Some 2.7 billion years ago in what is now Omdraaisvlei farm near Prieska, South Africa, a brief storm dropped mild rain on a new layer of ash laid down by a recent volcanic eruption (not unlike ash from the 2010 Eyjafjallajökull eruption in Iceland) forming tiny craters. Additional ash subsequently buried the craters and, over eons, hardened to become rock known as tuff. Closer to the present, other rainstorms eroded the overlying tuff, exposing a fossil record of raindrops from the Archean eon, and may now have revealed the density of early Earth’s atmosphere.
By scanning with lasers the craters created by ancient raindrops—and comparing the indentations with those made by water drops sprinkled onto a layer of similar ash today—physicist Sanjoy Som of the University of Washington in Seattle and his colleagues have derived a measurement of the pressure exerted by the primitive atmosphere. The scientists report in Nature on March 29 that the ancient air could not have been much denser than the present atmosphere—and, in fact, may have been much less so. (Scientific American is part of Nature Publishing Group.)
"Air pressure 2.7 billion years ago was at most twice present levels, and more likely no higher than at present," Som explains. The key to that determination is raindrop size. Back in 1851 pioneering geologist Charles Lyell suggested that measuring the fossilized indentations of raindrops might reveal details about the ancient atmosphere. These mini-craters are formed based on the size and speed of ancient raindrops. Because the atmosphere drags on each drop, constraining the speed of its descent based on its size, if one could determine an ancient raindrop’s size, one could determine how thick the atmosphere likely was.
Definitely an interesting article to read, and wonder more about Earth’s early atmosphere!