Richard A. Kerr, "A Better Atmosphere for Life" (2005)

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> <font face="Times New Roman" size="2">EARTH SCIENCE:<font>
> <font face="Times New Roman">A Better Atmosphere for Life<font>
> <b><font face="Times New Roman">Richard A. Kerr<font>
> <font face="Times New Roman">Thirty years ago, geochemists took away the primordial soup that biologists thought they needed to cook up the first life on Earth. Now, some atmospheric chemists are trying to give it back. They're suggesting that the early Earth could have held onto much more of its volcanic hydrogen--a key ingredient in the recipe for making the organic compounds that may have led to the first life. <font>

Creating the primordial organic goo used to be easy. If you combined the methane and ammonia seen in the still-primordial atmosphere of Jupiter, passed lightninglike sparks through the mixture, and added some water, voilà, complex organic compounds such as amino acids formed. But then in the 1970s geochemists spoiled the party by insisting that Earth's earliest atmosphere was nothing like Jupiter's. Earth's carbon would have been part of oxygen-rich carbon dioxide, and its nitrogen part of inert nitrogen gas, they said. And hydrogen seeping from the planet's interior would have quickly escaped to space. That left chemists with a thin gruel indeed. It had far too much oxygen, which destroys organics, and not enough of the hydrogen that enables carbon atoms to link up to form the complex polymers needed for life. In the lab, such mixtures yielded few organics, and simple compounds at that.

Now, atmospheric chemist Feng Tian of the University of Colorado, Boulder, and his colleagues argue that hydrogen on early Earth would have escaped much more slowly than has been assumed (Science, 13 May, p. 1014). Lacking the oxygen that absorbs solar energy, they point out, the outer fringes of the early atmosphere would have been far colder than they are today. With less energy jittering its atoms, much less lightweight hydrogen would have "boiled" away into space.

The researchers also figured out how to calculate the rate at which hydrogen would have been lost as wisps of the atmosphere flowed away into space. The mathematics of such supersonic flow had frustrated all previous attempts. Overall, hydrogen would have escaped at 1/100 the rate previously assumed, the group says. Rather than building to concentrations of just 0.1%, hydrogen might have reached 30%. That would make for a far more productive atmosphere than chemists have been coping with for 30 years. "The end result is you drop vast amounts of organic compounds into the ocean to make a soup," says the group's Brian Toon of the University of Colorado, Boulder.

"On the face of it, what they have produced is quite reasonable," says atmospheric chemist Yuk Yung of the California Institute of Technology in Pasadena. "It's a nice piece of work. It's going to make the biologists a lot happier." Astrobiologist David Catling of the University of Bristol, U.K., isn't so sure. "It would be rather premature," he says, to shift emphasis back to the prebiotic chemistry of a hydrogen-rich atmosphere and organic-goo-laced ocean. Tian and his colleagues "haven't dealt with all the factors that lead to hydrogen escape," says Catling. He suspects that a more sophisticated model would show that hydrogen escaped the early Earth at least as fast as it does today. Time will tell whether too many cooks spoil the primordial broth.