Raindrops that fell 2.7 billion years ago, in the Archaean eon, have not survived, of course, but indentations they made in volcanic ash remain to this day, preserved in the solidified ash, a rock known as tuff.
In 1851, pioneering geologist Charles Lyell suggested that ancient raindrop indentations could be used to determine ancient air density. No one seems to have followed up on that idea until recently, when astrobiologist Sanjoy M. Som, then of the University of Washington, Seattle, and four colleagues examined the imprints of 955 individual Archaean raindrops at Omdraaivlei, South Africa. Latex casts of the indentations were precisely measured using high-resolution, three-dimensional laser scanning.
The researchers then collected samples of fresh volcanic ash from the 2010 Eyjafjallajökull eruption in Iceland and weathered Pleistocene ash from Hawaii, both samples with grain-size distributions similar to that of the Archaean ash. Next, they released droplets of known mass onto the ash samples from a height of eighty-nine feet, high enough to assure that the drops reached their maximum speed, or terminal velocity, before impacting the ash, as the ancient natural raindrops would have. The old and new indentations were then compared.
The results help explain how the Sun, which was as much as 30 percent dimmer in the Archaean, could have kept Earth warm enough to hold liquid water, as it did. One thought was that the atmosphere was significantly denser than at present. If it was, it would have slowed the speed, and thereby lessened the impact, of raindrops. The results indicate, however, that the Archaean atmosphere was the same or likely even less dense than ours. That left a strong alternative hypothesis: that concentrations of highly effective greenhouse gases, such as methane, were much higher during the Archaean, trapping what heat the Sun did provide. (Nature)