Micrometeorites reveal oxygen in Earth’s atmosphere earlier than thought

New Scientist : A collection of 60 micrometeorites found in Australia reached Earth 2.7 billion years ago, which makes them the oldest known surviving meteorites. Andrew Tomkins of Monash University and his colleagues were surprised to find the micrometeorites to begin with; the fact that the rocks contain iron oxides is rewriting part of Earth’s early history. The oxides most likely formed as the meteorites passed through Earth’s upper atmosphere. The meteorites landed in a highly alkaline lake, which was completely anoxic at the bottom, and got preserved in layers of limestone. Scientists believe Earth’s atmosphere at that time had very low levels of oxygen, but all the supporting evidence is limited to the lower atmosphere. Several theories suggest that the upper atmosphere may have been oxygen-rich due to solar radiation splitting molecules of water, carbon dioxide, and sulfur dioxide.