Can micrometeorites tell us about Earth's ancient atmosphere?
Yes. Tiny micrometeorites preserved in 2.7-billion-year-old rocks were oxidised as they fell, suggesting the upper atmosphere then contained more oxygen than expected.
Micrometeorites, tiny space grains that melt as they fall through the atmosphere, react with the air on the way down. This makes them natural probes of the atmosphere at the time they fell.
When a metal-rich micrometeorite melts, oxygen in the air oxidises some of its iron, forming minerals such as magnetite and wüstite. The amount of oxidation depends on how much oxygen was in the upper atmosphere.
In 2016, researchers studied micrometeorites preserved in limestone from the Pilbara region of Western Australia, about 2.7 billion years old. At that time, before the Great Oxidation Event around 2.4 billion years ago, Earth's lower atmosphere is thought to have contained almost no oxygen. Yet the micrometeorites had been heavily oxidised. The researchers suggested that the upper atmosphere contained significant oxygen, perhaps produced by sunlight breaking apart carbon dioxide or water, or that other gases, such as carbon dioxide, did the oxidising.
Later studies argued that a carbon dioxide-rich atmosphere could explain the oxidation without much free oxygen, and that interpretation remains debated. Either way, the work showed that ancient micrometeorites can test models of Earth's early atmosphere.
Micrometeorites from different geological periods are now being collected from ancient sediments. They can also reveal changes in the flux of space dust, for example after large asteroid break-ups, such as the L-chondrite break-up about 470 million years ago, when extraterrestrial dust reaching Earth increased greatly.