The depth of the depressions in what was once volcanic ash suggest how fast the raindrops were travelling when they hit the ground, which is turn gives scientists information on how dense the atmosphere was almost 3 billion years ago.
The Earth was a difference place than what is seen today, scientists have said; it rotated more slowly on its axis, the moon appeared huge in the sky because it was much closer, sunlight was much weaker and the atmosphere was unable to support life.
The fossil raindrops suggest the atmosphere 2.7 billion years ago was likely about as dense as today, perhaps a bit less, researcher Sanjoy Som from NASA's Ames Research Center in California told the BBC
That supports the idea the ancient atmosphere must have had a strong concentration of greenhouse gases.
"There was probably quite a bit of nitrogen in the atmosphere, like today, but there was no oxygen," he said.
Without extra density in the atmosphere to trap heat, only the presence of greenhouse gases would provide a blanket to keep heat in and keep the Earth from turning into a snowball planet under a substantially weaker sun, he said.