Mathematics – Logic
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsm43a1471l&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #SM43A-1471
Mathematics
Logic
1150 Cosmogenic-Nuclide Exposure Dating (4918), 1160 Planetary And Lunar Geochronology, 2169 Solar Wind Sources, 6250 Moon (1221), 7599 General Or Miscellaneous
Scientific paper
The surface of the Moon is a uniquely accessible location for collecting samples of contemporary and fossil solar wind. Noble gases implanted by the solar wind have been particularly well studied, because the solar wind is the largest source of these elements in the lunar regolith. Elemental and isotopic abundances of noble gases in the present-day solar wind were first determined using foils exposed by the Apollo astronauts; improvements on these measurements are becoming possible only now, with the examination of Genesis mission samples. Lunar mineral grains and glasses collected during the Apollo missions are still unrivaled as records of the solar wind in the geological past. We have studied argon isotopes from 355 lunar impact spherules collected at the Apollo 12 and Apollo 14 landing sites. We used the 40Ar/39Ar isochron method to determine formation ages of individual specimens, and distinguished cosmogenic and solar contributions of ^{38}Ar and ^{36}Ar by correlating the releases of these isotopes with the laboratory-induced calcium proxy 37Ar. The results of our work are relevant for both solar science and lunar science. The isotopic composition and release patterns of solar argon from the spherules are most simply explained by the presence of two distinct implanted components, an isotopically light solar wind and a higher-energy, isotopically heavier component which we identify with so- called solar energetic particles. Though these two components have been discussed in the literature, their origins and the relationship between them remain enigmatic. Our measurements favor a solar wind ^{38}Ar/^{36}Ar ratio lighter than that of the terrestrial atmosphere. We find no evidence of a secular change in the isotopic composition of the solar energetic particle component, which dominates the record of implanted solar noble gases. In some models, the solar energetic particles come from the high-energy tail of the solar wind energy distribution, representing 3-30 ppm of the total flux. If this is correct, then the fluence of solar argon retained by the spherules, typically ~1 pmol/mm2 of ^{38}Ar, appears unreasonably large, unless the flux, energy distribution, or isotopic composition of the solar wind has changed over geological time. Both the cosmogenic and solar argon released from the spherules imply relatively efficient mixing of the upper regolith over hundreds of millions of years. All the spherules received an appreciable dose of solar corpuscular radiation, and the fluence of solar argon was nearly independent of spherule age. This suggests that, once spherules are buried, they are rarely or never returned to the top of the soil. Nevertheless, the cosmic ray exposure ages we infer for the spherules demonstrate that many spherules were cycled beneath the ~1 m penetration depth of galactic cosmic rays between their creation and collection in the shallow subsurface. Future missions to the Moon can benefit our understanding of the solar wind not only by analyzing solar atoms as they arrive, but also by extending the collection of lunar surface materials available for in situ analysis. In this paper, we review work we have already completed with Apollo samples, and suggest directions for future research.
Levine Jerome
Muller Richard A.
Renne Paul R.
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