hold?