A raindrop is temporary, leaving behind a damp blotch and no more. Even if it falls in just the right area to create an imprint, even if that imprint is preserved for billions of years, it's just the imprint of a raindrop…right? Well, a fossilized footprint can teach archaeologists about the creatures who roamed the early Earth, but a fossilized rain imprint can tell us about the early Earth itself, and shed some new light on those scourges of global warming: greenhouse gases.


Top image: Mea Culpa Merlin / Flickr

A long, long time ago, it rained in South Africa. Specifically, it rained over volcanic ash, which preserved the imprints of the drops for 2.7 billion years. Researchers from the University of Washington analyzed these imprints to calculate the density of the Earth's atmosphere billions of years ago.


In their Nature paper, the scientists wrote, "We interpret the raindrop fossils using experiments in which water droplets of known size fall at terminal velocity into fresh and weathered volcanic ash, thus defining a relationship between imprint size and raindrop impact momentum."

To determine the velocity with which the droplets hit the ash, researchers made latex molds of the imprints and then measured them with a laser scanner. Once they knew the velocity with which the ancient raindrops fell, the researchers could compare it to the velocity of modern raindrops, and calculate how the density of the atmosphere back then differed from today's.

"We followed published methods to predict theoretically from first principles how raindrop terminal velocity changes with air density, and thus how dimensionless momentum changes with air density. Given the measurement of the largest Ventersdorp imprint, we obtained the corresponding dimensionless momentum of the impacting drop using our experimental relationship. By assuming the dimension of the raindrop responsible for the largest imprint (bounded by the maximum diameter of 6.8mm), we quantified atmospheric density."

While it's interesting to discover that the atmosphere billions of years ago was less than twice as dense as it is today, this finding actually helps clear up an ancient mystery.


More than four billion years ago, when the Earth and its sun were both young, the sun shone less brilliantly than it does today, and couldn't have kept the Earth as warm. In fact, the temperatures on Earth should have been low enough to freeze water solid, making life well nigh impossible.

But liquid water did indeed exist on Earth even during that cooler period over four billion years ago. In a 2005 press release from NASA, astrobiologist Carl Pilcher explains:



"NASA is interested in how early the Earth had abundant liquid water. If oceans form early in a planet's history, then so can life. Learning how early oceans formed on Earth will help us understand where else oceans and perhaps even life may have formed in this solar system and in planetary systems around other stars."

So what kept Earth warm enough to make water – and life – possible? Researchers had hypothesized that either the atmosphere was thicker and better able to seal in heat, or that greenhouse gases were at a higher concentration.

But this new paper proves that the atmosphere was not significantly thicker than it is today, so greenhouse gases had to be the cause. They may be causing trouble with global warming today — but life on Earth owes its very existence to the liquid water that greenhouse gases made possible billions of years ago.


Via Nature, image via Nature