- Email: [email protected]
High molecular weight organic matter in martian meteorites
Planetary and Space Science 50 (2002) 711 – 716
www.elsevier.com/locate/pss
High molecular weight organic matter in martian meteorites M.A. Sephtona;∗ , I.P. Wrighta , I. Gilmoura , J.W. de Leeuwb , M.M. Gradyc , C.T. Pillingera a Planetary
and Space Sciences Research Institute, Open University, Milton Keynes, Buckinghamshire MK7 6AA, UK Institute for Sea Research, P.O. Box 59, Den Burg, Texel, 1790 AB, The Netherlands c Natural History Museum, Cromwell Road, London SW7 5BD, UK
b Netherlands
Received 1 November 2001; received in revised form 23 April 2002; accepted 4 July 2002
Abstract We have performed an investigation to detect high molecular weight organic matter in martian meteorites. Solvent-extracted samples of two Antarctic 5nds (ALH 84001, sub-sample 106 and EET A79001, sub-sample 351) and one non-Antarctic fall (Nakhla) were analysed by :ash pyrolysis–gas chromatography–mass spectrometry. Results suggest that our sub-sample of ALH 84001 contains no pyrolysable organic matter. In contrast, our samples of EET A79001 and Nakhla contain organic matter of high molecular weight, which releases aromatic and alkylaromatic hydrocarbons, phenol and benzonitrile as major compounds upon pyrolysis. The detection of similar pyrolysis products from Nakhla and EET A79001 indicates that these martian meteorites may have a common high molecular weight organic phase. Carbon isotopic measurements of individual molecules in the Nakhla pyrolysate, by :ash pyrolysis– gas chromatography–isotope ratio mass spectrometry, reveal that this high molecular weight organic matter has some similarities to that found in carbonaceous chondrites. At this point, an origin by terrestrial contamination cannot be unequivocally ruled out, but the data seem to support proposals that martian samples contain organic matter originating from meteoritic infall on Mars. The results suggest that a wider, pyrolysis-based study of martian meteorites would be a justi5able use of these precious samples. ? 2002 Elsevier Science Ltd. All rights reserved. Keywords: Mars; Meteorites; Organic; ALH 84001; EET A79001; Nakhla
1. Introduction Martian meteorites represent impact-ejected fragments of volcanic rocks from the surface of Mars (McSween, 1994). Establishing the presence of indigenous organic matter in these meteorites would have profound implications for the origin and distribution of life in the solar system. The detection of indigenous abiological organic matter in these samples would suggest that a reservoir of organic molecules has been available to developing prebiological systems on Mars, whereas the identi5cation of indigenous biological organic matter would indicate that life in the solar system is not exclusive to the Earth. In 1989, Wright et al. (1989) reported the detection of organic matter in the Antarctic martian meteorite EET A79001, sub-sample 239 from lithology B, in amounts thought to be in excess of those which could have been added by terrestrial contamination. They stressed the likely ∗ Correspondence author. Tel.: +44-0-1908-659-358; fax: +44-01908-858-022. E-mail address: [email protected] (M.A. Sephton).
martian provenance of this organic matter and advocated its further study. More recently, Grady et al. (1994) discovered that another Antarctic martian meteorite, ALH 84001, contains 200 ppm of carbon as organic matter and McKay et al. (1996) established that at least some of this organic matter is in the form of polyaromatic hydrocarbons (PAHs). Controversially, when considering the relevance of the PAHs alongside additional mineralogical phenomena, McKay et al. (1996) interpreted these molecules as the fossil remains of biological activity on Mars. Subsequently, Becker et al. (1997) not only detected PAHs in EET A79001 that were similar to those found in ALH 84001 by McKay et al. (1996), but also found them in several Antarctic carbonaceous chondrites and the Antarctic ice itself. After considering this evidence, Becker et al. (1997) suggested that the PAHs in martian meteorites are the result of either meteoritic infall on Mars and/or Earth, terrestrial contamination, or a mixture of both of these sources. However, in a more detailed study of the PAHs in ALH 84001, the contribution to these molecules from terrestrial contamination was deemed to be negligible (Clemmett et al., 1998). Further
0032-0633/02/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 2 - 0 6 3 3 ( 0 2 ) 0 0 0 5 3 - 3
712
M.A. Sephton et al. / Planetary and Space Science 50 (2002) 711 – 716
work investigated the amino acid content of ALH 84001 and drew attention to the similarity in compound distribution between extracts of the martian meteorite and the Antarctic ice (Bada et al., 1998). As a result, it was concluded that the amino acids in ALH 84001 are mainly terrestrial contaminants, but the possibility that small amounts of these molecules may be indigenous was also considered (Bada et al., 1998). In the same year 13 C and 14 C measurements were used to indicate that the majority of organic matter in ALH 84001 and EET A79001 is the result of terrestrial contamination (Jull et al., 1998). Intriguingly, Jull et al. (1998) also discovered that ALH 84001 contains an indigenous acid-insoluble carbonaceous phase that may be organic and which constitutes 20% of the carbon in the meteorite. This proposition was supported by further 13 C measurements and laser desorption mass spectrometry (LDMS), which identi5ed two types of aromatic organic matter in ALH 84001 (Becker et al., 1999). Terrestrial contamination appears to be associated with the carbonate globules while a high molecular weight extraterrestrial component, similar to that in carbonaceous chondrites, seems to be associated with the bulk matrix. Most recently, Jull et al. (2000) used 13 C and 14 C measurements to identify signi5cant amounts of acid-soluble and acid-insoluble indigenous organic matter in the non-Antarctic martian meteorite, Nakhla. Although, it appears that Nakhla also contains some terrestrial contamination (Glavin et al., 1999). An important consequence of the work of Jull et al. (1998, 2000) and Becker et al. (1999) is that refractory high molecular weight organic phases may exist in martian meteorites. If this is the case then analyses of these carbonaceous components have the potential to verify the presence of indigenous organic matter in these samples. The most commonly identi5ed terrestrial contaminants in meteorites are low molecular weight, solvent-soluble