A new study published in Nature from NASA's Exobiology and
Evoluationary Biology program investigators and their colleagues
looks at fossilized raindrops and what they indicate about
conditions on the early Earth. In ancient Earth history, the sun
burned as much as 30 percent dimmer than it does now.
Theoretically that should have encased the planet in ice, but
there is geologic evidence for rivers and ocean sediments
between 2 billion and 4 billion years ago. Scientists have
speculated that temperatures warm enough to maintain liquid
water were the result of a much thicker atmosphere, high
concentrations of greenhouse gases or a combination of the two.
Using evidence from fossilized raindrop impressions from 2.7
billion years ago, the team has deduced atmospheric pressure at
the time, demonstrating that an abundance of greenhouse gases
most likely caused the warm temperatures. Source: University of
Washington Last Updated: 29 May 2012