Yayoi N. Miura

Irradiation History of Itokawa Regolith Material Deduced from Noble Gases in the Hayabusa Samples

Laboratory analysis of samples returned from an asteroid establishes a direct link between asteroids and meteorites and provides clues to the complex history of the asteroid and its surface. Noble gas isotopes were measured in three rocky grains from asteroid Itokawa to elucidate a history of irradiation from cosmic rays and solar wind on its surface. Large amounts of solar helium (He), neon (Ne), and argon (Ar) trapped in various depths in the grains were observed, which can be explained by multiple implantations of solar wind particles into the grains, combined with preferential He loss caused by frictional wear of space-weathered rims on the grains. Short residence time of less than 8 million years was implied for the grains by an estimate on cosmic-ray–produced 21Ne. Our results suggest that Itokawa is continuously losing its surface materials into space at a rate of tens of centimeters per million years. The lifetime of Itokawa should be much shorter than the age of our solar system.

Terrestrial nitrogen and noble gases in lunar soils

The nitrogen in lunar soils is correlated to the surface and therefore clearly implanted from outside. The straightforward interpretation is that the nitrogen is implanted by the solar wind, but this explanation has difficulties accounting for both the abundance of nitrogen and a variation of the order of 30 per cent in the 15N/14N ratio. Here we propose that most of the nitrogen and some of the other volatile elements in lunar soils may actually have come from the Earths atmosphere rather than the solar wind. We infer that this hypothesis is quantitatively reasonable if the escape of atmospheric gases, and implantation into lunar soil grains, occurred at a time when the Earth had essentially no geomagnetic field. Thus, evidence preserved in lunar soils might be useful in constraining when the geomagnetic field first appeared. This hypothesis could be tested by examination of lunar farside soils, which should lack the terrestrial component.

Orthopyroxenite ALH84001 and shergottite ALH77005: Additional evidence for a martian origin from noble gases

Abstract Concentrations and isotopic ratios of noble gases have been determined for two martian meteorites, ALH84001 and ALH77005. ALH84001, a newly recognized martian orthopyroxenite, has a high 129 Xe 132 Xe ratio (≈2.1), which is similar to that observed for the glass portion (lithology C) of shergottite EETA79001. The elemental ratios of trapped Ar, Kr, and Xe, the cosmic-ray exposure age (14.5 ± 1.8 m.y.) and the KAr age (3–4 b.y.) of ALH84001 are not identical with those of EETA79001 or any other martian meteorite. Therefore, ALH84001 may not originate from the same area as the other martian meteorites. The noble gas data of ALH77005 shergottite are compatible with previously reported data. Argon, Kr, and Xe are heavily contaminated by elementally fractionated terrestrial atmospheric gases. A cosmic-ray exposure age of 2.9 ± 0.7 m.y. and a KAr age of 1.33 ± 0.13 b.y. were obtained for this meteorite.

Martian atmosphere-like nitrogen in the orthopyroxenite ALH84001

Abstract Isotopic and elemental compositions of nitrogen, neon, and argon in the Martian meteorite ALH84001 were studied. Two whole-rock samples of ALH84001,57 were analyzed with detailed stepped combustion experiments (from 200 to 1200°C with every 100°C and 200°C, respectively, with laser ablation experiments for melting the samples). Nitrogen with δ 15 N = 418 ± 44‰ and 366 ± 15‰ after correction for cosmogenic nitrogen were obtained at 800°C, which are the highest δ 15 N values ever found among Shergottites, Nakhlites, Chassignite meteorites. The heavy nitrogen in ALH84001 may have the same origin as that in the glass portions of two shergottites, EETA79001 and Zagami, and is likely to have been derived from the Martian atmosphere. However, when and in which phase Martian atmosphere-like nitrogen was trapped in ALH84001 is still unknown. One possibility inferred from the released temperatures is that nitrogen is sited in maskelynite. On the plot of δ 15 N versus Ar/N (diagrams of δ 15 N vs. 40 Ar/ 14 N and 36Ar/ 14 N), the data points for ALH84001 in this study form a linear trend, which apparently differs from that defined by glass portions of the shergottites EETA79001 and Zagami. The present trend also differs from the trend seen by Grady et al. for ALH84001. The 40 Ar/ 14 N and 36 Ar/ 14 N ratios obtained in this work are relatively smaller than those of the shergottites on such plots, indicating the following possibilities: (1) Martian atmospheric Ar/N ratio changed throughout geologic time (e.g., ≈4 Ga) without a large change in δ 15 N, that is, ALH84001 trapped ancient Martian atmospheric nitrogen, and Ar/N ratio was initially lower, or (2) different trapping sites or mechanisms between ALH84001 and the shergottite glasses made an apparent difference in the Ar/N ratio, and elementally fractionated gases were trapped in ALH84001. In the case (1), either increase in Ar concentration in the Martian atmosphere due to preferential Ar degassing from the Martian interior or preferential escape of atmospheric nitrogen must have occurred. Nitrogen with δ 15 N from −13‰ to +5‰, and with δ 15 N ≥ 200‰ were released at low and high temperatures, respectively. The former may have been derived from the Martian lithosphere or terrestrial atmosphere, and the latter must be of cosmogenic nitrogen. The largest amount of 40 Ar was released at temperatures similar to the largest release of heavy nitrogen of ≈800°C. According to reported K abundance in ALH84001, the majority of 40 Ar in our samples is considered to be of radiogenic origin.