Mystery Surrounding Odd Patterns of Oxygen in Solar System's Oldest Rocks Solved

Researchers have uncovered why some of the oldest rocks in the universe contain strange patterns of oxygen.

"Whatever the source of the anomaly must be a major process in the formation of the solar system, but it has remained a matter of contention," said Mark Thiemens, dean of the University of California, San Diego's division of physical sciences and professor of chemistry. "Our experiments essentially recreate the early solar system in that they take gas phase molecules and make a solid, a silicate that is essentially the building block of planets."

The researchers determined that a simple chemical reaction is able to create silicate dust with oxygen anomalies like those found in the solar system's oldest rocks -- a discovery they made after recreating the conditions in the nebula from which the Sun and planets were born.

The discrepancy was discovered 40 years ago in a meteorite that exploded over Mexico, but has since been confirmed in other meteorites. Some of the oldest objects in the solar system, these specimens are believed to date back nearly 4.6 billion years ago. Meanwhile, their mix between oxygen-16, the most common isotope with one neutron for every proton, and variants boasting an extra neutron or two, differs from those rocks from Earth, the Moon and Mars.

"Oxygen isotopes in meteorites are hugely different from those of the terrestrial planets," said Subrata Chakraborty, a project scientist in chemistry at UC San Diego and the lead author of the report. "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 are typically sorted based on their mass, with oxygen-17 and its just one extra neutron found in molecules half as often as oxygen-18, which has two extra neutrons. However, in the meteorites these showed up in equal amounts,independently of their masses.

One explanation for this was a physical chemical principle called symmetry, which the researchers put to the test by filling a chamber with oxygen, varying amounts of hydrogen and solid silicon monoxide -- the ingredients of the early solar system. They then vaporized a plume of silicon monoxide gas into the mix using a laser. It reacted with the oxygen and hydrogen to form silicon dioxide dust, the basis of silicate minerals like quartz.

In analyzing the dust, the researchers identified a mix of oxygen isotopes that matched those found in the meteorites.

"No mattter what else happened early on in the nebula, this is the last step in making the first rocks from scratch," Thiemens said. "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."