A long-standing palaeoclimate paradox ¦nally appears to have been solved.
No grass, no animals, no trees,” explained astrobiologist Sanjoy Som. “The moon was a lotcloser, so the tides were stronger. The planet was spinning faster, and there was nooxygen. For all intents and purposes, it’s essentially an exoplanet.
Also, back then the sun was roughly 70% less luminous and its energy production wasthought to be less than 85% of its current level. This ‘faint young sun’ would not have beenable to keep Earth’s surface above the freezing point of water.“But that was not the case. It was warm enough for liquid water to be present,” added Som,who directs the Blue Marble Space Institute of Science. “There’s plenty of evidence in therock record for rivers, for beaches, for rain.”
That warmth also meant that the planet could provide a habitat for life. The seas wereabounding with microbes, the only living things on the planet, which were transitioning toliving on land. Much like modern organisms, these microbes required warmth from the sunto survive.
This inconsistency is known as the faint young sun paradox, and people like Som believegreenhouse gases were the key. They probably kept the planet in a harmonious state bylocking in the heat from the weak sun. That said, opinions vary on the composition of theair during that distant period.
Until recently, the prominent theory was that an increase of nitrogen gas (N2) in theatmosphere doubled the air’s thickness, creating a greenhouse gas effect that kept our planet warm. This process is called pressure broadening and would have meant that sittingon a Neoarchean felt like swimming 45 feet underwater, due to the extra air pressure.
But then, in 2012, raindrops began to erode this idea. Som, then a graduate student at theUniversity of Washington, and his colleagues compared the imprints of 2.7-billion-year-oldraindrops left in South African volcanic ash with those left by contemporary ones todetermine air thickness and pressure in the Neoarchean era (thicker air causes raindropsto descend at a slower rate and thus make smaller indentations on the ground).
Based on the shape of the imprints, the team concluded that air pressure 2.7 billion yearsago likely was only half of what it is currently, suggesting that Earth was warm back thenas a result of a build-up of greenhouse gases like methane, ethane and/or carbonylsulphide – instead of a thicker atmosphere.
Although the project yielded a wide margin of error, a year later the ¦ndings were backed bya group of European scientists who examined quartz crystals that were buried in the seabed 2.7 billion years ago and contained gas bubbles from hydrothermal vents. Byperforming chemical analysis, they identi¦ed pockets with nitrogen and determined the airpressure. Their calculations corresponded with Som’s cautious estimates, implying a lessdense ancient atmosphere.
Meanwhile, Som and his colleagues were experimenting with a method that utilized lava asa barometer, PBS reported. Lava cools quickly from both the top down and the bottom upas it §ows, creating pockets within the molten material that get ¦lled with gases. However,the bubbles formed at the bottom of the solidi¦ed lava tend to be smaller than those at thetop, which is due to air pressure.
The primary factor affecting the size of the uppermost bubbles is the pressure exerted bythe weight of the atmosphere on the lava. As you move deeper into the lava §ow, the addedweight of the molten rock also contributes to the pressure, causing the bubbles to becomesmaller.
“So if you know the size of the bubble at the top and the bottom, and you know thethickness of the lava, you can calculate air pressure,” Som said.
The researchers obtained samples from Neoarchean lava §ows and used X-rays to assessthe relative size of the bubbles. The ¦ndings surpassed Som’s previous calculations basedon raindrops and showed that the ancient Earth’s pressure was low, ranging from 20 to 50percent of present levels. This suggests that most of Earth’s nitrogen was not present inthe atmosphere during that time, but was instead located in either the crust or the mantle.
Som believes that Earth’s atmosphere initially contained a large amount of nitrogen, andthe ¦rst bacteria utilized the gas for their survival. The oldest known microorganisms,dating back to 3.2 billion years, were probably nitrogen ¦xers that transformed the gas intoammonia.
As these microbes thrived, they reduced the amount of nitrogen in the atmosphere, leadingto its thinning – a process that continued until the Great Oxidation Event 2.4 billion yearsago, when cyanobacteria that favored photosynthesis started adding oxygen to theatmosphere, replenishing it.
Som is most excited by the implications the ¦ndings could have in the search for habitableplanets outside our Solar System.
“By taking snapshots of Earth through time, when things were very different compared totoday, it can give you another indication of existing worlds that could host life.” Som said.“The early Earth then becomes a fantastic lab to study habitable planets that are differentthan modern Earth.”
