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.

The largest raindrop ever measured in modern times was 6.8 millimeters around, Som notes, which is also the theoretical limit; larger raindrops break apart. Because the laws of physics were likely the same in the distant past, this suggests that raindrops were no bigger in the Archean and puts an upper limit how big the ancient drops could have been. Plus, such raindrops are exceedingly rare in modern storms—and tend to fall in powerful downpours, which in the Archean would have been more likely to have washed ash away rather than form craters that could be fossilized.

To determine the size of the ancient droplets, Som and his colleagues compared the fossilized imprints with the craters that formed when they released various-size droplets from 27 meters above similar ash taken from the 2010 Eyjafjallajökull eruption in Iceland as well as from Hawaii. They then turned these modern craters to "rock" "using hair spray and low-viscosity liquid urethane plastic." Based on the comparisons, they concluded that the size of ancient droplets fell in the range of 3.8 to 5.3 millimeters.

Plugging those numbers into the mathematical relationship between raindrop size, speed and atmospheric density suggests that the early Earth's atmosphere exerted at most twice as much pressure as the present day atmosphere—assuming raindrops of the maximum size and speed created the craters—and more likely was roughly the same or as little as half present pressure.

A better understanding of the properties of Archean Earth's atmosphere may help explain what's known as the "faint young sun" paradox. Billions of years ago, the sun emitted less radiation, roughly 85 percent of its present output, and therefore heated the planet less. Yet, the fossil records suggest abundant liquid water and other signs of a warm, "clement" climate, as Som and colleagues noted in the analysis. The simplest explanation for this is that Earth simply boasted an atmosphere thick with greenhouse gases. "The sky was probably hazy," from the gases, Som says, in addition to being ruled by a fainter sun that passed across the sky more quickly because Earth rotated faster then. Plus, the atmosphere lacked a significant quantity of oxygen (because there were no plants), potentially lightening the atmospheric pressure. "Earth back then looked nothing like it does today."

Consistent with the scenario suggested by this new calculation, research published in Nature Geoscience on March 18 suggests that the early atmosphere cycled through periods of a "hydrocarbon haze" that included greenhouse gases such as methane, better known today as natural gas. Such a hydrocarbon haze—potentially being re-created today—helped trap the heat of the faint, young sun, warming the Earth.

That explains the clement Earth, according to Som—high levels of stronger greenhouse gases, such as methane. "Our work suggests that it was indeed greenhouse gases that kept the planet warm," Som says, a process ongoing in modern times.

Of course, this judgment relies on an assumption—that average temperatures 2.7 billion years ago were roughly 20 degrees Celsius, based on the lack of evidence for ice in the geologic record of the time. "This may be a preservation bias," Som admits.

What was clearly preserved, however, are the fossil imprints of ancient rain. And that record reinforces the fact that early Earth was essentially an alien world compared with today's planet—one devoid of plant life; with a moon that orbited more closely, driving stronger tides; and a very different atmosphere. "Yet it was very much alive," Som notes, boasting a rich array of microbial life, including photosynthetic bacteria, the ancestors of modern plant life just a scant few hundred million years from loading the atmosphere with oxygen. A better understanding of this planet's proto-atmosphere may help scientists identify life on other planets—as well as better understand just how influential greenhouse gases can be.