Meteorites, particularly carbonaceous chondrites, have been known to frequently be enriched in heavy nitrogen (nitrogen with an atomic weight of 15, rather than 14, due to the presence of an additional neutron in the nucleus) since the 1970s, with notable examples being the Renazzo Meteorite, which fell in Italy in 1924, has a heavy nitrogen enrichment relative to that of atmospheric nitrogen of 170 ‰ (170 parts per thousand, indicating that for every 1000 atoms of heavy nitrogen in a sample of atmospheric nitrogen, an equivalent sample of nitrogen from the Renazzo Meteorite will contain 1170 atoms of heavy nitrogen), the Bencubbin Meteorite, which was found in Western Australia in 1930, with a heavy nitrogen enrichment of 973 ‰, the Bells Meteorite with a heavy nitrogen enrichment of 335 ‰, and ALH 850585, which was found in the Allen Hills of Antarctica in 1985, with a heavy nitrogen enrichment of 858 ‰. However, this enrichment in heavy nitrogen does not appear to be found throughout the meteorites, but to be coming from specific compounds within them. The Allende Meteorite, a 17.3 kg carbonaceous chondrite which fell in Mexico in February 1969, has a heavy nitrogen enrichment of -18.6 ‰ (i.e. it less heavy nitrogen than atmospheric nitrogen), yet when heated to 600°C has been shown to give off much higher levels of heavy nitrogen.
Work carried out on the Murchison Meteorite, which fell in Victoria, Australia in 1969, during the 1980s and 1990s suggested that the heavy nitrogen found in that meteorite might be bound up in organic molecules, with differing molecules having different proportions of heavy nitrogen. This is interesting because the most likely way in which atoms of the same element can become sorted by isotopic weight within different molecules is for these molecules to form in an aqueous environment (liquid water) where the protomolecules are sorted in some way over a long period of time.
In a paper published on the arXiv Database at Cornell University on 15 November 2018, Malcolm McGeogh of the PLEX Corporation, Tomáš Šamořil of TESCAN ORSAY HOLDING, David Zapotok of TESCAN USA Inc., and Julie McGeogh of the Department of Molecular and Cellular Biology at Harvard University, present the results of a study of the Allende Meteorite, as well as Acfer 086, a 17.5 kg carbonaceous chondrite found in Algeria in 1989, which sought to find the source of heavy nitrogen in these meteorites.
Samples were obtained by drilling 6 mm into the meteorites with diamond burrs in a clean room. These samples were then cleaned ultrasonically in deionised water, and dried, then split into samples for analysis by Time of Flight Secondary Ion Mass Spectrometry on a XEIA3 TESCAN Scanning Electron Microscope equipped with a Focused Ion Beam that used high resolution orthogonal scanning, looking in particular for ions with molecular masses of 24, 25, 26, and 27, which correspond to C₂⁻ (two carbon atoms), C₂H⁻ (two carbon atoms and one hydrogen atom), ¹²C¹⁴N⁻ (one carbon atom and one nitrogen atom), and ¹²C¹⁵N⁻ (one carbon atom and one heavy nitrogen atom).
In a paper published on the arXiv Database at Cornell University on 15 November 2018, Malcolm McGeogh of the PLEX Corporation, Tomáš Šamořil of TESCAN ORSAY HOLDING, David Zapotok of TESCAN USA Inc., and Julie McGeogh of the Department of Molecular and Cellular Biology at Harvard University, present the results of a study of the Allende Meteorite, as well as Acfer 086, a 17.5 kg carbonaceous chondrite found in Algeria in 1989, which sought to find the source of heavy nitrogen in these meteorites.
Samples were obtained by drilling 6 mm into the meteorites with diamond burrs in a clean room. These samples were then cleaned ultrasonically in deionised water, and dried, then split into samples for analysis by Time of Flight Secondary Ion Mass Spectrometry on a XEIA3 TESCAN Scanning Electron Microscope equipped with a Focused Ion Beam that used high resolution orthogonal scanning, looking in particular for ions with molecular masses of 24, 25, 26, and 27, which correspond to C₂⁻ (two carbon atoms), C₂H⁻ (two carbon atoms and one hydrogen atom), ¹²C¹⁴N⁻ (one carbon atom and one nitrogen atom), and ¹²C¹⁵N⁻ (one carbon atom and one heavy nitrogen atom).
