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Four and a half billion years ago, the Earth would have been difficult to recognize. Instead of the forests, mountains and oceans we know today, our planet’s surface was entirely covered in magma, the molten rock material that emerges when volcanoes erupt. The scientific community agrees on this. What is less clear is what the atmosphere was like at the time. New international research efforts led by Paolo Sossi, senior researcher at ETH Zurich and NCCR PlanetS, attempt to unlock some of the mysteries of Earth’s primeval atmosphere. The findings were published today in the journal Advances in science.
Producing magma in the laboratory
“Four and a half billion years ago, magma was constantly exchanging gas with the atmosphere above,” Sossi begins to explain. “The air and the magma have affected each other. So, you can learn from each other.”
To learn about the primordial atmosphere of the Earth, which was very different from what it is today, the researchers then created their own magma in the laboratory. They did this by mixing a powder that matched the composition of the Earth’s molten mantle and heating it. What sounds simple required the latest technological advances, as Sossi points out: “The composition of our mantle-like powder made it difficult to melt – we needed very high temperatures of around 2,000 ° Celsius.”
This required a special furnace, which was heated by a laser and inside which researchers could levitate magma by letting streams of gas mixtures flow around it. These gas mixtures were plausible candidates for the primordial atmosphere which, like 4.5 billion years ago, affected magma. Therefore, with each gas mixture flowing around the sample, the magma turned out to be slightly different.
“The key difference we were looking for was the oxidation of the iron contained in the magma,” Sossi explains. In less precise words: how rusty it is. When iron meets oxygen, it oxidizes and turns into what we commonly refer to as rust. Therefore, when the gas mixture the scientists blew on their magma contained a lot of oxygen, the iron inside the magma oxidized more.
This level of iron oxidation in cooled magma gave Sossi and his colleagues something they could compare to the naturally occurring rocks that make up the Earth’s mantle today – the so-called peridotites. The oxidation of iron in these rocks still has the influence of the primordial atmosphere imprinted within it. Comparing the natural peridotites with those of the laboratory then provided scientists with clues as to which of their gas mixtures came closest to Earth’s primordial atmosphere.
A new vision of the emergence of life
“What we found is that, after cooling from the magmatic state, the young Earth had a slightly oxidizing atmosphere, with carbon dioxide as its main constituent, as well as nitrogen and some water,” Sossi said. The surface pressure was also much higher, almost a hundred times that of today, and the atmosphere was much higher, due to the hot surface. These characteristics made it more similar to the atmosphere of Venus today than that of Earth today.
This result has two main conclusions, according to Sossi and his colleagues: the first is that the Earth and Venus started with fairly similar atmospheres, but the latter subsequently lost its water due to closer proximity to the sun and higher temperatures. associated. The Earth, however, has retained its water, mostly in the form of oceans. These absorbed much of the CO2 from the air, thereby reducing CO2 levels significantly.
The second conclusion is that a popular theory of the emergence of life on Earth now seems much less likely. This so-called “Miller-Urey experiment,” in which lightning interacts with certain gases (particularly ammonia and methane) to create amino acids – the building blocks of life – would have been difficult to accomplish. The gases needed were simply not abundant enough.
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