Ultraviolet photolysis of amino acids on the surface of icy Solar System bodies

Abstract

The icy worlds of the outer Solar System are of significant astrobiological interests due, in large part, to the evidence of liquid water beneath the surfaces of a number of jovian and saturnian satellites. Many of these surfaces are subject to various levels of particle and photon radiation. If molecular compounds of biological origin are present in the surface ice layer (originating either in situ or delivered from a subsurface aqueous environment), can they be detected as evidence of biological activity, or do they decompose too rapidly in the surface radiation environment? We present a wavelength resolved study of the ultraviolet photolysis of glycine and phenylalanine to address this question. Studying these reactions at multiple discreet wavelengths distinguishes the present work from previous matrix isolation studies using hydrogen flow lamps and continuum sources by resolving the important contribution of photons with energies much lower than Lyman-α (121.6nm). We find that although the half-lives of glycine and phenylalanine are essentially identical at 147nm, they diverge at 206nm and diverge significantly at 254nm with glycine having longer half-lives at these longer wavelengths. Scaling the results to account for the wavelength dependent variation in solar irradiance shows that despite the reduction of photon energies in the 200–250nm range, versus 147nm, it is the longer wavelengths that will dominate the destruction of amino acids in icy surfaces. It seems unlikely that organics can survive long enough on the surface of an icy planetary body to be detected without being frequently replenished from a shielded source such as a subsurface ocean.