Fossilized Rain Drops!

Today I read an intriguing study about fossilized rain drop impressions and how they can be used to measure earth's atmospheric pressure 2.7 billion years ago!

(Image: Verrisimilus/CC BY-SA 3.0)

Firstly, it is pretty amazing that something as delicate as a rain drop impression can fossilize and survive for 2.7 billion years! All you need are perfect conditions - ground is covered with a fresh covering of volcanic ash and rainfall is not too heavy or too light but just the right size. The impressions may then harden into into stone, preserving them for present day scientists.

The second and more beautiful aspect is the implication. It turns out that size of a rain drop is strongly correlated with surface tension, force of gravity, and most importantly atmospheric pressure. By careful measurement of the indentations and some simulation, the atmospheric pressure seen by the earliest microbial lifeforms can be extracted. The value of atmospheric pressure is also useful to extract the amount of insolation the young earth received.

While geologists did have some idea about the gas composition of the ancient atmosphere, before this study almost nothing was known about the pressure. The full paper is under peer-review.