hydrocarbons (e.g. Cronin and Pizzarello, 1990; Sephton et al., 2001). As a result, a meteorite which has had its low molecular weight organic assemblage removed by solvent extraction is more likely to contain only indigenous organic matter (Bandurski and Nagy, 1976). Biological activity on Earth can result in the production of chemical fossils, the majority of which are present as components of a high molecular weight, solvent-insoluble kerogen (De Leeuw and Largeau, 1993). Meteoritic organic matter, as found in carbonaceous chondrites, is also composed mainly of high molecular weight, solvent-insoluble macromolecular material (Bandurski and Nagy, 1976). Hence, if the organic matter in martian meteorites is the result of martian life (McKay et al., 1996) or the product of meteoritic infall on Mars or the Earth (Becker et al., 1997, 1999) then in both cases it can be expected to be present mainly as a high molecular weight component. In turn, if the provenance of the organic matter in martian meteorites is to be successfully determined then attention must be directed towards detecting the presence of high molecular weight organic matter in these samples. If such organic matter is detected, its chemical structure
can be used to help discriminate between its possible origins. The nature of high molecular weight organic matter in small samples can be studied using analytical pyrolysis (e.g. Hartgers et al., 1994). Pyrolysis uses thermal energy in an inert atmosphere to transform high molecular weight organic matter into lower molecular weight fragments that can be readily analysed using conventional gas chromatography-based equipment. The pyrolysis unit can be coupled to both a gas chromatography–mass spectrometer (GC–MS) to obtain structural information and to a gas chromatography–isotope ratio mass spectrometer (GC– IRMS) to measure the isotopic compositions of individual molecules (Sephton and Gilmour, 2001a, b). In this study, :ash pyrolysis–gas chromatography– mass spectrometry (Py–GC–MS) and :ash pyrolysis–gas chromatography–isotope ratio mass spectrometry (Py–GC– IRMS) were used in an attempt to detect and characterise high molecular weight organic matter in two martian meteorites collected from Antarctica, ALH 84001 and EET A79001, and one collected following its fall in a non-Antarctic environment, Nakhla. 2. Experimental 2.1. Samples Sample details and terrestrial histories are listed in Table 1. Prior to pyrolysis, between 50 and 350 mg of each of the samples were crushed in an agate pestle and mortar. Solvent-soluble low molecular weight compounds were then removed from the sample using ultrasonically aided extraction (hexane, dichloromethane and methanol; 1 ml × 5 min each), centrifugation (1000 rpm × 5 min) and removal of the supernatant :uids. The extracted residues were dried in a Cleansphere (CA 100, Safetech Ltd.). The preliminary extraction steps ensured that when the solid samples were subsequently analysed by Py–GC–MS, the volatile products consisted only of fragments of high molecular weight, solvent-insoluble organic matter and did not contain contributions from any low molecular weight, solvent-soluble organic matter. GC–MS analysis of the solvent extracts of these small samples failed to detect any organic molecules. Table 1 Meteorite samples subjected to solvent extraction and then pyrolysis–gas chromatography–mass spectrometry
Meteorite
Class
Fall date
Location
ALH 84001, sub-sample 106
SNC
Antarctica
EET A79001, sub-sample 351
SNC
Nakhla
SNC
13; 000 yr (±1000) BP 12; 000 yr (±2000) BP 1911
Antarctica Egypt
Fall dates for ALH 84001 and EET A79001 are from Jull et al. (1998) and references therein. BP, before present.
M.A. Sephton et al. / Planetary and Space Science 50 (2002) 711 – 716
713
Sand, previously roasted at 450◦ C in air for 8 h, was subjected to the same procedure and used as a procedural blank.
(a) procedural blank
2.2. Py–GC–MS 5
10
15
20
25
30
35
40
45
retention time (min)
(b) solvent extracted ALH 84001
relative intensity
Each sample was pressed on to a :attened ferromagnetic wire and pyrolysed by inductive heating for 10 s (Curie temperature 610◦ C). The pyrolysis products were separated with a Hewlett-Packard 5890 gas chromatograph equipped with a cryogenic unit and 5tted with a CP-Sil 5 capillary column (25 m × 0:32 mm × 0:45 m). The gas chromatograph oven was programmed to maintain a temperature of 0◦ C for 5 min, followed by a linear rise of 3◦ C min−1 from 0◦ C to 300◦ C. The 5nal temperature was held for 15 min. Compound identi5cation was performed using a VG Autospec Ultima mass spectrometer operated at 70 eV with a mass range m=z 50 –800 (mass resolution 1000). The Py– GC–MS system is capable of routinely detecting nanogram amounts of individual pyrolysis products. For each sample, once pyrolysis products were detected by the relatively sensitive Py–GC–MS system, the remaining sample was used in an attempt to obtain carbon isotopic data by Py–GC–IRMS.
5
10
15
20
25
30
35
40
45
retention time (min)
(c) solvent extracted EET A79001 C2 CN
5
10
15
20
OH
25
30
35
40
45
retention time (min)
(d) solvent extracted Nakhla
C2
2.3. Py–GC–IRMS
CN
Powdered samples were introduced as dry pellets into an on-line micro-furnace (pyrojector, SGE Ltd.) and pyrolysed at 610◦ C. The pyrojector was directly coupled to the split–splitless injector of a Varian 3400 gas chromatograph linked via a combustion interface to a Finnigan MAT Delta S/GC isotope ratio mass spectrometer. A detailed description of the Py–GC–IRMS system can be found in Sephton and Gilmour (2001a). Gas chromatography was performed on a 25 m × 0:32 mm × 0:5 m BPX5 capillary column (SGE Ltd.). The GC oven was held at 40◦ C for 2 min and subsequently programmed from 40◦ C to 300◦ C at 5◦ C min−1 . The 5nal temperature was held for 6 min. Limited amounts of sample restricted the number of successful analyses to a single run of Nakhla. All carbon isotopic ratios are expressed in the notation relative to the international PDB standard as follows: 13
CPDB =
13
C= 12 Csample − 1 × 1000 ((): 13 C= 2 C PDB
3. Results and discussion 3.1. Pyrolysis products The Py–GC–MS chromatogram for the roasted sand blank (Fig. 1a) reveals that, over the mass range monitored, no terrestrial contaminants are added to the samples
OH
5
10
15
20
25
30
35
40
45
retention time (min)
Fig. 1. Partial total ion current traces from :ash pyrolysis–gas chromatography–mass spectrometry analyses of (a) a procedural blank, and solvent extracted samples of the martian meteorites, (b) ALH 84001, (c) EET A79001 and (d) Nakhla.
