FOSSIL RAIN: The imprints of fossil raindrops from 2.7 billion years ago reveal details of the early Earth's atmosphere.
Image: Courtesy of Sanjoy Som
Some 2.7 billion years ago in what is now Omdraaisvlei farm near Prieska, South Africa, a brief storm dropped mild rain on a new layer of ash laid down by a recent volcanic eruption (not unlike ash from the 2010 Eyjafjallajökull eruption in Iceland) forming tiny craters. Additional ash subsequently buried the craters and, over eons, hardened to become rock known as tuff. Closer to the present, other rainstorms eroded the overlying tuff, exposing a fossil record of raindrops from the Archean eon, and may now have revealed the density of early Earth's atmosphere.
By scanning with lasers the craters created by ancient raindrops—and comparing the indentations with those made by water drops sprinkled onto a layer of similar ash today—physicist Sanjoy Som of the University of Washington in Seattle and his colleagues have derived a measurement of the pressure exerted by the primitive atmosphere. The scientists report in Nature on March 29 that the ancient air could not have been much denser than the present atmosphere—and, in fact, may have been much less so. (Scientific American is part of Nature Publishing Group.)
"Air pressure 2.7 billion years ago was at most twice present levels, and more likely no higher than at present," Som explains. The key to that determination is raindrop size. Back in 1851 pioneering geologist Charles Lyell suggested that measuring the fossilized indentations of raindrops might reveal details about the ancient atmosphere. These mini-craters are formed based on the size and speed of ancient raindrops. Because the atmosphere drags on each drop, constraining the speed of its descent based on its size, if one could determine an ancient raindrop's size, one could determine how thick the atmosphere likely was.
The largest raindrop ever measured in modern times was 6.8 millimeters around, Som notes, which is also the theoretical limit; larger raindrops break apart. Because the laws of physics were likely the same in the distant past, this suggests that raindrops were no bigger in the Archean and puts an upper limit how big the ancient drops could have been. Plus, such raindrops are exceedingly rare in modern storms—and tend to fall in powerful downpours, which in the Archean would have been more likely to have washed ash away rather than form craters that could be fossilized.
To determine the size of the ancient droplets, Som and his colleagues compared the fossilized imprints with the craters that formed when they released various-size droplets from 27 meters above similar ash taken from the 2010 Eyjafjallajökull eruption in Iceland as well as from Hawaii. They then turned these modern craters to "rock" "using hair spray and low-viscosity liquid urethane plastic." Based on the comparisons, they concluded that the size of ancient droplets fell in the range of 3.8 to 5.3 millimeters.
Plugging those numbers into the mathematical relationship between raindrop size, speed and atmospheric density suggests that the early Earth's atmosphere exerted at most twice as much pressure as the present day atmosphere—assuming raindrops of the maximum size and speed created the craters—and more likely was roughly the same or as little as half present pressure.
A better understanding of the properties of Archean Earth's atmosphere may help explain what's known as the "faint young sun" paradox. Billions of years ago, the sun emitted less radiation, roughly 85 percent of its present output, and therefore heated the planet less. Yet, the fossil records suggest abundant liquid water and other signs of a warm, "clement" climate, as Som and colleagues noted in the analysis. The simplest explanation for this is that Earth simply boasted an atmosphere thick with greenhouse gases. "The sky was probably hazy," from the gases, Som says, in addition to being ruled by a fainter sun that passed across the sky more quickly because Earth rotated faster then. Plus, the atmosphere lacked a significant quantity of oxygen (because there were no plants), potentially lightening the atmospheric pressure. "Earth back then looked nothing like it does today."
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Add Comment@Howard ...wow. There's abundant fossil evidence of liquid water throughout Earth's history, well prior to 2.7 billion years ago. The 1st most common element in the universe is hydrogen and the 3rd is oxygen, and their only favored, stable molecule is H2O; peroxides ain't stable. Earth's fossil record shows temperatures comparable to modern Earth's, give or take 20C, which only favors a limited range of chemicals for precipitation. And despite all that information available at a casual Google search, you decide that it is bad science to suggest water is the likely cause behind rain on a planet that's been covered in water for 4.5 billion years. Yeah, and maybe those aren't dollar bills in my wallet, they're winning lottery tickets.
Reply | Report Abuse | Link to this..interesting, but there's a big range on the possible atmospheric density. Otherwise much inference, but most people would take it as a safe bet that the raindrops were H2O.
Reply | Report Abuse | Link to thisI'm no climate scientist, but if the size of rain drops depends on the density of atmospheric pressure, why do we get rain drops in all sizes nowadays, even within the same region, season, time of day, etc?
Reply | Report Abuse | Link to this2.7 billion years and this is the only rock they found; I do not think so unless the rain back then was very selective in where it landed. This sounds like another African fallacy. In that length of time, that rock and that area would be buried under tons of space dirt and Earth dirt, and you are not going to convince me that in 2.7 billion years, there was no activity of any kind from anything in that area of the Earth. Did you forget that humans and all the other animals and birds sprang from Africa; another fallacy? That is a lot of activity for anything to survive that long laying on the ground, unless that Meerkat (check out Popular Science's same article http://www.popsci.com/science/article/2012-03/fossilized-raindrop-dimples-finger-greenhouse-gases-source-ancient-earths-warmth) was protecting that rock all that time...that is just as believable.
