UV Photolysis of Amino Acids in Water Ice: How Long Can They Survive on Europa?

We report the rate of decomposition by ultraviolet photolysis of 4 simple amino acids in a ~mm-thick crystalline water ice matrix at T=100K to constrain the surviv-ability of these important organic molecules within ice lying near the surfaces of outer solar system bodies. As a specific example, we focus on Europa where subsurface liquid water or " warm ice " may be extruded onto the surface from a liquid water reservoir at depth during resurfacing events associated with the globally distributed linea and " bands " , long linear " cracks " in a high albedo ice surface, e.g. [1, 2, 3, 4], or during activity in the " chaotic terrain " [5]. Europa's low density of impact craters indicates extensive resurfacing [6]. As an analog to some planetary ices, we freeze our ice samples from liquid solution which results in mm-thick samples of crystalline phase hexagonal ice that appears " white " due to multiple scattering from internal mi-crostructure. After irradiating an ice and amino acid mixture with a Argon mini-arc UV continuum light source [7], we used a derivatiza-tion technique based on a fluorescence reaction of amino acids, developed by Roth [8] and modified by Zhao [9], to directly measure the remaining fraction of amino acid. We measured ice samples with 0.14, 0.28 and 1.6 mm thickness, prepared from ~10-4 M solutions of glycine, D,L-aspartic, D,L-glutamic, and D,L-phenylalanine irradiated from 10 to 1020 minutes. We also measured UV transmission for phenylalanine ice samples in the 230-270 nm wavelength region to investigate the effects of irradiation and incorporation into a 100K ice matrix on the strong absorption bands of an aromatic amino acid. We find that the half-life for decomposition of the amino acid-ice samples is linearly proportional to their thickness as is expected for a layer with strong multiple scattering. Glycine is the most resistant to destruction and phenylalanine is the most easily destroyed. For the 1.6 mm thick samples under lab conditions, the half-life of glycine was 57 hours, aspartic 21 hours, glutamic 23 hours, and phenylalanine 8 hours. For similar ices under solar irradiation at Europa, these results can be expressed as a " penetration velocity " , the depth to which half of the amino acids are destroyed in a year. Depending on which amino acid and the portion of the UV spectrum that is responsible for the destruction , we conclude that half of …