Combined element (H and C) stable isotope ratios of methane in carbonaceous chondrites

Abstract

We have performed the first ever combined-element stable isotopic measurements of extraterrestrial molecules. Methane from two carbonaceous chondrites, Murchison and Cold Bokkeveld, was measured for its hydrogen and carbon isotopic compositions. The combined isotopic composition of meteoritic methane reveals information about the indigenous nature of volatile aliphatic organic matter in meteorites and its probable extraterrestrial source environment. Deuterium enrichments relative to the solar nebula 4.6 Gyr ago reflect a contribution from low-temperature chemical reactions in interstellar space. Similar carbon but different hydrogen isotopic compositions for methane in the two meteorite samples probably represent comparable primary origins but varying levels of secondary processing and exchange with isotopically light hydrogen. Our high precision laboratory-obtained measurements provide valuable reference points for observational spectroscopists undertaking astronomical investigations of the stable isotopic composition of extraterrestrial methane.

TITAN'S BULK COMPOSITION CONSTRAINED BYCASSINI-HUYGENS: IMPLICATION FOR INTERNAL OUTGASSING

“…Even though large quantities of carbon may have been brought by CI chondrites, it is unclear how much methane may be produced upon dissociation of organic matters in the interior. Interestingly, the D/H in methane released from carbonaceous chondrites by freeze-thaw disaggregation at low temperature (Butterworth et al 2004) is close to the D/H in atmospheric methane on Titan. This does not prove that chondritic methane significantly contributes to the atmospheric budget, but at least it indicates that a chondritic source is not incompatible with the available data.…”

Meteorite ablation products and their contribution to the atmospheres of terrestrial planets: An experimental study using pyrolysis-FTIR

“…Although free methane occurs at a level of 140 ppb in Murchison (CM2) (Yuen et al, 1984;Butterworth et al, 2004), corresponding to an expected mass of methane of $1 ng in the 7-10 mg samples of Murchison (CM2) analysed here, this is a mass well below the sensitivity of the instrumental setup ($1 lg), as established by the calibration curves of Court and Sephton (2009). Nevertheless, methane is produced by the pyrolysis of terrestrial organic materials such as coal and wood, so its absence in the pyrolysis products of these meteorites is noteworthy.…”

“…Nevertheless, the absence of methane still enables a maximum yield to be calculated. Using the estimate of the detection limit of methane of around 1 lg, as discussed in Section 2.2, the maximum yields of pyrogenic methane from our samples is calculated to be approximately 100 ppm, respectively, a level substantially greater than the $140 ppb abundance of free methane present in Murchison (Yuen et al, 1984;Butterworth et al, 2004). Consequently, these analyses allow us to state that the yield of methane upon the gasification of a carbonaceous asteroid of the composition of Murchison does not exceed about 100 ppm.…”

Ultraviolet-radiation-induced methane emissions from meteorites and the Martian atmosphere

“…21) and a more realistic 130 nm (see Supplementary Information), we obtain an 8.5-55% conversion of the accessible carbon to CH 4 (2.3-14.6 mg CH 4 g 21 ). This is about five orders of magnitude higher than total CH 4 release observed for carbonaceous chondrites during freeze-thaw disaggregation 22 . After redistributing the irradiated ground sample by shaking the reactor (Fig.…”

“…The carbon signature, d 13 C-CH 4 for CH 4 from Murchison, was in the range 257% to 232%, values that are commonly found for terrestrial sources including biogenic ones. This can be compared with carbon isotope signatures found for various types of organic matter in Murchison, which are in the range 213.4% to 136% for soluble organic matter and 213% for the (dominant) insoluble organic matter (IOM), respectively 22,26 . For bulk organic matter (BOM) and single organic compounds such as amino acids, d 13 C values ranging from 22% to 221% and from 21% to 141% have been reported 27 , while inorganic carbon (carbonate) shows average d 13 C values of 143% (refs 27, 28).…”