Air density 2.7 billion years ago limited to less than twice present levels by fossil raindrop imprints

Hello folks, a few weeks ago I published a paper on early Earth atmospheric pressure.

Sanjoy M. Som, David C. Catling, Jelte P. Harnmeijer, Peter M. Polivka & Roger Buick (2012) Air density 2.7 billion years ago limited to less than twice modern levels by fossil raindrop imprints, Nature 484, 359–362.

Here is the short version:

We calculate the air density 2.7 billion years ago by measuring fossil raindrop imprints. The thicker the air, the more slowly raindrops fall and the smaller the imprints become. The converse is true. Atmospheric density is obtained by coupling experiments of falling raindrops with theoretical predictions of raindrop dynamics. We find that atmospheric density was at most 2x present levels, but rainfall statistics favor an air density more probably between 0.5x - 1.05x present levels.

Or you can read an extended summary here. The paper can be found on Nature's website here, although i've attached it below to help discussion.

If this is a topic that interests you, I'd be delighted to talk about it more here and try to answer your questions!


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Sanjoy this is really neat!! I'd love to hear more about how you do the calculation. I remember you saying something about teaching measuring impact imprints for a class - do you have any sort of lesson plan built around this topic?

Hi Sara,

Thanks for your interest! The calculation is actually quite straight forward. Given that it is raindrop impact momentum that governs imprint size, and given that the largest possible drop size (~ 7mm in diameter) is independent of air density, one needs i) a relationship between raindrop impact momentum and imprint size, and ii) a relationship between air density and raindrop momentum (for a raindrop of given size).

The first relationship is obtained by experiments, which Peter Polivka and I did in a stairwell. We released raindrops of different sizes (and thus different impact momentum) onto a substrate similar to the one that the fossilized raindrop imprints are found in, and measured the resulting imprint dimensions. Now, from theory, one can calculate the impact momentum of raindrops falling at different air densities, so ii) is easily obtained from first principles.

So by measuring the largest fossilized imprints, we used the experimental relationship to find the corresponding momentum, and assuming that the largest imprints were formed by the largest raindrop allowed, used theory to related impact momentum found in i) with air density. That gave us the air density upper bound!

I don't have a lesson plan written, but raindrops are easily captured by placing a tray of flour in a rainstorm for a few seconds, so some fun can be had with that! It could be interesting for students to see how broadly raindrop sizes are in a typical rain event, and compare such distributions with those found in different climates or in different seasons! It's an idea I'd like to one day develop into a citizen science project.

Thanks for the detailed response!! I think that would make a fabulous citizen science project! I might adopt this as a lesson plan next time I do a classroom outreach event. It is just so cool how simple physics captures so much. Thanks for the inspiration! =D

Hey Sanjoy, 

Loved this paper when it came out, and I spoke to Roger last month about it when we were in Lyon together.  I have some pretty sexy imprints form 1 Ga rocks in Scotland that I'm pretty certain are not raindrops - seems like you guys would be perfect to get your advice.  

If I'm right and they're not imprints, then I think they may be giant bacterial colonies, something thats not been seen in these rock types before.  Nothing so exciting as atmospheric density, but it could be a neat story in the paleontological side of things?

Hey Leila,

Thanks! I'd very much enjoy chatting about those! Do you have some images?

I can show you in person, but here's a taster

Try again ;)


Hmmmm they look too uniform in size to be raindrop imprints, but how about I cross the hallway and check'em out :)

Whenever you like :)



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