Som et al., Nature, 2012
A 2.7-billion-year-old sedimentary rock from South Africa shows the imprints of ancient raindrops.
Ancient Raindrops Reveal Secrets of a Prehistoric Atmosphere
Author: Laura Komor
Source: Los Angeles Times
If you’ve ever taken a high school Earth Science class, you’ve probably heard of Charles Lyell. This 19th-century geologist did much more than further the theory of uniformitarianism. Lyell had an interesting way of determining the "thickness" of ancient atmospheres: by studying raindrop fossils. When a raindrop falls through a thin atmosphere, it will fall more quickly and leave a deeper dent than a raindrop traveling through a thicker atmosphere. Using Lyell's method, Sanjoy Som, an astrobiologist at NASA Ames Research Center in California, worked with colleagues to study ancient atmospheres. Som and his colleagues made casts of the impressions of 955 ancient raindrops that fell 2.7 billion years ago and were preserved in sedimentary rock in South Africa. The scientists then recreated those raindrops by sprinkling water on ash, artificially adjusting atmospheric conditions until they could match the fossils. Assuming that modern raindrops were the same size as those in prehistoric times, they estimated that the ancient raindrops probably existed in an atmosphere that was only 50 to 105 percent as thick as our atmosphere. This debunks the theory that our atmosphere at that time was too thick to allow liquid water to exist on Earth—rather a paradox, given the existence of raindrop fossils. We know that the Sun then was not as bright as it is now, and it was believed that the heat it emitted was not enough to thaw the ice-bound Earth. Essentially, scientists thought that without a thicker atmosphere, which would trap heat, the Earth would remain frozen. However, Som’s study has led geologists to believe that even though the prehistoric atmosphere might have actually been thinner than today’s, perhaps the concentration of certain heat-trapping greenhouse gases was higher in ancient times. A 2003 study by University of Maryland professor Alan Kaufman determined that carbon dioxide levels 1.4 billion years ago were at least 10 times, perhaps as much as 200 times, the carbon dioxide levels of today. It looks likely that 1.3 billion years prior, the thinner atmosphere had even higher concentrations of CO2, allowing the rain to fall.
Read on at Los Angeles Times
The original article is in the journal Nature
Read more about Kaufman's 2006 study at NASA Earth Observatory