Scientists Solve Oxygen Mystery in Solar System's Earliest Rocks

A mystery has long puzzled cosmochemists when it came to the formation of the solar system. Oxygen follows a strange, anomalous pattern in the oldest and most pristine rocks. This particular pattern probably results from a different chemical process than the well-understood reactions that form minerals containing oxygen on Earth. Now, researchers may have discovered the answers behind these different chemical processes.

The anomaly in oxygen was seen about forty years ago in a stony meteorite that exploded over Mexico. Within these meteorite samples, which are some of the oldest objects in the solar system, have a strange mix between oxygen-16 and variants with an extra neutron or two.

"Oxygen isotopes in meteorites are hugely different from those of the terrestrial planets," said Subrata Chakraborty, one of the researchers, in a news release. "With oxygen being the third most abundant element in the universe and one of the major rock forming elements, this variation among different solar system bodies is a puzzle that must be solved to understand how the solar system formed and evolved."

Oxygen isotopes usually sort out according to mass. For example, oxygen-17, with just one extra neutron, is incorporated into molecules half as often as oxygen-18, which has two extra neutrons. In these stony meteorites, though, the two heavier oxygen isotopes show up in equal proportions. The rates at which they are incorporated into these minerals forming these earliest rocks were independent of their masses.

In order to examine these ancient chemical processes a bit more closely, the scientists recreated conditions in the solar nebula, the swirl of gas that coalesced to form our star. They filled a hockey puck sized chamber with pure oxygen, varying amounts of pure hydrogen and a little black nugget of solid silicon monoxide. They then used a laser to vaporize a plume of silicon monoxide gas into the mix. This allowed them to see the reactions of gases that formed the earliest solid materials in the solar system.

So what did they find? The scientists collected and analyzed the dust created in the chamber and found that the mix of oxygen isotopes matched the anomalous pattern found in stony meteorites. This seemed to show what sort of reactions occurred in the early solar system.

"No matter what else happened early on in the nebula, this is the last step in making the first rocks from scratch," said Thiemens, one of the researchers, in a news release. "We've shown that you don't need a magic recipe to generate this oxygen anomaly. It's just a simple feature of physical chemistry."

The findings are published in the journal Science.