Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.280r&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 280
Computer Science
2
Scientific paper
The very small Salem meteorite contained high concentrations of cosmogenic radionuclides (e.g., ^56Co and ^26Al) made by solar-cosmic-ray (SCR) particles (Evans et al., 1987; Nishiizumi et al., 1990). The high activities for SCR-produced radionuclides imply that Salem had little ablation, was small in space, and its orbit was mainly <~1 AU (Nishiizumi et al., 1990). Light noble gases were measured in several samples of Salem by the Zurich group and gave an exposure age of 9.5 Ma, but no significant SCR-produced neon was observed, including a sample with very high ^26Al (Nishiizumi et al., 1990). One reason for not observing SCR-produced neon is a complex exposure history with the latest stage starting ~2 Ma ago (for the SCR production of ^26Al) and the earlier stage having shielding from the SCR, thus most neon in Salem would be produced by galactic-cosmic-ray (GCR) particles. An orbital change with Salem further from the Sun 22 Ma ago yields similar results. I calculated GCR and SCR production rates in Salem for several exposure scenarios. The solar-proton fluxes (protons are ~98% of the SCR) were from lunar rock 68815 (Nishiizumi et al., 1988) with exponential-rigidity spectra shapes (R-omicron) and omnidirectional fluxes above 10 MeV of 100 MV and 70 protons/(cm^2 s) and of 70 MV and 150 protons/(cm^2 s). The cross sections for producing neon were those in Reedy (1992) and, with the 100-MV spectrum, gave ^21Ne production rates as a function of depth in meteorites very similar to those calculated by Reedy (1987) for ^26Al. Rates for SCR-produced ^20Ne and ^22Ne were about twice as high. For a radius of 10 g/cm^2, these two fluxes gave similar rates for ^21Ne of 83 atoms/(min kg) at a depth of 1 g/cm^2 (about the GCR rate), the 70-MV spectrum is more productive (~2 times high) at the surface, and the 100-MV spectrum produces more at greater depths. Assuming that Salem had a simple exposure history as a 10- g/cm^2-radius meteoroid, SCR-produced Ne should be observable to pre-atmospheric depths of ~2 cm, where the SCR rate is ~0.3 of the GCR rate. The ^26Al activities suggest low ablation (assuming the 1-AU SCR fluxes used here), <~1 mm for the hottest sample and <~2 mm for the surface sample used for Ne analysis. At 1 mm below the surface, SCR production rates for neon are 2-3 times lower than at the surface. Erosion of Salem in space would lower concentrations of SCR-produced nuclides. Erosion rates <~2 mm/Ma are implied by the ^26Al and these 1-AU production rates. An erosion rate of 1 mm/Ma would imply that surface samples on Salem were, 10^7 years ago, 1 cm deep. Production rates of ^21Ne at 1-cm (3.45 g/cm^2) depth in a 13.5-g/cm^2-radius object were calculated to be only 25 and 37 atoms/(min kg) for the fluxes with R-omicron = 70 and 100 MV, respectively. Twice this erosion rate yields production rates 10 Ma ago of 10 and 21 atoms/(min kg). These calculated profiles 10 Ma ago for today's surface samples were flat. Solar-proton-produced neon would be hard to observe in Salem if its history was complex or if it had a significant erosion rate, especially with some ablation. A soft solar proton spectrum (low rigidity, like R-omicron = 70 MV) would enhance these effects as protons with such a spectrum are not very penetrating. The question of Salem's exposure history needs further examination, such as measuring 3.7-Ma ^53Mn, tracks, and stable cosmogenic nuclides like neon. Non-observation of significant SCR-produced neon in Salem is possible and might help constrain the modeling for SCR interactions in Salem (e.g., proton spectral shape). This work was supported by NASA and done under the auspices of the US DOE. References: Evans J. C. (1987) Lunar Planet. Sci., 18, 271. Nishiizumi K. et al. (1988) Proc. Lunar Planet. Sci. Conf. 18th, 79. Nishiizumi K. et al. (1990) Meteoritics, 25, 292. Reedy R. C. (1987) Lunar Planet. Sci., 18, 822. Reedy R. C. (1992) Lunar Planet. Sci.. 23 1133.
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