Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30s.534l&link_type=abstract
Meteoritics, vol. 30, no. 5, page 534
Other
Breccias, Carbon, Hydrogen, Isotopes, Lunar Samples, 10086, 12023, Samples, Lunar, Winds, Solar
Scientific paper
Early analyses of lunar soils have revealed the presence of ppm amounts of indigenous methane and ethane thought to originate from the interaction of carbon and hydrogen in the solar wind [1]. The carbon abundance of lunar soils are typically between 50 to 300 ppm with isotopic compositions ranging from 0 to +20 per mil [2]. Solar wind hydrogen is generally accepted to contain delta D = -1000 per mil i.e. O ppm deuterium content; as indicated from D/H measurements of lunar hydrogen, after correction for contamination effects of terrestrial water from the sample [3]. In addition, recent solar spectroscopic measurements have detected hydrogen bearing molecules e.g. water [4]. Hence, isotopic analyses of methane in lunar samples can provide valuable information about isotopic composition of solar wind implanted species. For this study, we have analyzed methane from lunar soil A12023 and a breccia fragment taken from soil A10086. The methane from these samples was released by stepped pyrolysis and introduced into a new static vacuum mass spectrometer, MIRANDA (capable of measuring the delta l3C of ng amounts of methane to a precision of +/- 0.2 per mil. Isotopic compositions and abundances are calculated from 17M/l6M ratios in methane from which the delta 13C values can be determined assuming that solar wind hydrogen is delta D = -1000 per mil. Results from A12023 are presented in Figure 1, results from A10086 are discussed elsewhere [5]. Both samples displayed a broad release of methane between 400 degrees C and 800 degrees C with yields of 1.3 ppm and 1.9 ppm for the soil and the breccia respectively, consistent with previous measurements of ppm amounts of methane [1]. Figure 1 illustrates that methane in A12023 is isotopically heavy with a plateau of delta 13C = +55 per mil at a temperature between 500 degrees C and 600 degrees C. This is consistent with the previous measurement of A10086 [5] which also revealed isotopically heavy carbon with delta 13C approximately of +60 per mil. There is a possibility of hydrogen isotope exchange between water in the sample and methane, although unlikely below 600 degrees C. The delta l3C values have also been recalculated using a delta D of -360 per mil, the average delta D of water extracted from lunar material [6]. In this case a delta l3C of +20 per mil is obtained for the methane, in closer agreement to previous analyses which have a maximum delta 13C = +15 per mil [7]. Our preliminary investigations have indicated that the carbon in the methane is isotopically distinct from other lunar carbon sources. However further analyses are required to measure both delta 13C and delta D in a variety of lunar samples to verify the hypothesis that the methane is of solar wind origin. References: [1] Abell P. I. et al. (1970) Nature, 226, 251-252. [2] Epstein S. and Taylor H. P.(1973) Proc. LSC 4th, 1559-1575. [3] Epstein S. and Taylor H. P. (1972) Proc. LSC 3rd, 1429-1454. [4] Wallace L. et al. (1995) Science, 268, 1155-1158. [5] Brilliant D. R. et al. (1994) LPS XXV, 173-174. [6] Merlivat L. et al. (1972) Proc. LSC 3rd, 1473-1477. [7] Chang S. et al. (1974) GCA, 38, 853-872.
Brilliant D. R.
Butterworth Anna L.
Lecluse Christine
Morse Andrew D.
Pillinger Colin T.
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