Primeval Precipitation; June 2012; Scientific American Magazine; by David Biello; 1 Page(s)

Some 2.7 billion years ago, in what is now Omdraaisvlei farm near Prieska, South Africa, a brief storm dropped rain on a layer of ash from a recent volcanic eruption. The raindrops, which formed tiny craters, were buried by more ash and, over aeons, that ash hardened into rock. Closer to the present, other rainstorms eroded the rock, exposing a fossil record of raindrops from the Archean era. Researchers are now studying these fossilized raindrops to learn more about early Earth’s atmosphere.

By using lasers to scan the craters—and comparing the indentations with those created today—astrobiologist Sanjoy Som of the NASA Ames Research Center and his colleagues have derived a measurement of the pressure exerted by the early atmosphere. The scientists reported online March 28 in Nature that the ancient air may have been less dense than the present-day atmosphere.