during the solvent-extraction procedures. Any organic entities released from the meteorite samples by pyrolysis ought to represent fragments of high molecular weight organic matter present in the meteorites. The result for ALH 84001 (Fig. 1b) indicates that this particular sub-sample contains no pyrolysable organic matter. Py–GC–MS analysis of EET A79001 (Fig. 1c) reveals that high molecular weight organic matter is present in this meteorite. The major pyrolysis products are predominantly aromatic and alkyl-substituted aromatic hydrocarbons, consisting of benzene, C1 –C2 alkylbenzenes (toluene and ethenylbenzene) and naphthalene. Oxygen-containing aromatic compounds are also represented by phenol. The nitrogen-containing aromatic compound, benzonitrile is also present in significant amounts. Py–GC–MS of Nakhla (Fig. 1d) indicates that this meteorite also contains high molecular weight organic matter. The pyrolysate contains all of the organic
714
M.A. Sephton et al. / Planetary and Space Science 50 (2002) 711 – 716
compounds identi5ed in the EET A79001 pyrolysate, plus the C12 aromatic hydrocarbon, biphenyl. For Nakhla, compounds with identical retention times to benzene, toluene, a C2 alkybenzene and benzonitrile were also detected by Py–GC–IRMS. 3.2. Signi:cance of the pyrolysis products The absence of pyrolysis products from ALH 84001 shows that this particular sample contains no pyrolysable high molecular weight organic matter. If this sub-sample is representative of the meteorite as a whole then this would suggest that the preterrestrial refractory carbonaceous component identi5ed in ALH 84001 by Jull et al. (1998) may not be organic, but rather a highly cross-linked material perhaps approximating to amorphous or poorly crystalline elemental carbon, or alternatively an acid resistant form of carbonate. However, it has been reported that the organic matter in ALH 84001 is distributed heterogenously throughout the meteorite (McKay et al., 1996) and LDMS analyses of this meteorite have previously revealed the presence of high molecular weight organic matter (Becker et al., 1999). Hence it is likely that our portion of ALH 84001 simply represents an organic-poor site. In contrast to ALH 84001, EET A79001 and Nakhla release signi5cant amounts of organic compounds following pyrolysis. The pyrolysis products from EET A79001 and Nakhla are qualitatively similar and indicate that the high molecular weight organic matter present in these two meteorites is composed of one- or two-ring aromatic cores connected by aliphatic and heteroatomic linkages. Interestingly, the pyrolysis products from these two meteorites are super5cially similar to those released by the pyrolysis of high molecular weight organic matter in carbonaceous chondrites (cf. Bandurski and Nagy, 1976; Sephton et al., 1998). On 5rst inspection, this appears to be consistent with suggestions that the organic matter in martian meteorites could be the result of the exogenous delivery of meteoritic organic matter to the martian surface (Becker et al., 1997, 1999; Jull et al., 2000). 3.3. Extra-terrestrial or terrestrial organic matter? The fact that the Nakhla pyrolysate has been detected twice, on two diLerent systems, suggests that the pyrolysis products are not experimental artefacts. Hence, high molecular weight organic entities do appear to be present in the Nakhla meteorite and are probably present in EET A79001. One unavoidable possibility which must be considered is that in part, or in total, the organic compounds released by pyrolysis of the martian meteorites are the result of terrestrial contamination. It has been suggested that for Antarctic martian meteorites, such as EET A79001, low molecular weight terrestrial molecules can be intro-
duced to the meteorites by Antarctic ice meltwater (Bada et al., 1998). It has also been proposed that low molecular weight terrestrial contaminants were introduced to the non-Antarctic martian meteorite Nakhla by sediment porewaters after this meteorite fell in an agricultural farmland region of the Nile River Delta (Glavin et al., 1999). The suggestion that Nakhla has been aLected in some way by terrestrial water is consistent with hydrogen isotopic data for water released from the meteorite at low temperatures during incremental heating (Leshin et al., 1996). Therefore, both EET A79001 and Nakhla may have been compromised by water-borne terrestrial contaminants. However, water-borne contamination is more likely to involve low, rather than the high molecular weight organic entities detected by Py–GC–MS. Furthermore, EET A79001 and Nakhla seem to contain structurally similar high molecular weight organic matter despite being collected from fall sites in very diLerent environments. Presumably, Antarctic and non-Antarctic environments should be characterised by diLerent types of terrestrial contamination. Therefore, although terrestrial contamination cannot be ruled out, the data is also consistent with the organic matter being present in these meteorites prior to their fall to Earth. 3.4. Carbon isotopes The proposal that Nakhla contains a preterrestrial high molecular weight organic phase is consistent with the 13 C and 14 C work of Jull et al. (2000) which indicated that Nakhla may contain an acid-insoluble organic component, not added by recent terrestrial contamination. It is likely that the pyrolysis products from Nakhla correlate with the components released from an acid-insoluble residue of this meteorite by stepped combustion at 200 –400◦ C (Jull et al., 2000). The carbon isotopic compositions of individual molecules can provide clues as to the source of the organic entities. For example, the Py–GC–IRMS analysis of Nakhla produces values that can be compared with those for their structurally identical counterparts from the high molecular weight organic matter in the Murchison carbonaceous chondrite (Table 2). It appears that individual aromatic molecules released by the pyrolysis of Nakhla have carbon isotopic compositions within the range of values observed for aromatic units in carbonaceous chondrites (Sephton et al., 1998, 2000). The isotopic compositions of individual molecules from Murchison and Nakhla, however, diLer by up to 5:5( but this is no greater than the diLerence in isotopic composition observed between structurally identical molecules from the high molecular weight organic matter in diLerent carbonaceous chondrites (Sephton et al., 2000). Hence, the isotopic data reveals that it is possible that the organic matter in Nakhla is the result of meteoritic infall on Mars.
M.A. Sephton et al. / Planetary and Space Science 50 (2002) 711 – 716 Table 2 Carbon isotopic compositions of individual molecules released from high molecular weight organic matter in the Nakhla meteorite by pyrolysis
Compound
13 CPDB (
Carbonate Benzene Toluene C2 alkylbenzene Benzonitrile
+11:6 −22.0 −21.6 −22.1 −18.0
Nakhla
()
(
13 CPDB ( ) Murchison
+38 −28.0 −24.6 −17.8 – −21.9 −23.5
Values for structurally identical molecules from Murchison are also listed. The Murchison organic data is from Yuen et al. (1984) and Sephton et al. (1998), except for the benzonitrile value which is from a Py–GC–IRMS analysis of an oL-line, anhydrously pyrolysed HF/HCl residue.
