An evolutionary connection between interstellar ices and IDPs? Clues from mass spectroscopy measurements of laboratory simulations
Introduction
Comparison of infrared (IR) telescopic observations of cold interstellar environments with low temperature laboratory ices suggest that interstellar and protostellar ices are often composed of H2O mixed with 5–15% CO, CO2, CH3OH, and NH3 (Ehrenfreund and Charnley, 2000 and references therein). In most environments where such mixed-molecular ices are observed, radiation is also expected. For example, interstellar ices should be exposed to significantly enhanced UV fields in star-forming regions, and ices near young stars in their T-Tauri phase should be exposed to the high energy particle fluxes and UV radiation (Giampapa and Imhoff, 1985). Even in dense molecular clouds with high visual extinction, radiative processing of these ices is expected from cosmic rays and cosmic ray-induced UV (Prasad and Tarafdar, 1983). Thus, astronomical mixed-molecular ices are predicted to experience energetic processing. Laboratory studies have shown that processing of relevant ice analogs in the laboratory by UV photolysis (Bernstein et al., 1995) and ion irradiation (Moore et al., 1996) produces organic compounds more complex than the simple starting materials.
Some of our past studies have shown that there are similarities between the organic compounds produced by energetic processing of ices and the molecules found in primitive meteorites (Bernstein et al., 2001, Bernstein et al., 2002, Bernstein et al., 2003). Of special relevance to this report is that the UV photolysis of an interstellar/protostellar/cometary ice analog with a composition of H2O:CH3OH:NH3:CO=100:50:1:1 yields both fluorescent and amphiphilic compounds that can spontaneously self-assemble into vesicles (Dworkin et al., 2001). These vesicles have very similar morphologies to those produced by amphiphiles found in primitive meteorites like the Murchison carbonaceous chondrite (Deamer and Pashley, 1988) and such structures may have been preserved in the Tagish Lake meteorite (Nakamura et al., 2002). High precision liquid chromatographic (HPLC) analysis of the laboratory material reported (Dworkin et al., 2001) showed dozens of unidentified peaks indicating a diverse population of fluorescent and UV-absorbing compounds. In this paper we present mass spectra of the ice analog derived organic mixture that formed vesicles before and after simulated atmospheric entry heating, and compare these to the mass spectra of two IDPs. Together, this suggests that the amphiphiles in meteorites and the organics in IDPs may have derived from radiative processing of interstellar or protostellar ices at very low temperatures. An interstellar origin of the organic matter in meteorites and interplanetary dust particles (IDPs) is consistent with the presence of deuterium enrichments, suggestive of low temperature processes (Sandford et al., 2001). Additionally, the mass spectra of the unheated residues indicate the presence of hundreds, rather than dozens, of compounds extending the richness of the organic material formed through these simulations.
Section snippets
Methods
The preparation techniques for the simulated interstellar ice photolysis residue samples are described in detail elsewhere (Allamandola et al., 1988; Dworkin et al., 2001). The starting gas-mixture (H2O:CH3OH:NH3:CO=100:50:1:1) roughly reflects the composition and concentrations of the major ice components observed in interstellar dense molecular clouds along the line of sight to high-mass protostars (d’Hendecourt et al., 1999). Different concentrations of the same mixture (all H2O-rich) have
Sample analysis
Fig. 1 shows the μL2MS of the non-volatile organic residue synthesized by the UV photolysis of a H2O:CH3OH:NH3:CO=100:50:1:1 ice mixture at 15 K in comparison to control experiments. It is seen that high molecular weight compounds are formed by the UV photolysis of simple ices at low temperature. The overall shape of the mass distribution is always found to be very similar for a given set of conditions. All UV photolyzed ice samples produce mass spectra having a distribution of peaks (one at
Conclusions and implications
We have presented two-step laser desorption mass spectra of the organic mixture produced in laboratory experiments designed to reproduce the photochemistry of realistic interstellar/protostellar/cometary ices both before and after heating designed to simulate atmospheric entry. This study represents a further molecular characterization of the same material for which fluorescent and amphiphilic properties had already been reported (Dworkin et al., 2001). We find that our laboratory organic
Acknowledgments
We very gratefully acknowledge Bob Walker for his excellent technical support, without whom these long duration experiments would not have been possible. This work was supported by NASA’s Exobiology (Grant 344-38-12-04), Astrobiology (Grant 344-50-92-02), and Origins of Solar Systems (Grant 344-37-44-01) Programs.
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