From Earth to the moon to Mars, the rocky structures of celestial bodies were built using the very same mixture of oxygen. But for 40 years now, researchers have been noticing something strange: meteorites from the early days of the Solar System are built with a different mixture of oxygen, one where an isotope of the element appeared far more frequently than it ought to. "It’s a big problem," cosmochemist Subrata Chakraborty tells The Verge. "It’s a really, really big problem. If you don’t understand the oxygen isotope, you don’t understand the formation and evolution of the solar system."

"It's a really, really big problem."

In a paper published today in Science, researchers from UC San Diego, led by Chakraborty, are finally explaining part of that mystery. Chakraborty says the distribution of oxygen found in these meteorites was a puzzle: they contained equal amounts of the isotopes oxygen-17 and oxygen-18, rather than having twice as much of the latter like other rocks.

While that may sound like a small difference, Chakraborty says knowing what caused this is important to understanding how the solar system formed. It’s such a puzzle that NASA even sent its Genesis probe out toward the sun in the early 2000s in part to address this very question. But as Chakraborty puts it, difficult experiments and available technology became something of a bottleneck in actually figuring out what was happening before now.

To begin solving it, Chakraborty and his team set out to simulate the environment of the early solar system. In a small chamber they recreated the gaseous makeup of a nebula, and using a type of ultraviolet laser, were able to form their own rocky solids in the way that meteorites would have formed around 4.6 billion years ago. As it turned out, the rocks that Chakraborty’s team created had the very same makeup as the anomalous meteorites.

"You don't need a magic recipe to generate this oxygen anomaly."

The researchers say that meteorites didn’t form this way because of some unexpected behavior present in the early universe. Rather, it appears that the reaction creating them is governed by molecular symmetry, a principle already known to scientists. "You don't need a magic recipe to generate this oxygen anomaly," Mark Thiemens, dean of physical sciences at UC San Diego, said in a statement. "It's just a simple feature of physical chemistry."

Chakraborty says that more research should be done into how minerals form under these circumstances, but learning the workings of oxygen — the third most abundant element in the universe — is a good start. "Oxygen is the building block of all rocky planets," Chakraborty says. "That’s why we are behind oxygen. That’s why NASA is behind oxygen."