3.5. Future directions Inevitably, studies on martian meteorites are limited by sample availability. However, the data generated in this study illustrate the potential of pyrolysis-based techniques to decipher the complex pre-terrestrial and terrestrial histories of these samples. Future work should involve replicate analyses of diLerent parts of the same meteorites to address the problem of sample heterogeneity. The use of larger samples would permit the identi5cation of minor components in the pyrolysates that are currently overprinted by background interference. Unequivocally constraining the origin of organic matter in martian meteorites will require more compound-speci5c isotopic measurements. The utility of 13 C, 13 N and D values for determining the provenance of individual organic entities in meteorites has been illustrated for the carbonaceous chondrites (Sephton and Gilmour, 2001b). 4. Conclusions Several provisional conclusions can be made using the present data set. At least some parts of certain martian meteorites appear to contain a high molecular weight organic component. Upon pyrolysis, this organic phase produces aromatic and alkylaromatic hydrocarbons, phenol and benzonitrile and therefore displays a super5cial similarity to the high molecular weight organic matter in carbonaceous chondrites. The carbon isotopic compositions of individual molecules within the Nakhla pyrolysate also show some similarities to those of their counterparts from carbonaceous chondrites. As the pyrolysate generated from the martian meteorite Nakhla has been detected twice, on two diLerent analytical systems, it appears unlikely that the pyrolysis products are experimental artefacts. The detection of this high molecular weight organic matter in both an Antarctic 5nd and a non-Antarctic fall appears consistent with suggestions that martian meteorites contain organic matter with
715
a preterrestrial origin, possibly delivered to the martian surface by meteoritic infall. Acknowledgements This work was supported by PPARC. We thank J. Gibson for help with sample preparation. M. Dekker, P. Slootweg and W. Pool are gratefully acknowledged for analytical assistance. AMWG provided the samples. We are grateful to O. Botta and C. Koeberl for their constructive reviews. This is NIOZ contribution number 3364. References Bada, J.L., Glavin, D.P., McDonald, G.D., Becker, L., 1998. A search for endogenous amino acids in martian meteorite ALH84001. Science 279, 362–365. Bandurski, E.L., Nagy, B., 1976. The polymer-like organic material in the Orgueil meteorite. Geochim. Cosmochim. Acta 40, 1397–1406. Becker, L., Glavin, D.P., Bada, J.L., 1997. Polycyclic aromatic hydrocarbons (PAHs) in Antarctic Martian meteorites, carbonaceous chondrites, and polar ice. Geochim. Cosmochim. Acta 61, 475–481. Becker, L., Popp, B., Rust, T., Bada, J.L., 1999. The origin of organic matter in the Martian meteorite ALH84001. Earth Planet. Sci. Lett. 167, 71–79. Clemmett, S.J, et al., 1998. Evidence for the extraterrestrial origin of polycyclic aromatic hydrocarbons (PAHs) in the martian meteorite ALH 84001. Faraday Discuss. 109, 417–436. Cronin, J.R., Pizzarello, S., 1990. Aliphatic hydrocarbons of the Murchison meteorite. Geochim. Cosmochim. Acta 54, 2859–2868. De Leeuw, J.W., Largeau, C., 1993. A review of macromolecular organic compounds that comprise living organisms and their role in kerogen, coal and petroleum formation. In: Engel, M.H., Macko, S.A. (Eds.), Organic Geochemistry. Plenum Press, New York, pp. 23–72. Glavin, D.P., Bada, J.L., Brinton, K.L.F., McDonald, G.D., 1999. Amino acids in the martian meteorite Nakhla. Proc. Natural Acad. Sci. USA 96, 8835–8838. Grady, M.M., Wright, I.P., Douglas, C., Pillinger, C.T., 1994. Carbon and nitrogen in ALH 84001. Meteoritics 29, 469. Hartgers, W.A., Sinninghe DamstOe, J.S., de Leeuw, J.W., 1994. Geochemical signi5cance of alkylbenzene distribution in :ash pyrolysates of kerogens, coals and asphaltenes. Geochim. Cosmochim. Acta 58, 1759–1775. Jull, A.J.T., Courtney, C., JeLrey, D.A., Beck, J.W., 1998. Isotopic evidence for a terrestrial source of organic compounds found in martian meteorites Allan Hills 84001 and Elephant Moraine 79001. Science 279, 366–369. Jull, A.J.T., Beck, J.W., Burr, G.S., 2000. Isotopic evidence for extraterrestrial organic matter in the martian meteorite Nakhla. Geochim. Cosmochim. Acta 64, 3763–3772. Leshin, L.A., Epstein, S., Stolper, M., 1996. Hydrogen isotope geochemistry of SNC meteorites. Geochim. Cosmochim. Acta 60, 2635–2650. McKay, D.S, et al., 1996. Search for past life on Mars: possible relic biogenic activity in Martian meteorite ALH84001. Science 273, 924–930. McSween, H.Y.J., 1994. What we have learned about Mars from SNC meteorites. Meteoritics 29, 757–779. Sephton, M.A., Gilmour, I., 2001a. Pyrolysis–gas chromatography–isotope ratio-mass spectrometry of macromolecular material in meteorites. Planet. Space Sci. 49, 465–471.
716
M.A. Sephton et al. / Planetary and Space Science 50 (2002) 711 – 716
Sephton, M.A., Gilmour, I., 2001b. Compound speci5c isotope analysis of the organic constituents in carbonaceous chondrites. Mass Spectrom. Rev. 20, 111–120. Sephton, M.A., Pillinger, C.T., Gilmour, I., 1998. 13 C of free and macromolecular aromatic structures in the Murchison meteorite. Geochim. Cosmochim. Acta 62, 1821–1828. Sephton, M.A., Pillinger, C.T., Gilmour, I., 2000. Aromatic moieties in meteoritic macromolecular materials: analyses by hydrous pyrolysis and 13 C of individual compounds. Geochim. Cosmochim. Acta 64, 321–328.
