Experimental study of erosion of methane ice by energetic ions and some considerations for astrophysics.

The rate of erosion and polymerization of methane ice under bombardment by hydrogen and helium ions is studied in the laboratory as a function of ion energy and ice film thickness. For the thinnest films studied (few hundred Angstroms), erosion of the entire film is accomplished with MeV-energy helium ions. Loss of both carbon and hydrogen is observed from thicker films, with the hydrogen to carbon content of the films decreasing with increasing irradiation exposures. In general, the initial alterations of the films, with hydrogen and carbon losses, are rapid. The effective "cross section" for initially producing hydrogen loss is ~5 x 10(-16)cm(2)). After a total incident dose of >~ 10(15cm(-2) of MeV energy He ions, the loss rates decrease significantly and the effective cross section is ~10 (-17)cm(2)). Thus, the state of ultimate polymerization of a pure methane film in an astrophysical particle environment will depend critically on the thickness of the ice layer as well as on the ion composition of the radiation environment.