THE atmosphere on primitive Earth was much richer in hydrogen than previously thought and could have been the source of the molecules that led to early life, a new analysis shows. The study lends credence to the long-standing suggestion that lightning in early Earth鈥檚 atmosphere helped form these prebiotic molecules.
In a classic experiment in 1953, American chemist Stanley Miller produced amino acids by zapping electricity through a mixture of methane, ammonia, hydrogen and water, which he thought mimicked the composition of Earth鈥檚 early atmosphere. Even a mixture low in methane and ammonia, but rich in hydrogen gave rise to such molecules. But later research indicated that hydrogen would have been less abundant in the early atmosphere than in such concoctions. Researchers argued that the expected blend of nitrogen and carbon dioxide would not have yielded prebiotic material.
Now Feng Tian of the University of Colorado in Boulder and his colleagues claim primitive Earth was indeed rich in hydrogen because the gas would have been trapped in the atmosphere, preventing it from escaping into space. Today, the upper atmosphere can reach temperatures above 700 掳C because atomic oxygen there absorbs ultraviolet radiation from the sun. The absorbed energy is transferred to hydrogen atoms, which then escape. But because the primitive Earth鈥檚 atmosphere was rich in carbon dioxide, which would have radiated the solar UV energy back into space, the temperatures would have remained below 350 掳C, Tian argues. This would have prevented hydrogen from escaping as readily as it does today. And as volcanoes kept spewing out hydrogen, the gas would have made up nearly 30 per cent of the atmosphere.
Advertisement
鈥淪o much hydrogen in the early atmosphere makes it far easier to produce organic molecules relevant to the origin of life鈥�
This would have significantly increased the rate at which organic compounds formed. Tian calculates that concentrations of organic compounds in the oceans would have been two to three orders of magnitude greater than previous estimates.
Astrobiologist Chris Chyba, of Stanford University in California, says the idea is credible. 鈥淭his is a very different atmosphere chemically from what has been the consensus model for the past few decades,鈥� he says. 鈥淧utting that much hydrogen into the early atmosphere makes it far easier to produce organic molecules relevant to the origin of life.鈥�
