Raindrop Fossils

Raindrop Fossils A meerkat sits on rock covered in fossil impressions of raindrops.
Photo Credit: Wlady Altermann/University of Pretoria

Fossilized raindrop impressions, preserved for 2.7 billion years, may reveal new information about Earth's history.


Prehistoric raindrops. I’m Bob Hirshon and this is Science Update.

Believe it or not, some fossils contain impressions of raindrops that fell billions of years ago. Astrobiologist Sanjoy Som, now at NASA-Ames Research Center, used them to study the early Earth’s atmosphere. He explains that if the air was denser back then, raindrops would have fallen more slowly.

And thus the raindrop impressions would be smaller versus if the air pressure was thin, then the drops would be falling faster, and the imprints would be larger.

While at the University of Washington, his team compared the fossilized raindrop impressions to new ones, which they made under precise conditions in the lab. Their work suggests that the air pressure back then was much closer to today’s than previously thought. Som says comparing conditions on the young Earth to the present may help us make predictions about planets beyond the solar system. I’m Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

When you think of fossils, you usually think about dinosaurs and other prehistoric life. But there are also fossils that contain signatures of physical processes, like fossilized raindrop impressions. It may sound hard to believe that something as impermanent as raindrop marks would last billions of years. However, they can, under the right conditions. For example, the raindrop impressions Som's team studied were formed when rain fell on a layer of fresh volcanic ash, and the impressions were sealed under another layer of volcanic ash. 

The raindrop impressions, found in present-day South Africa, date back 2.7 billion years. That's over half the age of the Earth itself, according to current estimates. Back then, the planet was very different from today: the sun was about 30 percent dimmer, for example, and only primitive microbial life existed—no plants or animals. The atmosphere also lacked oxygen. So in many ways, it was an alien planet compared with our own.

That's the reason Som and his colleagues became interested in learning more. Som's primary field is astrobiology, or the search for extraterrestrial life. In order to make predictions about which distant planets might harbor life, we need to know the conditions under which life can exist. We really have only one model for that: the present-day Earth. But if we can learn more about this otherworldly, prehistoric Earth environment, we can have a second and very different model for a life-supporting planet.

Their main objective was to determine how dense the early Earth's atmosphere was, compared with today's. From its density, you can begin to investigate aspects of its composition, as well as the corresponding effects it would have on climate. One mystery about this era is that although the sun was considerably less bright than it is today—so dim that all other things being equal, the Earth should have been encased in ice. Yet, it appears that temperatures then were much closer to the modern Earth's, because there's evidence that large bodies of liquid water existed then. Some scientists have hypothesized that the Earth had a much denser atmosphere two to three billion years ago, which trapped more of the heat from the sun than today's atmosphere does.  

The experiment that Som and his team performed was surprisingly low-tech, at least in certain ways. Basically, Som stood at the bottom of a stairwell with a tray of ash, carefully designed to mimic the ash that captured the raindrops over two billion years ago. An assistant dropped water droplets, carefully calibrated to mimic the size of raindrops, from seven stories above him. Although seven stories isn't nearly as high as the clouds, it's high enough for water droplets to reach terminal velocity. (Falling objects don't continue to accelerate indefinitely; once the downward force of gravity equals the upward drag force from the air, they stop accelerating and fall at a constant speed, called “terminal velocity.”) Therefore, the drops hit the ash exactly as fast as they would if they fell from a rain cloud.

This showed Som's team how big the impressions would be if that rain fell through today's atmosphere. As for the early atmosphere, they worked backwards from the size of the raindrop impression. The basic physical concept, as you heard, is that if the droplets had fallen through a denser atmosphere, they would have fallen more slowly. (To take a more extreme example, imagine a baseball falling through a tank of water instead of air). Slower-falling drops would create smaller impressions. Yet the impressions in the fossils were comparable in size to the ones Som created in the lab.

This suggests that the density of Earth's atmosphere, at this point in its history, was fairly close to today's. Which means there must be some other reason why it was warm enough for large bodies of liquid water, even though the sun was much dimmer. As to what that might be, that's for future studies to investigate. One possibility is that the atmosphere had high concentrations of greenhouse gases—the same gases that are causing climate change today—which would have warmed the planet without significantly affecting the atmosphere's density.

Now try and answer these questions:

  1. What are fossilized raindrop impressions? How did they form?
  2. Why is the early Earth like an alien planet?
  3. How did the experimenters use the raindrop impressions to draw conclusions about the early atmosphere's density?
  4. What if the raindrops in the experiment created much larger impressions than those in the fossils? What would that have implied?
You may want to check out the April 13, 2012, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: What two-billion-year-old raindrop fossils can tell us about conditions on the early Earth, what science is revealing about the artwork of Leonardo da Vinci and Vincent Van Gogh, and a breakthrough in materials science technology yields plastic that can repair itself.

Going Further

For Educators

In the lesson Fossils and Geologic Time, students learn about the development of the geologic time scale, as well the role fossils play in helping us understand Earth's history.

The Science NetLinks collection for Earth Science Week includes resources from a broad field of study that explores the way the forces of our planet intersect.

The Science NetLinks lesson Abrupt Climate Change helps students better understand paleoclimatology, or the study of the early Earth's climate.

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