2016-05-09. Earth’s ancient atmosphere was half as thick as it is today.

posted May 14, 2016, 11:58 AM by Alan Gould
By Roland Pease, Science. For GSS Life and Climate chapter 2. Excerpt: ...very little is known about how thick Earth’s ancient atmosphere once was. Now, a new study suggests that Earth’s atmosphere 2.7 billion years ago was between a quarter to half as thick as it is today. The finding could force scientists to re-examine nearly everything they know about Earth’s early atmosphere, from nitrogen-fixing cycles to how the young planet trapped enough heat to give rise life as we know it. Scientists have long assumed that Earth’s ancient atmosphere had a stronger greenhouse than today’s. That’s because the sun put out 20% less heat than it does today, and even with elevated levels of greenhouse gases, Earth would have struggled to keep global temperatures above freezing. A thicker atmosphere would have helped to compensate. But in recent research, this expected thickness hasn’t been found: A 2012 study of fossilized raindrops, for example, found that Earth’s early atmosphere was as little as half as thick as it is today. Now, the same team of scientists has come up with a more precise way of measuring early pressure:  gas bubbles trapped in ancient lava. In what is now the Australian outback, basalt lavas poured out over thousands of square kilometers billions of years ago, hardening when they hit seawater. The lavas contained dissolved gases that fizzed as they emerged onto Earth’s surface.... By measuring the size of the bubbles at the top—which were pushing against the weight of the atmosphere—and comparing them with the smaller bubbles at the bottom—which were pushing against both the atmosphere and the weight of the rock itself—the team came up with a proxy for ancient air pressure. ...low nitrogen levels, however, close off a nitrogen-boosted greenhouse effect as one favored solution to the problem of keeping Earth warm when the young sun was faint. Evidence has accumulated that levels of the known greenhouse gases, like carbon dioxide and methane, were inadequate to compensate for the low warmth of the early sun. Yet evidence of ancient liquid water makes it clear the planet had not chilled below freezing. ...Goldblatt concedes that the new findings, if confirmed, kill that solution. “With such little pressure, it’s going to be very hard to account for the warm temperatures,” he says. But he still wants more evidence: A series of measures from other basalts of different ages would be ideal....  http://www.sciencemag.org/news/2016/05/earth-s-ancient-atmosphere-was-half-thick-it-today
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