Tiny bubbles of air buried deep in the ice of Eastern Antarctica may contain bits of the Earth's atmosphere as it was 1.5 million years ago, according to a new report.
"Ice is a great medium for trapping air," said Ed Brook of Oregon State University, one of several authors of a paper in the journal Climate of the Past that describes where this ancient ice might be.
"It traps it without altering it very much."
Most of us think of ice forming when liquid water freezes, but the ice in the North and South poles formed from thousands of years of snowfall that never melted.
Over time, the weight of the newer snow compacts the individual snowflakes beneath it, causing them to grow together until they eventually form ice.
As the snow gets pushed together, the air between the individual snowflakes form long channels. Eventually those channels close off to form air bubbles, Brook explained.
"When you don't have any melting, you get this great preservation," he said.
But ancient ice, and the ancient air trapped within it, will be difficult to find. Even in the coldest places on our planet, most of it has melted, if not from the heat of the Sun, then from geothermal heat that arises from within the Earth.
And although we think of ice as fairly solid, the ice at the bottom of the polar ice sheets does in fact move, very slowly, out toward the oceans. And this movement can mix up the ice and the air.
So far, the oldest ice ever collected goes back 800,000 years. Scientists think that the even older ice may be about 3.2 kilometres beneath the Antarctic ice sheet.
While finding the oldest air and ice is kind of cool all on its own, the scientists are hoping that ice more than a million years old can help them solve a scientific mystery.
Over the last 800,000 years, the Earth has gone through cycles of ice ages and warm periods that repeat about every 100,000 years. But studies of ocean sediments have revealed that more than 900,000 years ago, the Earth cycled through ice ages and warm periods every 40,000 years.
Why the change? One hypothesis is that millions of years ago, there were higher amounts of greenhouse gases.
The best way to test that theory is to see if actual air from the time was indeed higher in greenhouse gases.
The international science team identified a few areas in the South Pole where the conditions may be right to find ice that formed more than 1 million years ago.
These are places where the ice sheet is not too thick, and where there is less geothermal heat flow.
"It is not like X-marks the spot though," Brook said.
"The point of the paper is to say these are the spots where we should look more carefully."
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