Sephton, M.A., Pillinger, C.T., Gilmour, I., 2001. Normal alkanes in meteorites: molecular 13 C values indicate an origin by terrestrial contamination. Precambrian Res. 106, 45–56. Wright, I.P., Grady, M.M., Pillinger, C.T., 1989. Organic materials in a martian meteorite. Nature 340, 220–222. Yuen, G., Blair, N., DesMarias, D.J., Chang, S., 1984. Carbon isotope composition of low molecular weight hydrocarbons and monocarboxylic acids form the Murchison meteorite. Nature 307, 252–254.
www.elsevier.com/locate/pss
High molecular weight organic matter in martian meteorites M.A. Sephtona;∗ , I.P. Wrighta , I. Gilmoura , J.W. de Leeuwb , M.M. Gradyc , C.T. Pillingera a Planetary
and Space Sciences Research Institute, Open University, Milton Keynes, Buckinghamshire MK7 6AA, UK Institute for Sea Research, P.O. Box 59, Den Burg, Texel, 1790 AB, The Netherlands c Natural History Museum, Cromwell Road, London SW7 5BD, UK
b Netherlands
Received 1 November 2001; received in revised form 23 April 2002; accepted 4 July 2002
Abstract We have performed an investigation to detect high molecular weight organic matter in martian meteorites. Solvent-extracted samples of two Antarctic 5nds (ALH 84001, sub-sample 106 and EET A79001, sub-sample 351) and one non-Antarctic fall (Nakhla) were analysed by :ash pyrolysis–gas chromatography–mass spectrometry. Results suggest that our sub-sample of ALH 84001 contains no pyrolysable organic matter. In contrast, our samples of EET A79001 and Nakhla contain organic matter of high molecular weight, which releases aromatic and alkylaromatic hydrocarbons, phenol and benzonitrile as major compounds upon pyrolysis. The detection of similar pyrolysis products from Nakhla and EET A79001 indicates that these martian meteorites may have a common high molecular weight organic phase. Carbon isotopic measurements of individual molecules in the Nakhla pyrolysate, by :ash pyrolysis– gas chromatography–isotope ratio mass spectrometry, reveal that this high molecular weight organic matter has some similarities to that found in carbonaceous chondrites. At this point, an origin by terrestrial contamination cannot be unequivocally ruled out, but the data seem to support proposals that martian samples contain organic matter originating from meteoritic infall on Mars. The results suggest that a wider, pyrolysis-based study of martian meteorites would be a justi5able use of these precious samples. ? 2002 Elsevier Science Ltd. All rights reserved. Keywords: Mars; Meteorites; Organic; ALH 84001; EET A79001; Nakhla
1. Introduction Martian meteorites represent impact-ejected fragments of volcanic rocks from the surface of Mars (McSween, 1994). Establishing the presence of indigenous organic matter in these meteorites would have profound implications for the origin and distribution of life in the solar system. The detection of indigenous abiological organic matter in these samples would suggest that a reservoir of organic molecules has been available to developing prebiological systems on Mars, whereas the identi5cation of indigenous biological organic matter would indicate that life in the solar system is not exclusive to the Earth. In 1989, Wright et al. (1989) reported the detection of organic matter in the Antarctic martian meteorite EET A79001, sub-sample 239 from lithology B, in amounts thought to be in excess of those which could have been added by terrestrial contamination. They stressed the likely ∗ Correspondence author. Tel.: +44-0-1908-659-358; fax: +44-01908-858-022. E-mail address: [email protected] (M.A. Sephton).
martian provenance of this organic matter and advocated its further study. More recently, Grady et al. (1994) discovered that another Antarctic martian meteorite, ALH 84001, contains 200 ppm of carbon as organic matter and McKay et al. (1996) established that at least some of this organic matter is in the form of polyaromatic hydrocarbons (PAHs). Controversially, when considering the relevance of the PAHs alongside additional mineralogical phenomena, McKay et al. (1996) interpreted these molecules as the fossil remains of biological activity on Mars. Subsequently, Becker et al. (1997) not only detected PAHs in EET A79001 that were similar to those found in ALH 84001 by McKay et al. (1996), but also found them in several Antarctic carbonaceous chondrites and the Antarctic ice itself. After considering this evidence, Becker et al. (1997) suggested that the PAHs in martian meteorites are the result of either meteoritic infall on Mars and/or Earth, terrestrial contamination, or a mixture of both of these sources. However, in a more detailed study of the PAHs in ALH 84001, the contribution to these molecules from terrestrial contamination was deemed to be negligible (Clemmett et al., 1998). Further
0032-0633/02/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 2 - 0 6 3 3 ( 0 2 ) 0 0 0 5 3 - 3
712
M.A. Sephton et al. / Planetary and Space Science 50 (2002) 711 – 716
work investigated the amino acid content of ALH 84001 and drew attention to the similarity in compound distribution between extracts of the martian meteorite and the Antarctic ice (Bada et al., 1998). As a result, it was concluded that the amino acids in ALH 84001 are mainly terrestrial contaminants, but the possibility that small amounts of these molecules may be indigenous was also considered (Bada et al., 1998). In the same year 13 C and 14 C measurements were used to indicate that the majority of organic matter in ALH 84001 and EET A79001 is the result of terrestrial contamination (Jull et al., 1998). Intriguingly, Jull et al. (1998) also discovered that ALH 84001 contains an indigenous acid-insoluble carbonaceous phase that may be organic and which constitutes 20% of the carbon in the meteorite. This proposition was supported by further 13 C measurements and laser desorption mass spectrometry (LDMS), which identi5ed two types of aromatic organic matter in ALH 84001 (Becker et al., 1999). Terrestrial contamination appears to be associated with the carbonate globules while a high molecular weight extraterrestrial component, similar to that in carbonaceous chondrites, seems to be associated with the bulk matrix. Most recently, Jull et al. (2000) used 13 C and 14 C measurements to identify signi5cant amounts of acid-soluble and acid-insoluble indigenous organic matter in the non-Antarctic martian meteorite, Nakhla. Although, it appears that Nakhla also contains some terrestrial contamination (Glavin et al., 1999). An important consequence of the work of Jull et al. (1998, 2000) and Becker et al. (1999) is that refractory high molecular weight organic phases may exist in martian meteorites. If this is the case then analyses of these carbonaceous components have the potential