Meteoritics & Planetary Science 43, Nr 1/2, 399–413 (2008) AUTHOR’S PROOF Abstract available online at http://meteoritics.org Detection of cometary amines in samples returned by Stardust Daniel P. GLAVIN1*, Jason P. DWORKIN1, and Scott A. SANDFORD2 1NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA 2NASA Ames Research Center, Moffett Field, California 94035, USA *Corresponding author. E-mail: [email protected] (Submitted 05 December 2006; revision accepted 01 February 2007) Abstract–The abundances of amino acids and amines, as well as their enantiomeric compositions, were measured in samples of Stardust comet-exposed aerogel and foil using liquid chromatography with UV fluorescence detection and time of flight mass spectrometry (LC-FD/ToF-MS). A suite of amino acids and amines including glycine, L-alanine, β-alanine (BALA), γ-amino-n-butyric acid (GABA), ε-amino-n-caproic acid (EACA), ethanolamine (MEA), methylamine (MA), and ethylamine (EA) were identified in acid-hydrolyzed, hot-water extracts of these Stardust materials above background levels. With the exception of MA and EA, all other primary amines detected in comet- exposed aerogel fragments C2054,4 and C2086,1 were also present in the flight aerogel witness tile that was not exposed to the comet, indicating that most amines are terrestrial in origin. The enhanced relative abundances of MA and EA in comet-exposed aerogel compared to controls, coupled with MA to EA ratios (C2054,4: 1.0 ± 0.2; C2086,1: 1.8 ± 0.2) that are distinct from preflight aerogels (E243- 13C and E243-13F: 7 ± 3), suggest that these volatile amines were captured from comet Wild 2. MA and EA were present predominantly in an acid-hydrolyzable bound form in the aerogel, rather than as free primary amines, which is consistent with laboratory analyses of cometary ice analog materials. It is possible that Wild 2 MA and EA were formed on energetically processed icy grains containing ammonia and approximately equal abundances of methane and ethane. The presence of cometary amines in Stardust material supports the hypothesis that comets were an important source of prebiotic organic carbon and nitrogen on the early Earth. INTRODUCTION et al. 1990; Chyba and Sagan 1992; Huebner and Boice 1992; Oró et al. 1992). The organic composition of these small solar After a seven-year mission, the Stardust spacecraft system bodies depends on the original composition, returned to Earth the first samples from a comet on January 15, processing, and mixing of the molecular cloud (dust, ice, and 2006. Over the next six months, the Stardust Preliminary gas) and solar nebula from which our solar system formed. It Examination Team (PET) analyzed the samples using a is currently believed that comets represent a mixture of variety of laboratory techniques to understand impact features, interstellar material that has been moderately to heavily mineralogy and petrology, elemental abundances, isotopic processed in the solar nebula (Ehrenfreund and Schutte 2000; distribution, and organic composition of the returned samples Irvine et al. 2000). To date, over 140 molecular species have (Brownlee et al. 2006). The organics found in the samples now been identified in the gas phase by their rotational were discussed in more detail in Sandford et al. (2006). Results spectrum in the interstellar medium (Lis et al. 2006). In from the Organics PET analyses of amino acids and amines in addition, over 20 organic species such as methane (CH4), material returned by Stardust will be discussed here. ethane (C2H6), ammonia (NH3), cyanic acid (HCN), formic Comets represent some of the most primitive material in acid (HCOOH), formaldehyde (H2CO), formamide our solar system and they were likely a major contributor of (HCONH2), acetaldehyde (CH3CHO), acetonitrile (CH3CN), the intense heavy bombardment that occurred on the early and methanol (CH3OH), have been identified by radio Earth (Chyba 1990). It is also thought that the delivery of spectroscopic observations of the comets Hale-Bopp and water and organic matter by comets and their fragments could Hyakutake (Crovisier et al. 2004; Crovisier and Bockelée- have been a significant source of the early Earth’s prebiotic Morvan 1999). These simple molecules would have provided organic inventory that led to the emergence of life (Chyba the organic reservoir to allow the formation of more complex 399 © The Meteoritical Society, 2008. Printed in USA. 400 D. P. Glavin et al. organic compounds in comets such as amino acids and amine Analyzer (CIDA) time of flight mass spectrometer instrument compounds (Allamandola 1998; Bernstein et al. 2002; on the Stardust spacecraft also detected a nitrogen-rich Muñoz-Caro et al. 2002). Glycine (NH2CH2COOH), the organic chemistry with large amounts of cyanide and lower simplest amino acid, has been reported in interstellar clouds abundances of oxygen in 29 separate spectra obtained from (Kuan et al. 2003), although this tentative detection remains comet 81P/Wild 2 dust impacting the target surface (Kissel et