Sunday, August 14, 2005

Earth's Early Atmosphere and the Search for Life on other Planets

Model Gives Clearer Idea Of How Oxygen Came To Dominate Earth's Atmosphere

Researchers interested in how earth's atmosphere came to be dominated by oxygen have come up with an interesting model to explain why there was a lag between the origin of photosynthesis and the domination of earth's atmosphere by oxygen.

There were several processes at work. First, gasses emitted from volcanoes combined with the oxygen and acted as an oxygen sink. Second, oxidation of iron from space bombardment acted as a second sink. Researcers found that varying the estimates of iron content in the earth's crust could change the time frame by up to a billion years in one direction or the other.

Here is how it works:

Earth's oxygen supply originated with cyanobacteria, tiny water-dwelling organisms that survive by photosynthesis. In that process, the bacteria convert carbon dioxide and water into organic carbon and free oxygen. But Claire noted that on the early Earth, free oxygen would quickly combine with an abundant element, hydrogen or carbon for instance, to form other compounds, and so free oxygen did not build up in the atmosphere very readily. Methane, a combination of carbon and hydrogen, became a dominant atmospheric gas.

With a sun much fainter and cooler than today, methane buildup warmed the planet to the point that life could survive. But methane was so abundant that it filled the upper reaches of the atmosphere, where such compounds are very rare today. There, ultraviolet exposure caused the methane to decompose and its freed hydrogen escaped into space, Claire said.

The loss of hydrogen atoms to space allowed increasingly greater amounts of free oxygen to oxidize the crust. Over time, that slowly diminished the amount of hydrogen released from the crust by the combination of pressure and temperature that formed the rocks in the crust.

"About 2.4 billion years ago, the long-term geologic sources of oxygen outweighed the sinks in a somewhat permanent fashion," Claire said. "Escaping to space is the only permanent escape that we envision for the hydrogen, and that drove the planet to a higher oxygen level."

The most intersting part of the article is the last sentence:

"There is interest in this work not just to know how an oxygen atmosphere came about on Earth but to look for oxygen signatures for other Earth-like planets," Claire said.

Note that the one thing missing in this search for life on other planets is Intelligent Design