Preserved imprints of raindrops that fell 2.7 billion years ago, frozen in volcanic ash. By measuring the splash marks, scientists calculated ancient Earth's atmosphere was less than half as dense as today's. This single rain shower answered a fundamental question about our planet's history.
Rygel, M.C., CC BY-SA 3.0, via Wikimedia Commons
A shower frozen in time
In the volcanic rock of South Africa's Ventersdorp Supergroup, geologists found a delicate event recorded in stone: the impact craters of a 2.7-billion-year-old rain shower. During the Neoarchean Eon, a time when Earth's air was unbreathable and life consisted of microbes, a volcano erupted, blanketing the landscape in a layer of fine ash. A brief rainstorm followed, speckling the fresh ash bed with tiny pits. Before these impressions could erode, a second volcanic blast sealed them beneath another layer, preserving them for billions of years.
These are not the rocks, but latex casts made from them, that scientists study. Researchers led by astrobiologist Sanjoy Som poured latex over the fossilized tuff to create detailed peels of the raindrop impressions. Back in the laboratory, high-precision laser scans measured the exact shape and size of the pits. By comparing these ancient imprints to modern experiments—where water drops were released down a stairwell into trays of recent volcanic ash—the team could reconstruct the conditions of the ancient downpour.
An atmosphere half as thick
The physics of a falling raindrop acts as a barometer. A drop's maximum speed, or terminal velocity, is a balance between the pull of gravity and the drag from the air it falls through. A thicker, denser atmosphere creates more drag, slowing a raindrop's descent and causing a smaller splash. In a thinner atmosphere, the same drop falls faster and hits harder, leaving a larger crater.
The analysis, published in Nature, concluded that the atmospheric density 2.7 billion years ago was between 50% and 105% of today's. This finding presents a paradox. During the Neoarchean, the Sun was about 20-30% dimmer than it is now. With such a faint Sun and a thin atmosphere, Earth should have been a frozen ball of ice. Yet, extensive geological evidence shows that liquid water and rivers were abundant.
The thin atmosphere indicated by the raindrops means that something else must have kept the planet warm. The solution is likely a powerful greenhouse effect. The Archean atmosphere must have contained much higher concentrations of potent greenhouse gases, like methane and carbon dioxide, to trap the faint sunlight and maintain temperatures above freezing. This single, ancient rain shower provides a physical constraint on atmospheric models of the early Earth, showing how our planet remained habitable.
💡Fun Facts
To calibrate their findings, scientists dropped water from a pipette down an 88-foot stairwell into trays of volcanic ash from Hawaii and Iceland.
The largest possible raindrop that can form naturally is about 6.8 millimeters in diameter; any larger and it breaks apart as it falls.
The idea of using fossilized raindrops to measure ancient atmospheric pressure was first proposed by geologist Charles Lyell in 1851.
The Ventersdorp Supergroup, where the fossils were found, is a massive geological formation up to 4,500 meters thick, composed mostly of ancient volcanic rock.
This is not a formal tourist site but a feature within a widespread geological formation. There are no opening hours for visiting the general area.
Admission
Free.
Accessibility
The specific fossil sites are not marked or developed for tourism. Access would require navigating to remote, rugged geological outcrops in the North West Province, likely with a 4x4 vehicle and hiking.
The Fossilized RaindropsNorth West Province, South Africa