Acfer 086 (A), Allende (B) meteorite samples milled via diamond burrs (C) to produce micron scale fragments (D). McGeogh et al. (2019).
McGeogh et al. note that the molecular mass of ¹²C¹⁵N (one carbon atom and one heavy nitrogen atom) is the same as that of ¹³C¹⁴N (one heavy carbon atom and one nitrogen atom), which could potentially be a problem. However, the ratio of heavy carbon to carbon in the Allende Meteorite has previously been determined, enabling this to be removed from the calculations. The proportion of heavy carbon to carbon in Acfer 086 is unknown, but McGeogh et al. felt able to assume that it is the same as in the Allende Meteorite, due to the similar composition (and therefore presumed origin) of these bodies.
Using this methodology McGeogh et al. found that nitrogen bound to carbon in the Allende Meteorite was enhanced in the heavier isotope by about 410 ‰ relative to terrestrial nitrogen, and that nitrogen bound to carbon in Acfer 086 was enhanced in the heavier isotope by about 1015 ‰ relative to terrestrial nitrogen.
Both the meteorites produced exactly twice as much CN⁻ as C₂⁻, and this ratio was consistent across all the samples, suggesting that all the ions were coming from a common source. The most likely source of this would be polymer amides with a backbone of covalently bonded carbon and nitrogen, which if broken into random chunks should produce CN⁻:C₂⁻ in the ratio 2:1. When C₂H⁻ ions are included in the analysis the ratio of carbon to nitrogen is 2.8 to 1 in the Acfer 086 Meteorite, and 2.64 to 1 in the Allende Meteorite. This is consistent with the presence of fragments of mixed amino acids in the sample, with common amino acids having carbon to nitrogen ratios of 2 to 1 (glycine, asparagine, and histidine), 2.5 to 1 (glutamine), and 3 to 1 (alanine, serine, cysteine, and lysine). Both meteorites have previously yielded poly-glycine amide chains, with occasional alpha carbon hydroxylations and methylations, making such a compound a likely source of the heavy nitrogen in the samples.
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McGeogh et al. note that the molecular mass of ¹²C¹⁵N (one carbon atom and one heavy nitrogen atom) is the same as that of ¹³C¹⁴N (one heavy carbon atom and one nitrogen atom), which could potentially be a problem. However, the ratio of heavy carbon to carbon in the Allende Meteorite has previously been determined, enabling this to be removed from the calculations. The proportion of heavy carbon to carbon in Acfer 086 is unknown, but McGeogh et al. felt able to assume that it is the same as in the Allende Meteorite, due to the similar composition (and therefore presumed origin) of these bodies.
Using this methodology McGeogh et al. found that nitrogen bound to carbon in the Allende Meteorite was enhanced in the heavier isotope by about 410 ‰ relative to terrestrial nitrogen, and that nitrogen bound to carbon in Acfer 086 was enhanced in the heavier isotope by about 1015 ‰ relative to terrestrial nitrogen.
Both the meteorites produced exactly twice as much CN⁻ as C₂⁻, and this ratio was consistent across all the samples, suggesting that all the ions were coming from a common source. The most likely source of this would be polymer amides with a backbone of covalently bonded carbon and nitrogen, which if broken into random chunks should produce CN⁻:C₂⁻ in the ratio 2:1. When C₂H⁻ ions are included in the analysis the ratio of carbon to nitrogen is 2.8 to 1 in the Acfer 086 Meteorite, and 2.64 to 1 in the Allende Meteorite. This is consistent with the presence of fragments of mixed amino acids in the sample, with common amino acids having carbon to nitrogen ratios of 2 to 1 (glycine, asparagine, and histidine), 2.5 to 1 (glutamine), and 3 to 1 (alanine, serine, cysteine, and lysine). Both meteorites have previously yielded poly-glycine amide chains, with occasional alpha carbon hydroxylations and methylations, making such a compound a likely source of the heavy nitrogen in the samples.
See also...
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