controversial (Snyder et al. 2005). Glycine was not identified al. 2004). It is possible that the volatile nitrogen-rich organic by radio spectroscopic measurements of the comets C/1995 component detected by CIDA was captured by the Stardust O1 (Hale-Bopp) and C/1996 B2 (Hyakutake) above detection comet-exposed aerogel during comet Wild 2 flyby. limits (Crovisier and Bockelée-Morvan 1999; Crovisier et al. Although CIDA did not detect any free amino acids, many 2004). Other simple amines such as methylamine (CH3NH2) of the oxygen-poor, nitrogen-rich precursor polymers that and ethylamine (CH3CH2NH2) have also not been detected in have been identified in some Wild 2 dust particles (Kissel these comets (Crovisier and Bockelée-Morvan 1999; et al. 2004; Sandford et al. 2006) may hydrolyze in acid to Crovisier et al. 2004). Only methylamine has been identified form more complex organic compounds. in the interstellar medium (Ehrenfreund and Charnley 2000). The PET analyses of organics in samples returned by Theoretical models of the coma of Hale-Bopp find that the Stardust were largely focused on particles that impacted the observed abundances of some organic molecules including aerogel and aluminum foil (Sandford et al. 2006). However, it HCN and CH3CN cannot be produced by gas phase chemistry, is also possible that Stardust returned a “diffuse” sample of so these species were probably present in the nuclear ice gas-phase organic molecules that struck the aerogel directly (Rodgers and Charnley 2001). Indeed, complex organic or diffused away from the grains after impact. To test this compounds can be formed in laboratory-simulated interstellar/ possibility, we investigated the abundances of primary amine cometary ices by UV or proton irradiation of simple ice compounds, as well as their enantiomeric composition, in mixtures that have been observed in the interstellar medium Stardust comet-exposed flight aerogel and foil using a highly and in comets (Allamandola et al. 1988; Moore and Hudson sensitive liquid chromatography with simultaneous UV 1998; Bernstein et al. 1995; Allamandola and Hudgins 2003). fluorescence detection and time of flight–mass spectrometry Analyses of the ice residues after irradiation using a variety of (LC-FD/ToF-MS) technique (Glavin et al. 2006). Here we analytical techniques demonstrate that these materials contain a report for the first time highly sensitive quantitative much more complex suite of volatile species than originally measurements of amino acids and amines in samples returned present in the ice as well as a refractory organic component. from a comet. In addition, hexamethylenetratramine (HMT, C6H12N4) was observed to be one of the major products produced by MATERIALS AND METHODS irradiation and warming of various astrophysically relevant ice mixtures (Bernstein et al. 1995; Muñoz-Caro and Schutte Chemicals and Reagents 2003; Muñoz-Caro et al. 2004). HMT will produce a variety of amino acids upon acid hydrolysis (Wolman et al. 1971) All glassware and sample handling tools were rinsed with and analyses of irradiated ice residues after acid hydrolysis Millipore water, wrapped in aluminum foil, and then heated in demonstrate that they do contain a rich mixture of amino acids air at 500 °C overnight. All of the chemicals used in this study not present in the original ice mixture (Bernstein et al. 2002; were purchased from Sigma-Aldrich and Fisher Scientific. A Muñoz-Caro et al. 2002; Elsila et al. 2007; R. Hudson, personal stock amino acid and amine solution (~10−5 M) was prepared communication). Therefore, it is possible that amino acids and/ by mixing individual standards (97–99% purity) in Millipore or their precursor materials (e.g., HMT) are present on (18.2 MΩ) water. The o-phthaldialdehyde/N-acetyl-L-cysteine interstellar dust or cometary grains that were subjected to (OPA/NAC) reagent used as a chemical tag for enantiomeric similar sources of radiation. separation and fluorescence detection of primary amine In situ measurements of the coma of comet 1P/Halley compounds was prepared by dissolving 4 mg OPA in 300 μL by the Giotto and Vega 1 and 2 probes’ mass spectrometers methanol (Fisher Optima), and then adding 685 μL 0.1 M (which measured the composition of gas and dust grains sodium borate buffer (pH 9) and 15 μL 1 M NAC. The sodium released by the nucleus) suggested the possible presence of tetraborate decahydrate (Na2B4O7⋅10H2O) powder a variety of organic species including aliphatic hydrocarbons, (SigmaUltra, 99.5–100% purity) used to prepare the sodium amines, nitriles, imines, and heterocyclic aromatic borate buffer was heated in air at 500 °C for 3 h to reduce amine compounds (Kissel and Krueger 1987). In addition, their contamination in the reagent. A 0.1 M hydrazine (NH2NH2) spectra also contained evidence for polymeric structures of solution used to remove excess OPA after derivatization was H2CO and HCN (Huebner 1987; Huebner et al. 1989; prepared by double vacuum distillation of anhydrous hydrazine Rodgers and Charnley 2001).