Samples whose isotope ratios don't fall on the slope-one-half line didn't result from mass-dependent processes. In 1973 the ratios of oxygen isotopes in carbonaceous meteorites, the oldest objects in the solar system, were found to vary significantly from those on Earth. Their graph line had a slope close to one. A decade later Thiemens and John Heidenreich found that ozone, the three-atom molecule of oxygen, showed a similar isotope trend, with a similar slope of one a relationship that was at least partly due to the molecule's chemical formation.
Sulfur isotope ratios are plotted in a similar way; the standard is an iron sulfide mineral called Diablo Canyon Troilite not native to Earth, however, but found in a fragment of the meteorite that created Arizona's Meteor Crater.
"Mass-independent processes suggest chemical reactions, whether in the lab, the stratosphere, or the early solar system," says Ahmed. "In the proto-solar system, bathed in intense ultraviolet light, these might have occurred on a grain of rock or ice or dust, or in just plain gas. The goal is to identify distinctive isotopic fractionations and examine the chemical pathways that could have produced them."
In the beginning
Since Thiemens's early work with ozone 30 years ago, his UC San Diego laboratory has perfected methods of recovering primordial samples from dust, meteorites, and the solar wind. Thiemens and Chakraborty were members of the science team for NASA's Genesis mission, and Chakraborty was able to extract mere billionths of a gram of oxygen from particles of the solar wind even after
|Contact: Paul Preuss|
DOE/Lawrence Berkeley National Laboratory