Meteorites that fell billions of years ago helped to understand what air was made of before life oxygenated it. The results were unexpected.
The achievement is described in a scientific article published in the journal PNAS.
Geologists usually determine the composition of the ancient atmosphere from the minerals that were on the surface at that time. So it was found, for example, that oxygen in the air began to accumulate 2.5 billion years ago. But what substances consisted of the early gaseous shell of the Earth and what were their proportions?
Data on this are scarce and contradictory. But now scientists have found a new source of information about those distant times.
"This is a promising new tool for determining the composition of the upper atmosphere billions of years ago," says first author Rebecca Payne of the University of Pennsylvania.
We are talking about iron meteorites no larger than grains of sand that fell 2, 7 billion years ago. When dropped, they heated up due to friction against the air and, as shown by chemical analysis, underwent oxidation.
Micrograph of an iron micrometeorite that fell 2.7 billion years ago.
Illustration by Andrew Tomkins.
What gas could have been an oxidizing agent in an oxygen-free era? The authors of the new study believe that it is carbonic acid. According to their calculations, the atmosphere at that time consisted of 25-50% CO2.
But here another difficulty arises. Such a huge concentration of carbon dioxide should provide a powerful greenhouse effect and, therefore, a warm climate. However, the geological layers of that time bear traces of glaciation.
Contradiction? Not necessary. Scientists have found an unexpected solution to this puzzle.
In their opinion, the share of carbon dioxide in the air was large, but the air itself was then much less than it is now. Therefore, in absolute terms, the amount of CO2 in the atmosphere was small, and the greenhouse effect was more than modest.
Let us recall that the atmosphere of those times did not contain oxygen, which now makes up 23% of the mass of dry air (at the surface of the Earth). But the absence of this gas alone does not yet make the gas shell as thin as required by the bold hypothesis of the authors. We have to assume that there was much less nitrogen in the air than it is now.
However, according to the authors, there are also independent data indicating a low nitrogen content in that era.