LC2C. Stay Current—Where Does Earth’s Atmosphere Come From?

2016-05-09. Earth’s ancient atmosphere was half as thick as it is today. By Roland Pease, Science. 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

2013-01-04.  Hydrogen-Nitrogen Greenhouse Warming in Earth’s Early Atmosphere | Robin Wordsworth and Raymond Pierrehumbert, Science Vol. 339 no. 6115 pp. 64-67. Excerpt: One of the most durable questions about Earth’s early climate arises from the faint young Sun effect: Because progressive accumulation of He in a star’s core causes its luminosity to increase with age, the solar energy incident on Earth was significantly lower [∼75% of present-day values 3.8 billion years ago (Ga)] during the Hadean and Archean eras. Because geological evidence shows that Earth was not in a globally glaciated, snowball state throughout this time, additional mechanisms must have been present to warm the climate. Previous explanations for this altered climate have included increased atmospheric ammonia or CH4, a decreased surface albedo, and changes in the distribution of clouds. However, all of these mechanisms have subsequently been shown to suffer important defects. …  on the pre-biotic/early Archean Earth, the atmospheric N2 content was around two to three times the present-day value. Hydrogen, the most abundant gas in the solar system, has previously been ignored in the Archean climate budget, presumably because it was long thought to be a minor constituent even in the early atmosphere. …It was long believed that the escape of H2 in the Archean was rapid, … However, recent numerical calculations imply that the rate of hydrodynamic H2 escape on the early Earth was more strongly constrained by the adiabatic cooling of the escaping gas, given a limited extreme ultraviolet (XUV) energy input. As a result, H2 could have been a major constituent (up to ∼30% by volume) of the Archean atmosphere unless surface or ocean biogeochemistry continuously removed it. …molecular hydrogen interacts strongly with infrared radiation via collision-induced absorption (CIA), the strength of which scales with the product of the densities of the two interacting gases. CIA has been well studied for the gas giant planets and Titan, where it dominates radiative transfer in the middle and lower portions of the atmosphere. On early Earth, N2 and H2 may both have been abundant in the atmosphere, so interacting pairs of N2-N2, H2-N2, and H2-H2 should all be considered as potential contributors to greenhouse warming. …CH4 greenhouse was believed necessary to solve the faint young Sun problem. In contrast, our results show that an early climate dominated by abiotic H2-N2 and CO2 warming is consistent with both observational and theoretical limits on atmospheric CO2 levels…. H2-N2 warming is also likely to be important in the search for biosignatures on super-Earth exoplanets, whose higher masses imply lower energy-limited hydrogen escape rates and larger typical atmospheric N2 inventories. Because incident XUV flux is a function of orbital distance, H2-N2 warming may be of particular importance to the habitability of terrestrial exoplanets that are far from their host stars.   …. Read the full article: http://www.sciencemag.org/content/339/6115/64.full  See also “How Was Early Earth Kept Warm?” by James F. Kasting, http://www.sciencemag.org/content/339/6115/44.full

2012-03-30.  Ancient raindrop fossils shed light on early atmosphere | By Amina Khan, Los Angeles Times.  Excerpt: Fossilized imprints of raindrops that were sealed into stone 2.7 billion years ago indicate that Earth’s early atmosphere could have been packed with greenhouse gases, according to new research that addresses a long-standing paradox of the planet’s early history.
About 2 billion years ago, the young sun was far less bright, emitting less than 85% of the light and heat it puts out today…. How did Earth manage to stay so warm? 
To get at the answer, a team of scientists from the University of Washington in Seattle adopted an unusual method employed in 1851 by a 19th-century geologist named Charles Lyell… Lyell suggested that the thickness of the ancient atmosphere could be pinned down by measuring the size of fossil raindrops…. 

2009 Dec 10. Our Atmosphere Came From Space Gases, Study Says. By Ker Than, for National Geographic News. Excerpt: The gases that make up Earth’s atmosphere came from a swarm of comets, not from bubbling volcanoes as long thought, a new study says. The new theory came about after scientists discovered that pristine samples of the elements krypton and xenon, recently collected from deep within the Earth, have the same chemical makeup as ancient meteorites….
It’s still true that volcanoes spewed out some gases, “but [that] contribution was insignificant” for the creation of Earth’s atmosphere,” Ballentine said.
… most of Earth’s krypton has remained unchanged since its arrival on our planet—allowing scientists to precisely study the conditions of early Earth.
Based on their research, Ballentine and colleagues claim that our atmosphere likely formed when gas and water-rich comets bombarded Earth, shortly after its formation 4.54 billion years ago.
…Scientists have already discovered that the comet barrage likely formed Earth’s oceans….

2003 September 17. Ancient Relatives of Algae Yield New Insights into Role of CO2 in Earth’s Early Atmosphere. NASA’s Earth Observatory. Greenhouse gas has been playing a critical role in warming our planet for billions of years, according to a new study that looks at the photosynthetic cycle by which plants convert light energy and CO2 into cellular tissue.