1. Cosmic ray exposure record in lunar meteorites Lunar Archive N. M. Curran *, R. Burgess, K. H. Joy SEAES, University of Manchester, Oxford Road, Manchester M13 9LP, UK * Corresponding author: natalie.curran@postgrad.manchester.ac.uk Lunar regolith and cosmic rays Lunar meteorites References and acknowledgement The Moon has some of the most extremely ancient surfaces in the Solar System, some up to ≈4.4 billion years old. Studying these surfaces can help us to understand the Earth-Moon formation event; the nature of small differentiated bodies; and the evolution and bombardment history of the Moon. Furthermore, the lunar surface preserves a record of the inner Solar System and the boundary between the Moon and the space environment [1]. Regolith is a term that applies to the layer of fragmental and unconsolidated material that covers the lunar surface and applies to many solar system bodies including Mars and Mercury. It is formed due to repeated and continuous bombardment of impactors, gradually creating a fine grained layer and ‘gardening’ the surface [2]. As the Moon has no atmosphere or magnetic field to protect it, the surface is a very hostile environment where all the energetic particles from the sun and outside our galaxy interact with the surface of the Moon [3]. Solar cosmic rays (SCRs) and galactic cosmic rays (GCRs) interact with the regolith producing cosmogenic daughter isotopes (Figure 1) [2]. The cosmogenic isotopes produced are an important tool for helping us understand the impact cratering processes and constraining the timing and duration of regolith formation. Figure 1: Schematic diagram illustrating the interaction of solar wind and cosmic rays with the lunar surface, adapted from reference [9]. The interactions between the cosmic rays and the surface generate secondary neutrons. These can be fast neutrons which can further interact with other surface material, decreasing their energy to epithermal and thermal neutrons (0.1-300eV and 0.1eV respectively). Neutrons can also be captured to form new isotopes (e.g. neutron capture of 130 Ba forming 131 Xe). Lunar regolith meteorites (figure 2) are ejected from the Moon by impacts (asteriodal and cometary) and delivered to the Earth [4]. Some lunar meteorites are excavated from depths up to 3 meters within the regolith. They are sourced from random localities on the lunar surface, including the lunar farside [5]. The transfer time of lunar meteorite from the Moon to the Earth is generally short, between 0.1-10.8 Myr, therefore, it is likely that most of the cosmogenic isotopes are produced during their residence on the lunar surface [4]. Methods Poster 5 Exposure ages All lunar meteorites experience at least one (sometimes more) exposures to cosmic rays during their history. Studying these effects allow us to determine how long the meteorite resided at or near the lunar surface, its shielding depth and time of ejection [5]. To determine the CRE age the amount of cosmogenic noble gases produced and their production rates, before ejection from the Moon, need to be known. The production rates of a nuclide are depth-dependant experimentally measured concentrations of the cosmogenic nuclide and depend on: (i) the flux and range of energies of both primary and secondary protons and neutrons from cosmic rays, (ii) the abundance and composition of the target element and (iii) shielding depth of the sample [6]. Calculating the CRE ages of lunar meteorites allows us to understand the history of each sample (simple or complex) and their parent regolith, in areas remote from the Apollo landing sites [7]. Future work will also focus on obtaining exposure ages and shielding depth for several Apollo 16 regolith samples. Top: ALHA 81005,99. A feldspathic regolith breccia. Right: ALHA 81005 (image credit NASA) Left: QUE 94281,50. A mixed feldspathic and basaltic breccia. Bottom: QUE 94281 (image credit NASA) Top: MIL 05035,34. A crystalline basalt (unbrecciated) Right: MIL 05035 (image credit NASA) Figure 3: Montaged back-scattered electron images and false colour X-ray maps of thin sections of MIL 05035,34 (top, from [8]) and MAC 88105,159 (bottom, Image: K.Joy). Ca = yellow, Mg = green, Si = blue, Fe = red, Al = white, Ti = pink and K = cyan Figure 2: Photographs of a selection of small (<1 cm) chips of ANSMET meteorites and their parent stone which will be used in this project. Figure 4: Photograph of the MAP mass spectrometer, in the Isotope Geochemistry and Cosmochemistry Group, SEAES, Manchester. [1] Heiken, G., Vaniman, D. & French, B. M. Lunar Sourcebook : a user's guide to the Moon. Cambridge University Press (1991). [2] Lucey, P. et al. Understanding the lunar surface and space-moon interactions. Rev Mineral Geochem 60, 83-+, doi:DOI 10.2138/rmg.2006.60.2 (2006). [3] Reedy, R. C., Arnold, J. R. & Lal, D. Cosmic-Ray Record in Solar-System Matter. Science 219, 127-135, doi:DOI 10.1126/science.219.4581.127 (1983). [4] Lorenzetti, S., Busemann, H. & Eugster, O. Regolith history of lunar meteorites. Meteorit. Planet. Sci. 40, 315-327 (2005). [5] Eugster, O.. Cosmic-ray exposure ages of meteorites and lunar rocks and their significance. Chemie Der Erde-Geochemistry 63 (1): 3-30 (2003) [6] Hohenberg, C. M., Marti, K., Podosek, F., Reedy, R. C and Shirck, J. R. Comparisons between observed and predicted cosmogenic noble gases in lunar samples. Proc. Lunar. Planet. Sci. Conf. 9 th, 2311-2344 (1978). [7] Wieler, R. Cosmic-ray-produced noble gases in meteorites. Rev Mineral Geochem 47, 125-170, doi:DOI 10.2138/rmg.2002.47.5 (2002). [8] Joy, K. H. et al. The petrology and geochemistry of Miller Range 05035: A new lunar gabbroic meteorite. Geochim Cosmochim Ac 72, 3822- 3844, doi:DOI 10.1016/j.gca.2008.04.032 (2008). [9] http://www.fas.org/irp/imint/docs/rst/Sect19/Sect19_13a.html We thank the NASA Meteorite Working Group for loaning us the samples and are grateful to the ANSMET teams who collected them. MAC 88105,159 We aim to investigate meteorites with mixed provenances (figure 2) to provide an understanding of the regolith in different lunar terrains. Twelve lunar meteorites from the ANSMET meteorite collection will be used in this study. A fraction of each meteorite will be analysed using FE-SEM and EMPA for compositional and petrographic data (e.g. figure 3). This compositional data will be used in the calculation of the production rates for each sample. Each meteorite will then be analysed for cosmogenic noble gas isotopes (e.g. 3 He, 21 Ne, 38 Ar, 83 Kr, 126 Xe) using the MAP mass spectrometer (figure 4). This isotopic data will be used to calculate the CRE age and shielding depth of each meteorite.