One of the biggest unknowns about the environment of the ancient Earth, billions of years ago when life was first finding a foothold, is just how thick the atmosphere was. Geochemists can get some idea of the composition of different gases in the air at the time, but there's no way of judging what the atmospheric pressure was. And this is crucial for understanding how much insulation our young planet received.
But now, David Catling at the University of Washington in Seattle has come up with an intriguing way for measuring the thickness of Earth's primordial atmosphere: study the ancient raindrop impressions formed in some sediments before they turned to stone.
Catling found some of the remarkable fossilised raindrop impressions in an ancient bed of volcanic ash in South Africa. The tiny craters are rarely preserved in the geologic record, says Catling, because to do so you typically need a plain of freshly erupted volcanic ash and a timely shower of rain - but not too heavy a downpour, else the droplets will run together and the ash will be washed away. In other words, you require a perfect rainstorm of lucky conditions.
The volcanic ash that Catling and his team studied is the oldest known record of one such perfect storm: it is 2.7 billion years old, formed at a time when the Earth was deep in the Archaean aeon and populated only with slimy biofilms of microbes.
What Catling realised is that simple physics determines the maximum size of a raindrop. Key factors are the surface tension of water, the strength of gravity and the density of air: the thicker the primordial atmosphere, the better it would have been at breaking apart falling rain into smaller droplets.
Only that final factor will have changed over the planet's history, and so by measuring the size of raindrops you can indirectly measure atmospheric density. Catling's collaborator, Roger Buick, also at the University of Washington, created impressions of these spattered rocks with latex peels, and then used a laser scanner and computer software to measure the crater sizes.
The final piece of the puzzle was linking the size of water droplets to the size of the craters they had caused. Catling's graduate student, Sanjoy Som, was crucial here, leaning out of a seventh-storey window to pipette water down onto a tray of Icelandic volcanic ash below.
Graham Shields - a geologist at University College London, who was at the Life and The Planet meeting at the Geological Society in London where Catling presented his work last week - agrees that it is a fascinating study. "This is some very exciting work. We don't have any measurable proxies for the atmospheric pressure, and until now no one's thought of using raindrop size. I'll certainly be very interested to see these results."
And what are those results? Catling is understandably tight-lipped right now, because the paper is about to go through peer review, and the ramifications of the discovery could be very wide-ranging.
"The earliest measurement we've got of the Earth's atmospheric pressure is from 1644 when Evangelista Torricelli invented the barometer," says Catling. "We hope to be able to push this back by quite a bit."
Author: Lewis Dartnell | Source: New Scientist [May 11, 2011]
But now, David Catling at the University of Washington in Seattle has come up with an intriguing way for measuring the thickness of Earth's primordial atmosphere: study the ancient raindrop impressions formed in some sediments before they turned to stone.
Catling found some of the remarkable fossilised raindrop impressions in an ancient bed of volcanic ash in South Africa. The tiny craters are rarely preserved in the geologic record, says Catling, because to do so you typically need a plain of freshly erupted volcanic ash and a timely shower of rain - but not too heavy a downpour, else the droplets will run together and the ash will be washed away. In other words, you require a perfect rainstorm of lucky conditions.
The volcanic ash that Catling and his team studied is the oldest known record of one such perfect storm: it is 2.7 billion years old, formed at a time when the Earth was deep in the Archaean aeon and populated only with slimy biofilms of microbes.
What Catling realised is that simple physics determines the maximum size of a raindrop. Key factors are the surface tension of water, the strength of gravity and the density of air: the thicker the primordial atmosphere, the better it would have been at breaking apart falling rain into smaller droplets.
Only that final factor will have changed over the planet's history, and so by measuring the size of raindrops you can indirectly measure atmospheric density. Catling's collaborator, Roger Buick, also at the University of Washington, created impressions of these spattered rocks with latex peels, and then used a laser scanner and computer software to measure the crater sizes.
The final piece of the puzzle was linking the size of water droplets to the size of the craters they had caused. Catling's graduate student, Sanjoy Som, was crucial here, leaning out of a seventh-storey window to pipette water down onto a tray of Icelandic volcanic ash below.
Graham Shields - a geologist at University College London, who was at the Life and The Planet meeting at the Geological Society in London where Catling presented his work last week - agrees that it is a fascinating study. "This is some very exciting work. We don't have any measurable proxies for the atmospheric pressure, and until now no one's thought of using raindrop size. I'll certainly be very interested to see these results."
And what are those results? Catling is understandably tight-lipped right now, because the paper is about to go through peer review, and the ramifications of the discovery could be very wide-ranging.
"The earliest measurement we've got of the Earth's atmospheric pressure is from 1644 when Evangelista Torricelli invented the barometer," says Catling. "We hope to be able to push this back by quite a bit."
Author: Lewis Dartnell | Source: New Scientist [May 11, 2011]
