The pre-atmospheric size of martian meteorites was calculated based on 80Kr produced by epithermal secondary cosmic-ray produced neutrons of 30-300 eV energy. For seven meteorites we obtained minimum radii of 22-27 cm, corresponding to 150-270 kg

Busemann, H.

Eugster, O.

Lorenzetti, S.

English

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Open Research OnlineThe Open University’s repository of research publicationsand other research outputsThe pre-atmospheric size of Martian meteoritesConference or Workshop ItemHow to cite:Eugster, O.; Busemann, H. and Lorenzetti, S. (2002). The pre-atmospheric size of Martian meteorites. In:33rd Lunar and Planetary Science Conference, 11-15 Mar 2002, Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1096.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukTHE PRE–ATMOSPHERIC SIZE OF MARTIAN METEORITES. O. Eugster, H. Busemann, and S. Lorenzetti, Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland (eugster@phim.unibe.ch) According to model calculations the upper limit of the radius of fragments ejected from Mars by asteroidal or cometary impact  is about 2m, corresponding to a mass of about 100 tons [1]. In the present work we determined the lower limit of the pre-atmospheric size of martian meteorites based on the neutron flux induced by the comic-ray (CR) interaction with meteoritic matter.  We measured the Kr isotopic composition in eight martian meteorites. In all of them we observe effects induced by secondary cosmic-ray (CR) produced neutrons. Concentrations of neutron induced 80Kr, 80Krn, are obtained from   80Krn = 83Krc [(80Kr/83Kr)c–(80Kr/83Kr)s].         (1)  The index 'c' denotes cosmogenic Kr and 's' spallation Kr. For (80Kr/83Kr)s we adopt 0.765 [2]. The results are given in Table 1.  Table 1.  Neutron induced 80Krn  (10-12cm3STP/g), Br concentration [6,7] (ppm), and CRE age, T (Ma).         Meteorite 80Krn Br T         Los Angeles 0.98 0.7   3.04 QUE 94201 0.27 0.36   2.51 Shergotty 0.44 0.94   2.81 Zagami 0.24 0.79   3.15 Nakhla 0.99 4.05 10.8 Chassigny 0.44 0.12 11.3 ALH 84001 0.43 – 14.7 Sayh al Uhaymir 005 0.18 0.28 1.2      The production rate of 80Krn is calculated from   P(80Krn) = 80Krn/(Br Â 7           (2)  The Br concentration and T are given in Table 1 and P(80Krn) in Table 2. Marti et al. [2] have shown that 80Krn is produced by epithermal neutron capture of 79Br in the energy range 30–300 eV. The neutron flux, φn, is then obtained from  φn = P'(80Krn)/σ79,                           (3)    where P'(80Krn) = 2.33 Â -23P(80Krn) and has the dimension neutrons cm-2s-1. The resonance integral σ79 is 110 Â -24cm2 [3]. Fig. 1 demonstrates the resulting neutron fluxes. They are shown in relation to the fluxes of > 5 MeV neutrons that were calculated based on cosmogenic 21Ne [8]. Two scenarios are possible how the meteoritic material might have experienced the observed neutron fluxes: (a) during a pre–exposure to cosmic rays in the regolith before ejection from Mars, and (b) as a meteoroid during the transfer from Mars to Earth.   Table 2.  Production rate P(80Krn) (10-12cm3STP/g, Ma), epithermal neutron flux φn (neutrons cm-2s-1), slowing down density q (neutrons cm-3s-1), and minimum pre-atmospheric radius Rmin (cm).           Meteorite P(80Krn) φn q Rmin           Los Angeles 0.46 0.93 0.033 26 QUE 94201 0.30 0.61 0.021 24 Shergotty 0.17 0.35 0.012 23 Zagami 0.094 0.19 0.0067 23 Nakhla 0.023 0.047 0.0016 22 Chassigny 0.32 0.65 0.023 25 Sayh al Uhaymir 005 0.55 1.12 0.039 27       The neutron flux as a function of depth within the martian regolith has not been measured but Lingenfelter et al. [4] give these data for the lunar regolith. We show that scenario (a) can be excluded because the expected neutron fluxes in the meteoritic  material would be an order of magnitude higher than observed. For scenario (b) we obtain a slowing down density, q, for epithermal neutrons of  q = φn Â ξΣtot,                           (4)  where ξΣtot = 0.0354 cm-1 for chondrites [5]. As the relevant element abundances in chondrites and martian meteorites are similar, this value is used here and we obtain  q = 0.0716 P(80Krn).                          (5)  Based on the calculations of Eberhardt et al. [5] minimum radii, Rmin, are obtained:   Rmin = Ro + aq,                             (6)  Lunar and Planetary Science XXXIII (2002) 1096.pdfwhere Ro = 22 cm and a = 118 cm4s. Fig. 1 shows the minimum pre–atmospheric radii as calculated from the measured q values. The lower limit of the resulting radii (Table 2) is in the range of 22–27 cm. Taking the density of the martian meteorites into account we estimate the minimum pre–atmospheric masses to have been in the range of 150–270 kg.    Fig. 1 Secondary neutron flux ratio for 30–300 eV neutrons (from 80Kr) to >5 MeV neutrons (from 21Ne) vs. flux of 30–300 eV neutrons in martian meteorites. LA, Los Angeles; C, Chassigny; Q, QUE 94201; S, Shergotty; Z, Zagami; N, Nakhla; SaU, Sayh al Uhaymir 005.    Fig. 2. Minimum pre–atmospheric radii of martian meteorites calculated from the measured slowing down densities q for epithermal CR produced neutrons. For meteorite names see caption to Fig. 1.  Acknowledgement: This work was supported by the Swiss NSF. References: [1] Nyffeler B. (1999) Diploma thesis, Univ. of Bern. [2] Marti K. et al. (1966) Z. Naturforsch. 21a, 398. [3] Hughes D. J. and Schwartz B. (1958) Neutron Cross Sections. BNL 325, 2nd ed. and Suppl. No. 1 (1960). [4] Lingenfelter R. E. et al. (1972) Earth Planet. Sci. Lett. 16, 355. [5] Eberhardt P. et al. (1963) In Earth Science and Meteoritics, pp. 143–168, North Holland. [6] Meyer (1988) Mars meteorite compendium – 1988. LSC # 27672, LBJ Space Center, Houston, pp. 237. [7] Dreibus G. et al. (2000) Meteorit. Planet. Sci. 35, A49. [8] Eugster et al. (2002) Meteorit. Planet. Sci. 37, in press. Lunar and Planetary Science XXXIII (2002) 1096.pdfMartian Meteorites: O. Eugster et al.

The pre-atmospheric size of Martian meteorites

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The pre-atmospheric size of Martian meteorites

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