But life did evolve, so greenhouse gases must have been around to warm the Earth. Evidence from the geologic record indicates an abundance of the greenhouse gas carbon dioxide. Methane probably was present as well, but that greenhouse gas doesn't leave enough of a geologic footprint to detect with certainty. Molecular oxygen wasn't around, indicate rocks from the era, which contain iron carbonate instead of iron oxide. Stone fingerprints of flowing streams, liquid oceans and minerals formed from evaporation confirm that 3 billion years ago, Earth was warm enough for liquid water.

Now, the geologic record revealed in some of Earth's oldest rocks is telling a surprising tale of collapse of that greenhouse - and its subsequent regeneration. But even more surprising, say the Stanford scientists who report these findings in the May 25 issue of the journal Geology, is the critical role that rocks played in the evolution of the early atmosphere.

''This is really the first time we've tried to put together a picture of how the early atmosphere, early climate and early continental evolution went hand in hand,'' said Donald R. Lowe, a professor of geological and environmental science who wrote the paper with Michael M. Tice, a graduate student investigating early life. NASA's Exobiology Program funded their work. ''In the geologic past, climate and atmosphere were really profoundly influenced by development of continents.''

The record in the rocks

To piece together geologic clues about what the early atmosphere was like and how it evolved, Lowe, a field geologist, has spent virtually every summer since 1977 in Sout
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Contact: Dawn Levy
dawnlevy@stanford.edu
650-725-1944
Stanford University
3-Jun-2004