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Project Progress

• NAI-GCA support in 2015 helped us continue our work on amino-acid stability. In 2015, we completed our first set of radiation experiments to measure the destruction rate of glycine in CO₂ ice. Our results showed that glycine at a subsurface depth of 5-10 meters embedded in CO₂-ice on Mars would not survive more than 100 million years, but could survive for 2-4 Gyr if stored in H₂O-ice. This work continued activities from 2014 and was recently published in Icarus (Gerakines, P. A., & Hudson, R. L. (2015). The radiation stability of glycine in solid CO₂ – In situ laboratory measurements with applications to Mars. Icarus, 252, 466–472. doi:10.1016/j.icarus.2015.02.008).

• We built on last year’s study of IR band strengths of ices made of organic molecules, as part of our interest in the carbon inventory of the outer Solar System and the interstellar medium. To extend this work we examined CH₄ the simplest hydrocarbon, looking at each of its three solid phases. We discovered that all of the previous low-temperature publications on solid CH₄ were for the compound’s crystalline phases and not the amorphous one that was usually claimed and which is more astronomically relevant. Reassessments of results from many earlier laboratory and observational papers are needed in light of our new work. Figure 1 shows spectral markers that are now known to distinguish amorphous and crystalline CH₄ (Gerakines, P. A., & Hudson, R. L. (2015). Infrared spectra and optical constants of elusive amorphous methane. The Astrophysical Journal, 805(2), L20. doi:10.1088/2041-8205/805/2/l20).

• Building on our CH₄ study, we turned to the other molecular extreme of carbon oxidation, CO₂, finding that the situation regarding IR intensities and spectra was even worse than with CH₄. Again, what had been assumed by astrochemists for about 30 years to be IR spectra of amorphous CO₂ was actually for the crystalline solid. We succeeded in producing the amorphous material and publishing the results, the first such work available for CO₂ (Gerakines, P. A., & Hudson, R. L. (2015). First infrared band strengths for amorphous CO₂, an overlooked component of interstellar ices. The Astrophysical Journal, 808(2), L40. doi:10.1088/2041-8205/808/2/l40).

• In collaboration with scientists in NASA Goddard’s Astrobiology Analytical Laboratory we completed a new project concerning heterocycle chemistry in meteorites. Our first molecular targets were nicotinic acid and its isomers. An initial paper was published in 2014, followed by a more-detailed one in this reporting year (Smith, K. E., Gerakines, P. A., & Callahan, M. P. (2015). Metabolic precursors in astrophysical ice analogs: implications for meteorites and comets. Chem. Commun., 51(59), 11787–11790. doi:10.1039/c5cc03272e). In that paper we demonstrated that cosmic radiation acting on ices containing CO₂ and pyridine, two known meteoritic molecules, would produce quinolinic and nicotinic acids, molecules that participate in the nicotinamide adenine dinucleotide (NAD) biosynthetic pathway. Figure 2 shows chromatograms of radiation-produced nicotinic acid (vitamin D) and two of its heterocyclic isomers.