Atmospheric pressure is a key characteristic of the climate on Earth and on other planets. Mars, for example, is dry and cold partly because of its very thin atmosphere - the low air pressure means the planet is unable to trap heat or retain water vapor.

Air pressure also can affect the chemical reactions that life depends upon. "The pressure is just as important as the temperature," says Roger Buick of the University of Washington.

Although there are several ways to try to gauge what the temperature was like on the ancient Earth, there are currently no "paleobarometers" for measuring the pressure of the past. "Nobody knows what the pressure was before 1643, when Evangelista Torricelli built the first barometer," Buick says.

To help remedy this, Buick has been awarded a grant from the NASA's Astrobiology: Exobiology and Evolutionary Biology program to conduct paleobarometry studies on rocks that formed during the Archean period when the Earth was less than half the age it is now.

The goal is to determine the air pressure back then to within an accuracy of 0.1 bar (the current atmospheric pressure is approximately equal to 1 bar ).

Archean conditions
The Archean period lasted from 3.8 to 2.5 billion years ago. Liquid water was present, as were single-celled organisms like bacteria and archaea. These life forms were likely anaerobic (not breathing oxygen) because -- as many scientists contend -- the atmosphere did not become oxygen-rich until a few hundred million years after the Archean. However, a small minority of researchers believe oxygen levels rose much earlier, Buick says.

Another uncertainty concerns how the Archean Earth remained warm despite the fact that the sun was 25 to 30 percent weaker back then.

"The big problem with Archean climate is the so-called faint young sun problem," says geoscientist Jim Kasting of Penn State University. A bigger greenhouse effect on Earth must have compensated for the weaker sun, otherwise the global temperature would have dropped below freezing. However, data from ancient soils (paleosols) indicate the amount of the greenhouse gas carbon dioxide back then was limited.

"It is difficult to keep Earth warm without requiring more CO2 than is permitted by the paleosol data," Kasting says.

The solution may be methane gas. One molecule of methane has the same greenhouse effect as 25 molecules of carbon dioxide. But just what mixture of methane, carbon dioxide and other greenhouse gases provided the Archean "heat blanket" is anybody's guess.

"You can assume almost anything you like," Buick says. That's why he and his colleagues have devised two techniques for putting solid constraints on what constituted Earth's ancient air.

Lava bubbles
The first method will study cavities found inside lava rocks (basalt) from the Archean period. Because lava cools quickly, it traps gas bubbles inside, which eventually give rise to the cavities. During formation, the size of the bubbles is set by two factors: the atmospheric pressure and the amount of lava weighing down on the bubble.

The resulting cavity widths range from 5 millimeters near the top to about a millimeter at the bottom. By comparing how the sizes vary with depth inside the basalt, Buick and his colleagues hope to estimate the air pressure when the lava cooled.

This technique has been used before as a paleoaltimeter for younger rocks, indicating at what altitudes the rocks formed. Buick's team, including Sanjoy Som and David Catling, also of the University of Washington, will try to measure the cavities in some of the oldest rocks that are known. "Archean rocks are very rare, and well-preserved ones even more so," Buick says.

Luckily, he and his colleagues have located several samples of 2.7-billion-year-old basalt with near-perfect cavities from northwest Australia. The holes have subsequently filled in with other minerals, so to measure the original cavity sizes the researchers will use a CAT scan operated by Whitey Hagadorn of Amherst College.

Rain craters
The other paleobarometry technique draws on the fact that raindrops are limited by air friction in how big they can grow. The higher the atmospheric pressure is, the greater the air friction and the smaller the maximum size that is possible. Buick explains that the maximum size is rarely reached in places that drizzle a lot, like Seattle where he lives. But in semi-arid areas, raindrops can grow to the Earth's current limit of 6.5 millimeters in diameter.

Obviously, there are no raindrops surviving from the Archean, but last year Buick and his colleague Jelte Harnmeijer discovered rocks in South Africa that formed out of rain-soaked mud around 2.7 billion years ago. When the mud dried and hardened to rock, it retained craters that were the fossilized imprints of raindrops.

"They look like little meteor impacts, a few millimeters across," Buick says. But he's confident that rain, not micrometeorites or hail, made these craters because there are no signs of bouncing or rolling as one would expect from a solid projectile. Moreover, the location of some of the little holes inside the rock rules out simple erosion.

He has taken rubber casts of the craters and now plans to determine their volume using a laser scanner. From the largest in the sample, he and his collaborators should be able to estimate the atmospheric pressure when the rain storm broke.

Previously, soil scientists determined how big a crater forms from a given raindrop size, but this never has been applied to ancient rocks because no one has ever before seen well-preserved raindrop craters from that long ago. Buick is so thrilled with his rare and precious find that he prefers to keep the exact location secret, only saying that it is located in an area "guarded by baboons, jackals and other fierce animals."

Implications
"Roger's proposal is the first effort of which I am aware to directly determine Archean atmospheric pressure," Kasting says.

"I'm very much looking forward to seeing what he comes up with." If a high pressure is found for the Archean, that would be further evidence of greater greenhouse warming and a warmer climate back then. Combined with other constraints, Buick expects an Archean pressure reading will improve estimates for how much carbon dioxide, methane, oxygen and other gases were available on the early Earth.

This could have important implications for our understanding of what sort of life existed during this period. A high estimate of methane, for example, could confirm the presence of "methanogens" (methane-producing archaea) among the Archean life forms.

However, if Buick were to find a pressure that was consistent with a high oxygen level, that could mean the rise of photosynthesizing cyanobacteria occurred earlier than many would have expected.

Interestingly, these techniques might one day be used to measure the past pressures on other worlds. The Viking landers caught glimpses of gas bubbles in martian basalt rocks. And there is good evidence that some mixture of hydrocarbons rain.

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