Very occasionally, Earth gets bombarded by a large meteorite. But every day, our planet gets pelted by space dust, micrometeorites that collect on Earth’s surface.

A University of Washington team looked at very old samples of these small meteorites to show that the grains could have reacted with carbon dioxide on their journey to Earth. Previous work suggested the meteorites ran into oxygen, contradicting theories and evidence that the Earth’s early atmosphere was virtually devoid of oxygen. The new study was published this week in the open-access journal Science Advances.

“Our finding that the atmosphere these micrometeorites encountered was high in carbon dioxide is consistent with what the atmosphere was thought to look like on the early Earth,” said first author Owen Lehmer, a UW doctoral student in Earth and space sciences.

At 2.7 billion years old, these are the oldest known micrometeorites. They were collected in limestone in the Pilbara region of Western Australia and fell during the Archean eon, when the sun was weaker than today. A 2016 paper by the team that discovered the samples suggested they showed evidence of atmospheric oxygen at the time they fell to Earth.

That interpretation would contradict current understandings of our planet’s early days, which is that oxygen rose during the “Great Oxidation Event,” almost half a billion years later.