A strange mix of oxygen found in a stony meteorite that exploded February 8, 1969 over Pueblito de Allende, Mexico has puzzled scientists ever since. Small flecks of minerals lodged in the stone and thought to date from the beginning of the solar system have a pattern of oxygen types, or isotopes, that differs from those found in all known planetary rocks, including those from Earth, its Moon and meteorites from Mars.

Now scientists from UC San Diego and Lawrence Berkeley National Laboratory have eliminated one model proposed to explain the anomaly: the idea that light from the early Sun could have shifted the balance of oxygen isotopes in molecules that formed after it turned on. When they beamed light through carbon monoxide gas to form carbon dioxide, the balance of oxygen isotopes in the new molecules failed to shift in ways predicted by the model they report in the September 5 issue of Science.

"It's solar system forensics. We're understanding a little about how it got made," said Mark Thiemens, Dean of the Division of Physical Sciences and a professor of chemistry and biochemistry at UC San Diego, who directed the project. The results pare down the potential explanations for how gas and dust coalesced to form the planets and will help this team and others interpret samples of the solar wind returned by NASA's Genesis spacecraft.

Scientists think the early Sun emitted intense far-ultraviolet light. Light energy at these very short wavelengths will dislodge oxygen atoms from molecules, freeing them to hook up with others in new combinations. In the process, the oxygen atoms absorb some of the energy.

This is how gases became dust and then larger minerals that collided and continued to build to form the planets. Oxygen, the most abundant element in the solar system, is a player in almost all of these reactions.

Each oxygen isotope responds to a unique set of light wavelengths. An abundance of a particular oxygen isotope within in a cloud of gas molecules will quench the light at its preferred wavelengths, shielding gas molecules farther along the light's path. Other wavelengths, including those that dislodge different oxygen isotopes, will continue unimpeded, favoring the inclusion of these rarer isotopes in new molecules.

The balance of oxygen isotopes found in the Allende meteorite is tipped toward the most abundant one, 16O. Planetary rocks have relatively more rarer heavier oxygen isotopes, as though rare isotopes were preferred as the planets formed.

"We decided to directly test this idea that photoshielding could change the isotope ratios," said Subrata Chakraborty, a postdoctoral fellow at UC San Diego and first author of the paper.

The team focused an intense beam of far-ultraviolet light generated by the Lawrence Berkeley National Laboratory's Advanced Light Source into a tube filled with carbon monoxide gas. The light knocked some of the oxygen atoms free, allowing them to recombine with other carbon monoxide molecules to form carbon dioxide. Chakraborty then collected and analyzed the carbon dioxide to determine the balance of oxygen isotopes in the new molecules.

By precisely controlling the wavelength of the light, the scientists were able to set up conditions that should have resulted in oxygen isotope mixes that matched either those found on Earth or in the Allende meteorite.

Wavelengths known to be absorbed by 16O should result in carbon dioxide molecules enriched with the heavier forms of oxygen. They tested two of these wavelengths: one enriched the mix; the other did not.

Wavelengths not absorbed by 16O should result in a mix that matched that found in the Allende meteorite. Again, of the two the team tested, one did and one did not. "Some process is altering the mix, but it can't be photoshielding," Chakraborty said.

Samples returned by the GENESIS spacecraft will have to be interpreted in light of these results, Thiemens said. By analyzing samples of the Sun's outer atmosphere captured from the solar wind, the mission aims to determine the original composition of the solar nebula, the swirl of dust and gas that formed the solar system. Measurements by Thiemen's research group and others will help to resolve the chemical mismatch between the meteorite inclusions and planetary rocks.

Several other models have been proposed to explain the anomaly--including the idea that an exploding star could have blasted in an extra dose of 16O--only to have been discarded when experimental evidence showed them to be unlikely.

The only one left standing, according to Thiemens, is an idea called molecular symmetry that says an atom flanked by two oxygen isotopes is more likely to become a stable molecule if the two isotopes are mismatched. This quieter process would also favor the formation of molecules that included rarer oxygen isotopes.

"There's no violence," Thiemens said. "It doesn't require a star blowing up or turning on to cast a nebula-wide shadow. It's symmetry."

Musahid Ahmed of Lawrence Berkeley National Laboratory and Teresa Jackson of UC San Diego are co-authors of the paper. NASA and the Department of Energy funded the project.