2.7 billion-year old fossilised raindrops

Som et al, Nature, 2012

Imprints of raindrops found on rocks in South Africa show it was raining 2.7 billion years ago. Other exposed rocks in the same area which are more than 3 billion years old contain features associated with flowing or standing water, including ripple marks and mud cracks, glassy 'pillow' lavas and algae-like microfossils. Indirect evidence suggests the planet had terrestrial oceans as early as 4.4 billion years ago.

The raindrops also indicate what Earth’s early atmosphere was like. At that time, the Sun was about 30 percent dimmer and its energy output is estimated to have been less than 85% of what it is today. This ‘faint young Sun’ should not have been able to be warm enough to keep Earth's surface above the freezing point of water, despite evidence to the contrary. Scientists have suggested that early Earth may have been kept warm by a much thicker atmosphere, high concentrations of greenhouse gases, or a combination of the two. Recent research also suggested that it was collisions between hydrogen and nitrogen molecules in the atmosphere that kept early Earth warm, however this model did not explain fossilized raindrop imprints on Earth that date back to 2.7 billion years ago.

The fossilized raindrops suggest that the atmosphere on Earth 2.7 billion years ago was full of greenhouse gases like methane. The impressions of the raindrops were preserved in volcanic ash that later fossilized. Researchers from the University of Washington sought to discover how fast the raindrops were falling. 

In Earth’s present-day atmosphere, the biggest raindrops fall about 9 meters per second (about 30 feet per second). Raindrops falling through a thicker atmosphere would put more drag on the raindrops and lower their speed. This would mean that the same-size raindrops falling through a thicker atmosphere would leave a smaller imprint.

The researchers compared the fossilized raindrop imprints with ones modeled under the force of Earth’s present day atmosphere and used different amounts of water and a substrate similar to what existed at the time the imprints were fossilized; recent volcanic ashfall in Hawaii. Judging by the shape of the imprints, the researchers concluded that the atmospheric density 2.7 billion years ago was most likely 50 to 105% of that today. 

However, the largest possible raindrops are very rare, so the imprints may likely have been created by raindrops smaller than the maximum size. This suggests the air pressure was similar or lower than that of present day Earth. The results as a whole suggest that Earth was warm 2.7 billion years ago as a result of a build-up of greenhouse gases like methane, ethane and/or carbonyl sulphide. 

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