to verify the presence of indigenous organic matter in these samples. The most commonly identi5ed terrestrial contaminants in meteorites are low molecular weight, solvent-soluble hydrocarbons (e.g. Cronin and Pizzarello, 1990; Sephton et al., 2001). As a result, a meteorite which has had its low molecular weight organic assemblage removed by solvent extraction is more likely to contain only indigenous organic matter (Bandurski and Nagy, 1976). Biological activity on Earth can result in the production of chemical fossils, the majority of which are present as components of a high molecular weight, solvent-insoluble kerogen (De Leeuw and Largeau, 1993). Meteoritic organic matter, as found in carbonaceous chondrites, is also composed mainly of high molecular weight, solvent-insoluble macromolecular material (Bandurski and Nagy, 1976). Hence, if the organic matter in martian meteorites is the result of martian life (McKay et al., 1996) or the product of meteoritic infall on Mars or the Earth (Becker et al., 1997, 1999) then in both cases it can be expected to be present mainly as a high molecular weight component. In turn, if the provenance of the organic matter in martian meteorites is to be successfully determined then attention must be directed towards detecting the presence of high molecular weight organic matter in these samples. If such organic matter is detected, its chemical structure
can be used to help discriminate between its possible origins. The nature of high molecular weight organic matter in small samples can be studied using analytical pyrolysis (e.g. Hartgers et al., 1994). Pyrolysis uses thermal energy in an inert atmosphere to transform high molecular weight organic matter into lower molecular weight fragments that can be readily analysed using conventional gas chromatography-based equipment. The pyrolysis unit can be coupled to both a gas chromatography–mass spectrometer (GC–MS) to obtain structural information and to a gas chromatography–isotope ratio mass spectrometer (GC– IRMS) to measure the isotopic compositions of individual molecules (Sephton and Gilmour, 2001a, b). In this study, :ash pyrolysis–gas chromatography– mass spectrometry (Py–GC–MS) and :ash pyrolysis–gas chromatography–isotope ratio mass spectrometry (Py–GC– IRMS) were used in an attempt to detect and characterise high molecular weight organic matter in two martian meteorites collected from Antarctica, ALH 84001 and EET A79001, and one collected following its fall in a non-Antarctic environment, Nakhla. 2. Experimental 2.1. Samples Sample details and terrestrial histories are listed in Table 1. Prior to pyrolysis, between 50 and 350 mg of each of the samples were crushed in an agate pestle and mortar. Solvent-soluble low molecular weight compounds were then removed from the sample using ultrasonically aided extraction (hexane, dichloromethane and methanol; 1 ml × 5 min each), centrifugation (1000 rpm × 5 min) and removal of the supernatant :uids. The extracted residues were dried in a Cleansphere (CA 100, Safetech Ltd.). The preliminary extraction steps ensured that when the solid samples were subsequently analysed by Py–GC–MS, the volatile products consisted only of fragments of high molecular weight, solvent-insoluble organic matter and did not contain contributions from any low molecular weight, solvent-soluble organic matter. GC–MS analysis of the solvent extracts of these small samples failed to detect any organic molecules. Table 1 Meteorite samples subjected to solvent extraction and then pyrolysis–gas chromatography–mass spectrometry
Meteorite
Class
Fall date
Location
ALH 84001, sub-sample 106
SNC
Antarctica
EET A79001, sub-sample 351
SNC
Nakhla
SNC
13; 000 yr (±1000) BP 12; 000 yr (±2000) BP 1911
Antarctica Egypt
Fall dates for ALH 84001 and EET A79001 are from Jull et al. (1998) and references therein. BP, before present.
M.A. Sephton et al. / Planetary and Space Science 50 (2002) 711 – 716
713
Sand, previously roasted at 450◦ C in air for 8 h, was subjected to the same procedure and used as a procedural blank.
(a) procedural blank
2.2. Py–GC–MS 5
10
15
20
25
30
35
40
45
retention time (min)
(b) solvent extracted ALH 84001
relative intensity
Each sample was pressed on to a :attened ferromagnetic wire and pyrolysed by inductive heating for 10 s (Curie temperature 610◦ C). The pyrolysis products were separated with a Hewlett-Packard 5890 gas chromatograph equipped with a cryogenic unit and 5tted with a CP-Sil 5 capillary column (25 m × 0:32 mm × 0:45 m). The gas chromatograph oven was programmed to maintain a temperature of 0◦ C for 5 min, followed by a linear rise of 3◦ C min−1 from 0◦ C to 300◦ C. The 5nal temperature was held for 15 min. Compound identi5cation was performed using a VG Autospec Ultima mass spectrometer operated at 70 eV with a mass range m=z 50 –800 (mass resolution 1000). The Py– GC–MS system is capable of routinely detecting nanogram amounts of individual pyrolysis products. For each sample, once pyrolysis products were detected by the relatively sensitive Py–GC–MS system, the remaining sample was used in an attempt to obtain carbon isotopic data by Py–GC–IRMS.
5
10
15
20
25
30
35
40
45
retention time (min)
(c) solvent extracted EET A79001 C2 CN
5
10
15
20
OH
25
30
35
40
45
retention time (min)
(d) solvent extracted Nakhla
C2
2.3. Py–GC–IRMS
CN
Powdered samples were introduced as dry pellets into an on-line micro-furnace (pyrojector, SGE Ltd.) and pyrolysed at 610◦ C. The pyrojector was directly coupled to the split–splitless injector of a Varian 3400 gas chromatograph linked via a combustion interface to a Finnigan MAT Delta S/GC isotope ratio mass spectrometer. A detailed description of the Py–GC–IRMS system can be found in Sephton and Gilmour (2001a). Gas chromatography was performed on a 25 m × 0:32 mm × 0:5 m BPX5 capillary column (SGE Ltd.). The GC oven was held at 40◦ C for 2 min and subsequently programmed from 40◦ C to 300◦ C at 5◦ C min−1 . The 5nal temperature was held for 6 min. Limited amounts of sample restricted the number of successful analyses to a single run of Nakhla. All carbon isotopic ratios are expressed in the notation relative to the international PDB standard as follows: 13
CPDB =
13
C= 12 Csample − 1 × 1000 ((): 13 C= 2 C PDB
3. Results and discussion 3.1. Pyrolysis products The Py–GC–MS chromatogram for the roasted sand blank (Fig. 1a) reveals that, over the mass range monitored, no terrestrial contaminants are added to the samples
OH
5
10
15
20
25
30
35
40
45
retention time (min)
Fig. 1. Partial total ion current traces from :ash pyrolysis–gas chromatography–mass spectrometry analyses of (a) a procedural blank, and solvent extracted samples of the martian meteorites, (b) ALH 84001, (c) EET A79001 and (d) Nakhla.
