History of trace gases in presolar diamonds inferred from ion-implantation experiments.

Diamond grains are the most abundant presolar grains found in primitive meteorites. They formed before the Solar System, and therefore provide a record of nuclear and chemical processes in stars and in the interstellar medium. Their origins are inferred from the unusual isotopic compositions of trace elements-mainly xenon-which suggest that they came from supernovae. But the exact nature of the sources has been enigmatic, as has the method by which noble gases were incorporated into the grains. One observation is that different isotopic components are released at different temperatures when the grains are heated, and it has been suggested that these components have different origins. Here we report results of a laboratory study that shows that ion implantation (previously suggested on other grounds) is a viable mechanism for trapping noble gases. Moreover, we find that ion implantation of a single isotopic composition can produce both low- and high-temperature release peaks from the same grains. We conclude that both isotopically normal and anomalous gases may have been implanted by multiple events separated in space and/or time, with thermal processing producing an apparent enrichment of the anomalous component in the high-temperature release peak. The previous assumption that the low- and high-temperature components were not correlated may therefore have led to an overestimate of the abundance of anomalous argon and krypton, while obscuring an enhancement of the light-in addition to the heavy-krypton isotopes.

Experimental simulations of the photodecomposition of carbonates and sulphates on Mars.

There is indirect spectroscopic evidence for the presence of sulphates and carbonates on the martian surface, and such minerals are also found in SNC meteorites, which are thought to be of martian origin. But although carbonates are expected to be abundant in the martian regolith, attempts to detect them directly have been unsuccessful. Here we report laboratory studies of the decompostion of calcium carbonate and magnesium sulphate under ultraviolet irradiation, which mimic the conditions under which photodecomposition of surface minerals by solar ultraviolet light might occur on Mars. We find that, even for a low abundance of carbonate minerals in the martian regolith, the rate of CO2 release due to photodecomposition is higher than the rate of CO2 loss from the atmosphere by solar-wind-induced sputtering processes, making this process a potential net source of atmospheric CO2 over time. SO2 is also released from the sulphate, albeit more slowly. The rate of carbonate degradation is high enough to explain the apparent absence of these compounds at the martian surface.