History of the Solar Environment

K. Marti¹, B. Lavielle²

¹University of California at San Diego, Dept. of Chemistry, 9500 Gilman Drive, La Jolla, CA 92093-0317
²CNRS; Laboratoire de Chimie Nucléaire Analytique et Bio-environnementale (CNAB); Universités Bordeaux 1 & 2 - UMR 5084 CNRS; Domaine Le Haut Vigneau - BP 120; 33175 Gradignan Cedex, France

Galactic cosmic rays (GCR) provide information on the solar neighborhood during the suns motion in the galaxy. There is now considerable evidence supporting a model of GCR acceleration by shock waves of supernova in active star-forming regions (OB associations) in the galactic spiral arms. During times of passage into star-forming regions an increase in the GCR-flux is expected. Recent data from the Spitzer Space Telescope are shedding light on the structure of the Milky Way and of its star-forming spiral arms. Flux changes should have left records in solar system metallic detectors. Iron meteorites with GCR-exposure times of several hundred million years have long been considered to represent potential detectors of GCR-flux variations (e.g. Voshage, 1962). Variable concentration ratios of GCR-produced stable and radioactive nuclides, with the latter depending on the integration times of the studied nuclides, were reported by Lavielle et al. (1999). These data indicate a recent 38% GCR-flux increase. Potential flux recorders consisting of different pairs of nuclides which measure average fluxes on different time scales are being investigated (Lavielle et al., 2007; Mathew and Marti, 2008). Important characteristics of two pairs of recorders (⁸¹Kr-Kr and ¹²⁹I-¹²⁹Xe) are properties which make them self-correcting for GCR-shielding (flux variability within the iron meteorites of varying sizes). The ⁸¹Kr-Kr method (Marti, 1967) uses Kr isotope ratios for these corrections, while ¹²⁹Xe accumulates the total isobaric yield from decay of ¹²⁹I, since this nuclide is produced by secondary neutron reactions on Te in troilites of iron meteorites. The two chronometers provide records of average GCR fluxes over the 1 and 100 million year time scales, respectively. We discuss briefly improved sensitivities due to recent advances which lower the detection limit of ⁸¹Kr to a few thousand atoms.

References
Lavielle B., Marti K., Jeannot J.-P., Nishiizumi K., and Caffee M. W. (1999) Earth Planet. Sci. Lett. 170, 93-104.
Lavielle B., Gilabert E., and Thomas B. (2007) MAPS 42-S, A92.
Mathew K. J. and Marti K. (2008) Galactic cosmic-ray-produced 129Xe and 131Xe excesses in troilites of the Cape York iron meteorite (submitted to MAPS).
Marti K. (1967) Phys. Rev. Lett. 18(7), 264-266.
Voshage H. (1962) Z. Naturforsch 17a, 422-432.