Ancient Raindrops provide Insight into our Early Atmosphere

Chris - What was the atmosphere like on the Earth nearly 3 billion years ago? Well that's a pretty tough question to answer, but incredibly, some fossilised raindrops or rather, the patterns that they left behind when they fell 2.7 billion years ago have enabled scientists to reconstruct some aspects of what the air that rain fell through would've been like. And with us to explain how, from the University of Washington in Seattle is Roger Buick who's one of the authors on the study. Hello, Roger.

Roger - Good day!

克里斯,o what are you actually trying to understand about the early atmosphere?

Roger - What we really want to know is why the Earth wasn't an ice ball 3 billion years ago. The Sun was only 80% as bright then as it is now and if the atmosphere was the same as it is now on the Earth, the planet should've been completely frozen over.

Chris - Yet we've got geological evidence of their having been running water and quite a balmy climate.

Roger - There's abundant geological evidence for liquid water - oceans, rivers, lakes, and now rain as well. Either we had to have a much denser atmosphere, or else we had to have a whole lot more greenhouse gases in the atmosphere to keep us warm. And there's been a debate going on for 20 odd years as to what sort of atmospheric warming mechanism was active on the early Earth.

Chris - And obviously, without the capacity to go back 3 billion years or so and take a snapshot of that atmosphere, we're forced to rely on indirect measures which I guess is what you've done in this paper inNaturethis week. You've used an indirect measure to work out what the atmosphere must have characteristically been like.

罗杰——是的,直到托里拆利发明了barometer, we've really got no real readings of what atmospheric pressure was like early in Earth history. The atmosphere has a very, very indirect and faint impact on rocks that can get preserved from early in Earth history. So, to be able to register anything about the atmosphere from early in Earth's times is really quite remarkable.

克里斯,o how did you actually do this?

Roger - Well, we found some literature records of raindrop imprints in rocks, almost 3 billion years old. There's a number of places in the world where there are these ancient raindrop craters and the best ones that we were able to come across were in South Africa, out in the Karu-velt of South Africa. And so, we went there and took samples of them and made latex peels off them and measured them. Now there's a relationship between the size of raindrop imprints and the speed at which the rain fell, and the speed at which the rain fell is influenced by the atmospheric density. The denser the atmosphere, the slower the raindrops fall, so the smaller the little craters that they form.

克里斯,o how do you work out based on the rock samples that you've got from 3 billion years ago, how big the crater should be in proportion to the speed of the raindrops? How did you close that gap in our knowledge?

Roger - Well, the big variable is the nature of the material being impacted. The South African raindrop imprints were in volcanic ash, so what we did was get some equivalent modern volcanic ash from the Icelandic eruption a couple of years ago and did experiments by dropping artificial raindrops down a 7-floor stairwell onto a pie plate full of this volcanic ash...

Chris - It's high tech stuff, this! And so, that was your sort of proxy measure. You could see based on how fast or therefore how much energy, how much momentum, the raindrops had and based on how big a splatter imprint they make you can extrapolate from the latex peels from the South African rocks, how fast the raindrops must've been going 2.7 billion years ago.

Roger - Exactly. The other variable of course is the size of the raindrop. As anybody British will know, raindrops vary greatly in size depending on the intensity of the rainfall.

Chris - Well at the moment, we're in drought in the southeast, so we don't have any rain at all. Zero is the size! So what did you do to constrain that?

Roger - We do know that there is an absolute maximum size to which raindrops can get. It's a proportion of the interplay between the surface tension of the water and the force of gravity trying to pull it to bits. And so, the largest that's ever been recorded on the Earth and the theoretical maximum size from physics is 6.8 mm. So, that sets a maximum bound on what the atmospheric pressure could've been, but it's unlikely that the absolute maximum raindrop size was achieved. Probably more like 5 mm was the realistic maximum raindrop size.

克里斯,o putting all that together, what does this tell us about the atmosphere 2.7 billion years ago when these - what are now rocks - were just volcanic ash and this moderate shower was falling on them?

Roger - Well, it us tells that the maximum possible atmospheric pressure was about 1.6 bars, so 1.5 times what it is now, but more likely, it was 1.1 bar, 1.1 atmospheres, well maybe even as low as 0.6 of a bar, so only 6 tenths of our current atmospheric pressure.

克里斯,o very, very similar to what we have today which argues that we couldn't have had just this blanketing smog of CO₂, giving us an artificial greenhouse effect to keep the Earth warm then.

Roger - That's right. Most likely, we had to have some other sort of greenhouse gas other than just carbon dioxide in the atmosphere to warm the Earth. Maybe something like methane, though that's problematic; if you have high levels of methane in the atmosphere, you produce a haze of organic particles formed by ultraviolet light and that reflects sunlight and has an anti-greenhouse effect. Other possible greenhouse gases would be, my favourite is laughing gas - nitrous oxide. There is some evidence that there was a biological nitrogen cycle producing laughing gas, back as far as about 2.7 billion years ago. So, laughing gas is something like 100 times more effective as a greenhouse gas than carbon dioxide. So we might have had some methane and some laughing gas in the atmosphere as well.