Radiation-Induced Processing of Hydrocarbons in Environments Relevant to Pluto

An understanding of the formation of the larger molecules in the outer solar system, by radiation induced processing of more primitive constituents, has implications relating to the evolution of the solar system. Organic residues formed by cosmic ray irradiation on cosmic ices may also have some exobiological significance, directly relating to the process from which life began on Earth. Since Pluto is one of the most primitive, and well-preserved, bodies in the solar system, its surface chemistry is of particular relevance. This project was an attempt to correlate recent astronomical data with a radiation model for compound formation. Spectroscopic observations of Pluto suggest the presence of ethane, presumably caused by cosmic ray irradiation of methane trapped in solid nitrogen at the surface. This project used near-infrared (NIR) spectroscopy to examine this process in laboratory analogs. Samples of nitrogen doped to appropriate low concentrations with methane and for carbon monoxide were deposited at 50 K or below, and the resulting films irradiated with varied doses of 1 MeV protons. The formation of ethane, and other products, was observed at radiation dosages consistent with levels experienced at the planet=s surface. However, neither the intensity nor bandwidths of the spectroscopic signals compared very well with the NIR telescopic data. These facts lend some support to a model suggesting two different terrains on the planet, one with low methane concentration, and another with much higher levels. Irradiation of the latter might account for the ethane signals observable from Earth. The laboratory irradiation experiments also resulted in the formation of a residue, stable at high temperatures, consistent with models of organic polymer formation on icy bodies in the outer solar system. Some attempts at determining the composition of the residue were also performed. The original document contains color images.