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.

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. 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. To figure out the size of the ancient droplets, Som and his colleagues got creative. They collected ash from the 2010 Eyja­fjallajökull eruption in Iceland, as well as from Hawaii, and released various-size droplets from 27 meters above it. They then turned these modern craters to “rock” by using hair spray and low-viscosity liquid urethane plastic. Based on comparisons between the ancient and new craters, they concluded that the size of ancient droplets ranged from 3.8 to 5.3 millimeters.

Plugging those numbers into the mathematical relations among raindrop size, speed and atmospheric density suggests that the early Earth’s atmosphere probably exerted the same or as little as half the present pressure.

This finding sheds light on yet another early Earth mystery known as the “faint young sun” paradox: billions of years ago the sun emitted less radiation and therefore heated the planet less, yet the fossil record suggests that the climate was warm. But if the atmosphere was no denser than it is now how did it hold so much heat? The simplest explanation is that Earth boasted an atmosphere rich with greenhouse gases able to trap a large amount of heat per molecule. Those gases likely originated from volcanoes and microbial life. “The sky was probably hazy” from these gases, Som says.

Consistent with this scenario, research published online March 18 in Nature Geoscience suggests that the early atmosphere cycled through periods of a “hydrocarbon haze” that included potent greenhouse gases like methane. Such a haze—potentially being re-created today—helped to trap the young sun’s heat, making life comfortable for microbes—and may offer a signal of life on other planets as well.

This article was published in print as "Primeval Precipitation."