When you hear about geologists studying records of Earth’s distant past, you probably picture something substantial—layers of rock or mineralized fossils. Raindrops are unlikely to come to mind. But that’s exactly what the authors of a new paper in Nature studied—2.7 billion year old imprints of raindrops.

There’s a long-standing mystery about the climate of the early Earth known as the “faint young Sun paradox." The Sun burned about 20 percent less brightly in its youth, which should have put the Earth below the freezing point of water. The rock record begs to differ, however. There’s plenty of evidence for liquid oceans on the Earth, and indicators point to a planet even warmer than the present day. So what explains the disparity?

Higher concentrations of greenhouse gases have been implicated, and a lower planetary albedo (or reflectivity) has been suggested. Another intriguing hypothesis is that the early atmosphere (which, given the lack of oxygen, was even more nitrogen-rich than it is today) was much denser. Increased atmospheric density (and therefore pressure) can enhance the impact of greenhouse gases by broadening the range of infrared wavelengths the gas absorbs.

It has been difficult to assess these hypotheses because the right kinds of evidence are hard to come by. Atmospheric density, in particular, is difficult to extract from interrogation of the rock record.

But not impossible. To accomplish this, researchers took a cue from Charles Lyell, the famous English geologist and trusted friend of Charles Darwin. In an 1851 paper describing what appeared to be raindrop imprints preserved in Carboniferous age sedimentary rocks, he wrote, “From such data [on the size of raindrop imprints] we may presume that the atmosphere of one of the remotest periods known in geology corresponded in density with that now investing the globe, and that different currents of air varied then, as now, in temperature, so as to give rise by their mixture to the condensation of aqueous vapour.”

Researchers have found raindrop imprints in 2.7 billion year old rocks made of volcanic ash in South Africa. The raindrops must have fallen while the ash was still soft and before it was covered by another layer of finer-grained ash. It appears that the rainfall didn’t last too long and wasn’t very intense, as there aren’t many overlapping imprints.

The size of a raindrop imprint depends on a couple things: the size of the raindrop responsible and its momentum upon impact. As it turns out, we don’t expect raindrop size to be different in a denser atmosphere. There’s a maximum size (around 6.8 mm in diameter) at which raindrops flatten and fragment, and that depends on properties of water like surface tension. Momentum changes with velocity, of course, and the terminal velocity of a falling raindrop depends on air density. For droplets of the same size, a larger imprint implies lower air density.

To develop a proper calibration, the researchers performed laboratory experiments with similar ashes (one being ash from the 2010 eruption of Eyjafjallajökull in Iceland). They hit the ash with droplets of varying sizes to alter momentum, and measured the resultant imprints.

Using the largest imprints in the South African rocks, they calculated the terminal velocity of the droplets that made them. They calculated this both for a maximum-size droplet and for drops of average size (3.8-5.3 mm). The larger droplet puts an upper limit on the density of Earth’s atmosphere 2.7 billion years ago at 2.3 kg/m3, or about double the modern value of 1.2 kg/m3. An average-sized droplet assigns a more likely range of 0.6 to 1.3 kg/m3.

This means that if higher atmospheric density had an impact on the early Earth’s temperature, it was too limited to explain the planet's warmth on its own. That likely steers the conversation back to elevated greenhouse gases. In addition, while some data suggests that the early ocean could have reached temperatures as high as 85°C, this upper limit on atmospheric density rules out extreme greenhouse gas concentrations, making these temperatures very, very unlikely.

A little rain can sometimes bring clearer skies. You could say the same is true for our picture of the early Earth’s climate.

Nature, 2012. DOI: 10.1038/nature10890 (About DOIs).