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Project Progress

In the Cosmic Ice Laboratory we simulate the low-pressure, low-temperature environment of space using a high-vacuum chamber and a cryostat. Ice samples are prepared on a pre-cooled mirror inside the cryostat and then exposed to energetic processing. They can be irradiated with 1-MeV protons to simulate cosmic-ray bombardment, photolyzed to simulate vacuum-ultraviolet (UV) exposure, or irradiated with 10-keV electrons to simulate a variety of magnetospheric and circumstellar environments. Our work this past year focused on the radiation chemistries of nitrile and water-nitrile ices, the amino acids valine and isovaline, and on H₂O-rich ices containing O₂. These studies were motivated by IR observations of acetonitrile (CH₃CN) in Titan’s atmosphere, cometary comae, and interstellar regions, by the chemical analysis and identification of isovaline in meteorites, and by UV observations of O₂ in H₂O-rich ices on Europa. In each case, the material we investigated was the same as that found in a specific low-temperature space radiation environment.

A major study to unravel the solid-phase radiation chemistry of nitrile and water-nitrile ices was completed during the past year. Zack Posun (GCA 2006 intern) performed laboratory experiments that showed unequivocally that HCN is a radiation product of CH₃CN, by using isotopically-labeled starting materials, and that the cyano (C∫N) group is radiolytically oxidized in H₂O-ice to form the cyanate ion (OCN) which in turn is converted into CO₂ by further oxidation. Spectra of isotopically-labeled ketenimine, H2C2NH, also were obtained, supporting this molecule’s earlier identification in Hudson and Moore (Icarus 2004, 172, 466) and our prediction that it is an interstellar species. Ketenimine’s recent discovery in the star-forming region Sagittarius B2(N) by Lovas et al. (Ap. J. 2006, 645, L137) attests to the predictive power of this experimental approach. Building on our previous work, we have continued to examine organic refractory residues from irradiated nitriles. Analyses done in conjunction with Dr. Jason Dworkin (GCA) revealed a suite of amino acid precursors. Figure 1 is taken from a manuscript describing these experiments, which has been submitted for publication.
A study of the formation, destruction, racemization, and thermal stability of the amino acids isovaline and valine has been initiated. Isovaline is of astrobiological interest since its enantiomers were found in CM carbonaceous chondritic meteorites (e.g., Murchison and Murray) in apparently equal ratio (racemic form) and in higher abundance than glycine (the simplest amino acid). Meteoritic isovaline, which is not biologically-active, is thought to have an abiotic origin. Initial proton irradiations of D- and L-valine at 10 K (Figure 2) showed that about half of the target molecules were destroyed with a dose of ~1 eV molecule_1, corresponding to a half-life of ~3 _ 10⁶ years in dense-cloud environments. Chemical analyses of the post-radiation residues showed that racemization had occurred, but not isomerization. Ariel Lewis (GCA 2007 intern) helped with our continuing study of the low-temperature radiolytic syntheses of valine and isovaline from iso- and sec-butylamine in the presence of CO₂.

In an exciting new result, Paul Cooper (NPP Fellow) obtained direct IR evidence for O-atom production during irradiation of frozen H₂O-rich mixtures containing O₂. Cooper identified 16O-containing ozone isotopologues in irradiated H₂¹⁶O + ¹⁸O₂ ices. The ozone variants found are ⁶⁶⁶O₃, ⁶⁶⁸O₃, ⁶⁸⁶O₃, ⁸⁸⁶O₃, and ⁸⁸⁸O₃ (⁶⁶⁶O₃ is ¹⁶O¹⁶O¹⁶O, ₆₆₈O₃ is ¹⁶O¹⁶O¹⁸O, etc.). Figure 3 shows the growth of the relevant absorptions in the 10-mm (wavelength) region as a function of radiation dose. These observations support the idea that O₂ detected on Europa’s surface, for example, is produced by the radiation-induced chemical decomposition of H₂O into H₂ molecules and O atoms. Photolytic production of O from gas-phase H2O is well known, but the corresponding process had not been identified in condensed-phase water, until now.

New proposal funded for astrobiology-motivated research

Other activities, including outreach, during the past year

Reviews

These are an indication of our continued involvement in a variety of astrobiologically-related efforts and of our standing in the professional community.

Reviewed fifteen manuscripts for publication (Astrobiology, Origin of Life and the Evolution of the Biosphere, Astrophysical Journal, Astronomy & Astrophysics, Icarus, Journal of Physical Chemistry, Chemical Physics, Chemical Physics Letters, Journal of Chemical Education, and American Mineralogist)

On-site reviewer for research proposals submitted to a NASA panel

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