Reply | Report Abuse | Link to thisThere had to have been other rain storms and wind storms that would had eroded that dust before it could fossilize, which would probably take thousands of years, and if it was already a rock, the rain will not pit it like that. I think you need to drum up another explanation for those pit in that rock, because not even Mrs. Jones' third-grade science class will believe this one.
It's always amusing when someone with a bit of school science starts to blast away at people who know more than them.
Reply | Report Abuse | Link to this"There's abundant fossil evidence of liquid water throughout Earth's history, well prior to 2.7 billion years ago."
Yes, but there is no evidence that it was undiluted or pure - or evidence that it rained pure H2O either.
Science is about Evidence, not smart arse'd amateurs.
I'd like an answer for that too-- I was just thinking yesterday that I was being pounded by really big raindrops then they turned to hail stones.
Reply | Report Abuse | Link to thisRe: “...Archean Earth's atmosphere...lacked a significant quantity of oxygen (because there were no plants)...”
Reply | Report Abuse | Link to thisGeologists Harry Clemmey and Nick Badham seem to have disputed the earlier part of the quote:
“Geological evidence often presented in favor of an early anoxic atmosphere is both contentious and ambiguous. The features that should be present in the geological record had there been such an atmosphere seem to be missing...[T]he dogma has arisen that Earth’s early atmosphere was anoxic, probably highly reducing...”
[1982. Oxygen in the Precambrian Atmosphere: An Evaluation of the geological Evidence. GEOLOGY, March, p. 141]
Geologist R. M. Stainforth seems to have disputed the latter:
“The discovery of pollen and spores in beds considered Precambrian (Proterozoic) has received brief notice in geological journals and the press….As stated, we offer no solution to the paradox. It is clear, however, that botanist Dunsterville in his hunt for rare orchids stumbled on a highly intriguing geological problem.”
[1966. Occurrence of Pollen and Spores in the Roraima Formation of Venezuela and British Guiana. NATURE, Vol. 210, April 16, pp. 292, 294]
Nice piece of work. This will be a very useful constraint for astrobiology! (Astrobiology student here.)
Reply | Report Abuse | Link to this@ drevabernat:
We don't get rain drops "in all sizes", that is why this work could be done. The constraint used is on maximum size of rain drops, which is a physical constraint due to the behavior of falling drops of water.
For the distribution of sizes you have to add how they originate, since smaller drops will not break apart during fall and contribute to the size distribution. Then indeed you have to ask climate scientists, or at least cloud formation experts.
@ JamesDavis:
Your comment is fractally wrong, so no one can reply to all errors. The short reply is that all you claim is erroneous. What preserves ash is usually more ash, so it was bound to happen at some time that just one short rain fall would be preserved by the next eruption.
@ HowardB:
We have plenty of evidence that the oceans were at least as salty as today. A sky filled with organics would add plenty of sea water organics too, despite the organisms thriving on it.
That doesn't mean clouds and precipitation is especially dirty. What evaporates to make up clouds is water molecules.
@ Bill_Crofut:
You insert spurious references. It is however true that a reducing atmosphere was (and is) arguable, today scientists believe the early atmosphere was at least neutral IIRC.
Torbj�rn Larsson, OM,
Reply | Report Abuse | Link to thisIf you agree that "...a reducing atmosphere was (and is) arguable..." then why are my references spurious?
What does a >>BELIEF<< in the early atmosphere have to do with science?
Reply | Report Abuse | Link to thisRainwater usually contains chloride, nitrate, and sulfate anions, as well as phosphate and nitrite anions. That is what rainfall currently contains. It is very possible that the composition was significantly different 2.7 billion years ago.
And I quote, "There had to have been other rain storms and wind storms that would had eroded that dust before it could fossilize, which would probably take thousands of years, There had to have been other rain storms and wind storms that would had eroded that dust before it could fossilize, which would probably take thousands of years . . ." Maybe you should tell that to the people of Pompeii and Herculaneum. It didn't take them very long to become fossilized - or at least their holes.
Reply | Report Abuse | Link to thisAnd if you can't find them, tell me - or my wife or my kids or any one of the millions of people who were around on May 18, 1980 when Mt. St. Helens blew. The ash (at our house) started in the afternoon and kept falling into the evening. The next morning it was pretty well done except for the silvery haze in the air. About 2 or 3 days later there was a rain which cleared up the air but firmed up and put a crust on the top of the ash. I don't remember a lot about the precipitation schedule for the rest of the spring and summer - too busy worrying about our animals, getting to work and other places without ruining our vehicle engines, etc.
We were lucky, St. Helens was small and it only seriously erupted once. However if Rainier, Mazama (Crater Lake), or worst-case Yellowstone had erupted and the ash was of a different constituency it could easily have rested on top of the crust and differentially eroded (much later). Remember, this was a very different earth and 2.7 billion years ago is a long time. If they were off by just a little, say .1 billion years - that is 100,000,000 years. 100 million years ago the dinosaurs were just really getting goodly started. A lot of time for just the right sequences of events to happen.
Another commentator talked about pollen and spores in the precambrian. The precambrian didn't end until over 2 billion years later then the 2.7 billion years we are talking about here. A lot of stuff happens in over 2 billion years. There was almost undoubtedly life 2.7 bya but it was very different from what we have now, definitely anaerobic.
bbremner,
Reply | Report Abuse | Link to thisMy commentary also included the quote from geologists Clemmey and Badham who seem to have called into question an anoxic atmosphere for Earth at any time in the past.