during the solvent-extraction procedures. Any organic entities released from the meteorite samples by pyrolysis ought to represent fragments of high molecular weight organic matter present in the meteorites. The result for ALH 84001 (Fig. 1b) indicates that this particular sub-sample contains no pyrolysable organic matter. Py–GC–MS analysis of EET A79001 (Fig. 1c) reveals that high molecular weight organic matter is present in this meteorite. The major pyrolysis products are predominantly aromatic and alkyl-substituted aromatic hydrocarbons, consisting of benzene, C1 –C2 alkylbenzenes (toluene and ethenylbenzene) and naphthalene. Oxygen-containing aromatic compounds are also represented by phenol. The nitrogen-containing aromatic compound, benzonitrile is also present in significant amounts. Py–GC–MS of Nakhla (Fig. 1d) indicates that this meteorite also contains high molecular weight organic matter. The pyrolysate contains all of the organic
714
M.A. Sephton et al. / Planetary and Space Science 50 (2002) 711 – 716
compounds identi5ed in the EET A79001 pyrolysate, plus the C12 aromatic hydrocarbon, biphenyl. For Nakhla, compounds with identical retention times to benzene, toluene, a C2 alkybenzene and benzonitrile were also detected by Py–GC–IRMS. 3.2. Signi:cance of the pyrolysis products The absence of pyrolysis products from ALH 84001 shows that this particular sample contains no pyrolysable high molecular weight organic matter. If this sub-sample is representative of the meteorite as a whole then this would suggest that the preterrestrial refractory carbonaceous component identi5ed in ALH 84001 by Jull et al. (1998) may not be organic, but rather a highly cross-linked material perhaps approximating to amorphous or poorly crystalline elemental carbon, or alternatively an acid resistant form of carbonate. However, it has been reported that the organic matter in ALH 84001 is distributed heterogenously throughout the meteorite (McKay et al., 1996) and LDMS analyses of this meteorite have previously revealed the presence of high molecular weight organic matter (Becker et al., 1999). Hence it is likely that our portion of ALH 84001 simply represents an organic-poor site. In contrast to ALH 84001, EET A79001 and Nakhla release signi5cant amounts of organic compounds following pyrolysis. The pyrolysis products from EET A79001 and Nakhla are qualitatively similar and indicate that the high molecular weight organic matter present in these two meteorites is composed of one- or two-ring aromatic cores connected by aliphatic and heteroatomic linkages. Interestingly, the pyrolysis products from these two meteorites are super5cially similar to those released by the pyrolysis of high molecular weight organic matter in carbonaceous chondrites (cf. Bandurski and Nagy, 1976; Sephton et al., 1998). On 5rst inspection, this appears to be consistent with suggestions that the organic matter in martian meteorites could be the result of the exogenous delivery of meteoritic organic matter to the martian surface (Becker et al., 1997, 1999; Jull et al., 2000). 3.3. Extra-terrestrial or terrestrial organic matter? The fact that the Nakhla pyrolysate has been detected twice, on two diLerent systems, suggests that the pyrolysis products are not experimental artefacts. Hence, high molecular weight organic entities do appear to be present in the Nakhla meteorite and are probably present in EET A79001. One unavoidable possibility which must be considered is that in part, or in total, the organic compounds released by pyrolysis of the martian meteorites are the result of terrestrial contamination. It has been suggested that for Antarctic martian meteorites, such as EET A79001, low molecular weight terrestrial molecules can be intro-
duced to the meteorites by Antarctic ice meltwater (Bada et al., 1998). It has also been proposed that low molecular weight terrestrial contaminants were introduced to the non-Antarctic martian meteorite Nakhla by sediment porewaters after this meteorite fell in an agricultural farmland region of the Nile River Delta (Glavin et al., 1999). The suggestion that Nakhla has been aLected in some way by terrestrial water is consistent with hydrogen isotopic data for water released from the meteorite at low temperatures during incremental heating (Leshin et al., 1996). Therefore, both EET A79001 and Nakhla may have been compromised by water-borne terrestrial contaminants. However, water-borne contamination is more likely to involve low, rather than the high molecular weight organic entities detected by Py–GC–MS. Furthermore, EET A79001 and Nakhla seem to contain structurally similar high molecular weight organic matter despite being collected from fall sites in very diLerent environments. Presumably, Antarctic and non-Antarctic environments should be characterised by diLerent types of terrestrial contamination. Therefore, although terrestrial contamination cannot be ruled out, the data is also consistent with the organic matter being present in these meteorites prior to their fall to Earth. 3.4. Carbon isotopes The proposal that Nakhla contains a preterrestrial high molecular weight organic phase is consistent with the 13 C and 14 C work of Jull et al. (2000) which indicated that Nakhla may contain an acid-insoluble organic component, not added by recent terrestrial contamination. It is likely that the pyrolysis products from Nakhla correlate with the components released from an acid-insoluble residue of this meteorite by stepped combustion at 200 –400◦ C (Jull et al., 2000). The carbon isotopic compositions of individual molecules can provide clues as to the source of the organic entities. For example, the Py–GC–IRMS analysis of Nakhla produces values that can be compared with those for their structurally identical counterparts from the high molecular weight organic matter in the Murchison carbonaceous chondrite (Table 2). It appears that individual aromatic molecules released by the pyrolysis of Nakhla have carbon isotopic compositions within the range of values observed for aromatic units in carbonaceous chondrites (Sephton et al., 1998, 2000). The isotopic compositions of individual molecules from Murchison and Nakhla, however, diLer by up to 5:5( but this is no greater than the diLerence in isotopic composition observed between structurally identical molecules from the high molecular weight organic matter in diLerent carbonaceous chondrites (Sephton et al., 2000). Hence, the isotopic data reveals that it is possible that the organic matter in Nakhla is the result of meteoritic infall on Mars.
M.A. Sephton et al. / Planetary and Space Science 50 (2002) 711 – 716 Table 2 Carbon isotopic compositions of individual molecules released from high molecular weight organic matter in the Nakhla meteorite by pyrolysis
Compound
13 CPDB (
Carbonate Benzene Toluene C2 alkylbenzene Benzonitrile
+11:6 −22.0 −21.6 −22.1 −18.0
Nakhla
()
(
13 CPDB ( ) Murchison
+38 −28.0 −24.6 −17.8 – −21.9 −23.5
Values for structurally identical molecules from Murchison are also listed. The Murchison organic data is from Yuen et al. (1984) and Sephton et al. (1998), except for the benzonitrile value which is from a Py–GC–IRMS analysis of an oL-line, anhydrously pyrolysed HF/HCl residue.
