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Earth’s primitive atmosphere, a Venusian hell

In an article published in the journal Science Advances, a team of international researchers reveal that the Earth's primitive atmosphere must have been extremely hot and hostile to life, as it was saturated with carbon dioxide, a far cry from the welcoming atmosphere we know today, composed mainly of oxygen and nitrogen.

Earth’s primitive atmosphere, a Venusian hell

Artist's view of the Earth today and 4.5 billion years ago. (© Tobias Stierli / PRN Planets)

Publication date: 26/11/2020

Press, Research

Related themes : Origins

Until now, it was thought that the Earth’s primitive atmosphere, i.e. the atmosphere present at the time of its formation, must have been composed of certain gases such as ammonia and methane, which could have facilitated the development of life on Earth by reacting with lightning to produce amino acids (the famous “Miller-Urey” experiment), the latter being one of the main building blocks of complex life. But the actual existence of such an atmosphere has never yet been verified, as reproducing the extreme conditions of the Earth’s beginnings in the laboratory has proved particularly complicated.

Recreating a primitive Earth in the laboratory

To answer this enigma, Paolo Sossi and his colleagues first created rocks in the laboratory with a chemical composition identical to that of the rocky part of our planet (crust and mantle). They then subjected them to extreme temperatures (nearly 2000°C) at the IPGP in a laser-heated oven, with the aim of reconstituting miniature molten planets just 2 mm in diameter. These mini-planets were then placed in a state of levitation within gaseous currents with compositions consistent with the different atmospheres possibly present at the time of the Earth’s formation. The effect of these gases, and therefore of the atmospheric composition, was then recorded in the molten rock through the ratio of oxidised iron to reduced iron present in these molten silicate micro-planets.

Miniature planets in fusion: reconstitution using a laser-heated furnace at nearly 2000°C at the IPGP (© IPGP-Université de Paris)
Close-up view of the experiment with the hot magma sample surrounded by gas and held in suspension (© P. Sossi / ETH Zurich)

An unfriendly primitive atmosphere

The research team then looked for a match with the same oxidised iron/reduced iron ratios found today in the rocks of the Earth’s mantle. Surprisingly, they discovered that our planet’s first atmosphere could not have contained the hydrogen-rich gases essential for life (ammonia and methane), but rather carbon monoxide and carbon dioxide. What’s more, the proportions and pressures of these gases in the Earth’s primitive atmosphere were probably similar to those observed on the planet Venus today, the latter containing around 96.5%CO2, 3.5% N2 and a pressure 92 times greater than that of the Earth.

So why do these two planets seem so different today? Paolo Sossi and his colleagues suggest that the Earth’s position in the ‘habitable zone’ of our solar system probably enabled it to retain liquid water on its surface in the form of the first oceans, which absorbed a large part of its initialCO2-rich atmosphere. Indeed, the Earth’s temperate location, together with its large size, contributed to the establishment of mild conditions on its surface over long geological periods. On the other hand, Venus, which receives almost twice as much solar radiation as our planet, was unable to maintain its initial water content because of the constantly high temperatures on its surface.

Can amino acids, the essential ingredients of life, develop in such atmospheres? The mystery remains.

Ref : Sossi, P.A., Burnham, A. D., Badro, J., Lanzirotti, A., Newville, M. & O’Neill, H.St.C. Redox state of Earth’s magma ocean and its Venus-like early atmosphere. Science Advances. doi: 10.1126/sciadv.abd1387

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