nai

Project Progress

Our research focuses on chemical reactions and physical properties of icy materials in extraterrestrial environments dominated by low-temperature and low-pressure conditions. Ionizing radiation sources are used to simulate space radiation conditions and to drive reactions. Infrared (IR) spectroscopy is used to monitor the chemistry in situ within a high-vacuum chamber. Our main tools for initiating chemistry are MeV and keV ion sources and UV lamps. With these tools we can investigate a wide variety of interstellar, circumstellar, and solar-system chemistry.

Over the past year we have continued our work on amino acids, moving away from molecular formation in interstellar ices to molecular destruction on planetary bodies. We also have followed up our earlier study of the radiation stability of sulfuric acid hydrates, as relate to Europa, by examining hydrate reformation in irradiated ices. Finally, we have extended our earlier work on reference mid-IR spectra, by addressing the specific case of solid-phase acetylene (C₂H₂), for application to Spitzer data.

Amino Acid Destruction by Radiolysis

In previous papers we have reported on the radiolytic and photolytic destruction of small molecules known to be in the interstellar medium (ISM). Recently, we have turned our attention to similar measurements on amino acids as a preliminary step toward examining even larger species. With NAI support, we have expanded substantially on our previous year’s work by measuring the radiolytic destruction rates of the amino acids glycine, alanine, and phenylalanine at three temperatures, both in the pure solid state and in the presence of water-ice. Our long-term goal is to obtain rate data for the radiolytic destruction of amino acids in various astronomical environments, such as on Mars, Titan, and the ISM. Figure 1, from our recent Icarus (2012) paper, compares the destruction of pure glycine and glycine trapped in H₂O-ice. Surprisingly, such data from in-situ measurements below room temperature are otherwise unavailable. The figure clearly and quantitatively shows that the rate of glycine destruction is smaller when the glycine molecules are surrounded by H₂O-ice. Figure 2 extends this conclusion to other two other amino acids, one aliphatic and one aromatic, at several concentrations and temperatures. The G value plotted is a standard quantity for measuring radiation-induced changes (large G ==> large change). Two conclusions that can be drawn are (1) that an amino acid’s destruction rate is smallest when H₂O molecules are present to absorb part of the incident radiation’s energy and (2) that the destruction rates actually are smaller at higher temperatures. Extensions to more Mars-like conditions are underway in our laboratory.

Radiolysis of Sulfuric-Acid Hydrates

Last year we completed a Europa-related study of the radiation products and destruction rates of sulfuric acid (H₂SO₄), sulfuric acid monohydrate (H₂SO₄ • H₂O), and sulfuric acid tetrahydrate (H₂SO₄ • 4 H₂O). We identified the main radiation products as H₂O, SO₂, (S₂O₃)_x, H₃O⁺, HSO₄^–, and SO₄^2–. This past year we extended our sulfur studies to the post-irradiation recovery (reformation) of these forms of sulfuric acid. Figure 3, taken from our recent Icarus paper, shows that the recovery is almost 100% in this case. Similar results were obtained for H₂SO₄ and H₂SO₄ • H₂O.

The astrobiological significance of this work is that these oxygen-rich species are not irreversibly destroyed by ionizing radiation. As irradiated sulfurous material is transported below Europa’s surface, it slowly will be warmed, reform sulfate ions, and be made available for sub-surface redox chemistry. We now are working to quantify the rates of sulfate-ion formation, especially from thermally-induced reactions between H₂O₂ and SO₂. Extensions of this work to other strong oxidants (e.g., perchlorates, ozone) are planned. This work was done with support from the Goddard Center for Astrobiology as well as NASA’s Planetary Geology & Geophysics program.

Other Work

In the previous year, our paper on the IR optical constants of nitriles related to Titan was published. This past year we used the same procedures and equipment to record an extensive set of spectra for acetylene (C₂H₂) in four different ice matrices, and we are now extending this work to the generation of acetylene’s IR optical constants. Along these lines, in the past year we have measured, for the first time, the refractive index of hydrocarbons in the visible region, a necessary first step for determining their optical constants at IR wavelengths. Such data are needed for quantifying the abundances of organic molecules in the outer solar system, and perhaps elsewhere.