3.5. Future directions Inevitably, studies on martian meteorites are limited by sample availability. However, the data generated in this study illustrate the potential of pyrolysis-based techniques to decipher the complex pre-terrestrial and terrestrial histories of these samples. Future work should involve replicate analyses of diLerent parts of the same meteorites to address the problem of sample heterogeneity. The use of larger samples would permit the identi5cation of minor components in the pyrolysates that are currently overprinted by background interference. Unequivocally constraining the origin of organic matter in martian meteorites will require more compound-speci5c isotopic measurements. The utility of 13 C, 13 N and D values for determining the provenance of individual organic entities in meteorites has been illustrated for the carbonaceous chondrites (Sephton and Gilmour, 2001b). 4. Conclusions Several provisional conclusions can be made using the present data set. At least some parts of certain martian meteorites appear to contain a high molecular weight organic component. Upon pyrolysis, this organic phase produces aromatic and alkylaromatic hydrocarbons, phenol and benzonitrile and therefore displays a super5cial similarity to the high molecular weight organic matter in carbonaceous chondrites. The carbon isotopic compositions of individual molecules within the Nakhla pyrolysate also show some similarities to those of their counterparts from carbonaceous chondrites. As the pyrolysate generated from the martian meteorite Nakhla has been detected twice, on two diLerent analytical systems, it appears unlikely that the pyrolysis products are experimental artefacts. The detection of this high molecular weight organic matter in both an Antarctic 5nd and a non-Antarctic fall appears consistent with suggestions that martian meteorites contain organic matter with
715
a preterrestrial origin, possibly delivered to the martian surface by meteoritic infall. Acknowledgements This work was supported by PPARC. We thank J. Gibson for help with sample preparation. M. Dekker, P. Slootweg and W. Pool are gratefully acknowledged for analytical assistance. AMWG provided the samples. We are grateful to O. Botta and C. Koeberl for their constructive reviews. This is NIOZ contribution number 3364. References Bada, J.L., Glavin, D.P., McDonald, G.D., Becker, L., 1998. A search for endogenous amino acids in martian meteorite ALH84001. Science 279, 362–365. Bandurski, E.L., Nagy, B., 1976. The polymer-like organic material in the Orgueil meteorite. Geochim. Cosmochim. Acta 40, 1397–1406. Becker, L., Glavin, D.P., Bada, J.L., 1997. Polycyclic aromatic hydrocarbons (PAHs) in Antarctic Martian meteorites, carbonaceous chondrites, and polar ice. Geochim. Cosmochim. Acta 61, 475–481. Becker, L., Popp, B., Rust, T., Bada, J.L., 1999. The origin of organic matter in the Martian meteorite ALH84001. Earth Planet. Sci. Lett. 167, 71–79. Clemmett, S.J, et al., 1998. Evidence for the extraterrestrial origin of polycyclic aromatic hydrocarbons (PAHs) in the martian meteorite ALH 84001. Faraday Discuss. 109, 417–436. Cronin, J.R., Pizzarello, S., 1990. Aliphatic hydrocarbons of the Murchison meteorite. Geochim. Cosmochim. Acta 54, 2859–2868. De Leeuw, J.W., Largeau, C., 1993. A review of macromolecular organic compounds that comprise living organisms and their role in kerogen, coal and petroleum formation. In: Engel, M.H., Macko, S.A. (Eds.), Organic Geochemistry. Plenum Press, New York, pp. 23–72. Glavin, D.P., Bada, J.L., Brinton, K.L.F., McDonald, G.D., 1999. Amino acids in the martian meteorite Nakhla. Proc. Natural Acad. Sci. USA 96, 8835–8838. Grady, M.M., Wright, I.P., Douglas, C., Pillinger, C.T., 1994. Carbon and nitrogen in ALH 84001. Meteoritics 29, 469. Hartgers, W.A., Sinninghe DamstOe, J.S., de Leeuw, J.W., 1994. Geochemical signi5cance of alkylbenzene distribution in :ash pyrolysates of kerogens, coals and asphaltenes. Geochim. Cosmochim. Acta 58, 1759–1775. Jull, A.J.T., Courtney, C., JeLrey, D.A., Beck, J.W., 1998. Isotopic evidence for a terrestrial source of organic compounds found in martian meteorites Allan Hills 84001 and Elephant Moraine 79001. Science 279, 366–369. Jull, A.J.T., Beck, J.W., Burr, G.S., 2000. Isotopic evidence for extraterrestrial organic matter in the martian meteorite Nakhla. Geochim. Cosmochim. Acta 64, 3763–3772. Leshin, L.A., Epstein, S., Stolper, M., 1996. Hydrogen isotope geochemistry of SNC meteorites. Geochim. Cosmochim. Acta 60, 2635–2650. McKay, D.S, et al., 1996. Search for past life on Mars: possible relic biogenic activity in Martian meteorite ALH84001. Science 273, 924–930. McSween, H.Y.J., 1994. What we have learned about Mars from SNC meteorites. Meteoritics 29, 757–779. Sephton, M.A., Gilmour, I., 2001a. Pyrolysis–gas chromatography–isotope ratio-mass spectrometry of macromolecular material in meteorites. Planet. Space Sci. 49, 465–471.
716
M.A. Sephton et al. / Planetary and Space Science 50 (2002) 711 – 716
Sephton, M.A., Gilmour, I., 2001b. Compound speci5c isotope analysis of the organic constituents in carbonaceous chondrites. Mass Spectrom. Rev. 20, 111–120. Sephton, M.A., Pillinger, C.T., Gilmour, I., 1998. 13 C of free and macromolecular aromatic structures in the Murchison meteorite. Geochim. Cosmochim. Acta 62, 1821–1828. Sephton, M.A., Pillinger, C.T., Gilmour, I., 2000. Aromatic moieties in meteoritic macromolecular materials: analyses by hydrous pyrolysis and 13 C of individual compounds. Geochim. Cosmochim. Acta 64, 321–328.
Sephton, M.A., Pillinger, C.T., Gilmour, I., 2001. Normal alkanes in meteorites: molecular 13 C values indicate an origin by terrestrial contamination. Precambrian Res. 106, 45–56. Wright, I.P., Grady, M.M., Pillinger, C.T., 1989. Organic materials in a martian meteorite. Nature 340, 220–222. Yuen, G., Blair, N., DesMarias, D.J., Chang, S., 1984. Carbon isotope composition of low molecular weight hydrocarbons and monocarboxylic acids form the Murchison meteorite. Nature 307, 252–254.