Laboratory studies of radiation effects in water ice in the outer solar system
Introduction
The most common occurrence of radiolysis of water is in space, where low-temperature water ice—in objects ranging from interstellar dust to comets and icy satellites—is subject to irradiation by high-energy ultraviolet light and atomic particles. Most of what we know about the surfaces of those icy bodies derives from studies of the reflected solar light (reflectance spectra). The sampled depth (optical skin) varies between a few microns or less in the mid-infrared and vacuum ultraviolet to centimeter in the visible due to scattering from mixed minerals and impurities. The impacts by energetic particles in this layer alter the reflectance due to radiolysis, which includes the creation of new chemical species, the ejection of molecules and atoms from the surface (desorption or sputtering), and the generation of lattice defects (Johnson, 1998; Baragiola, 2003b). The temperatures of interest are from 30 to 160 K; in this range, the growth temperature of the ice is the main factor determining its crystallographic phase (Sceats and Rice, 1982; Baragiola, 2003a). Condensation below 190 K but above ∼135 K, leads to cubic crystalline ice (Ic), which transforms into hexagonal ice above 160−200 K. Icy deposits grown on substrates colder than ∼130 K are amorphous; this phase (Ia) is metastable and converts to Ic at a rate that depends on temperature. Crystallization of Ia is observed above 125 K in laboratory time scales.
A crucial characteristic of Ia grown from the vapor phase is its microporosity, which may affect radiolysis because radiation products may be trapped in pores, and because porous ice in space will absorb other gases that may affect the radiolytic processes. For growth below 60 K, the micropores in the bulk of the ice are connected to the surface; above about 60 K, there is no connection of the micropores to the surface and gas adsorption capacity drops dramatically (Baragiola, 2003a).
Section snippets
Radiolysis and its consequences
Many molecular mechanisms have been proposed to explain radiolysis of ice based on detailed information from studies in the gas and liquid phases (Ferradini and Jay-Gerin, 1999). A main aspect in the radiolysis of ice is an enhanced cage effect due to the rigidity of the lattice, which results in lower radiation yields (Spinks and Woods, 1990; Johnson and Quickenden, 1997). Understanding is difficult because possible reactions involve a large number of species such as electrons, and H, OH, O, H2
Radiation-induced amorphization of crystalline ice
Electron diffraction has been used to show that crystalline ice can be rendered amorphous below 70 K by irradiation with high-energy electrons (Dubochet and Lepault, 1984) and 110−400-nm UV light (Kouchi and Kuroda, 1990). Differences in infrared spectra between amorphous and crystalline phases have been used to study amorphization by energetic ions (Strazzulla et al., 1992, Hudson and Moore, 1992) and UV light (Leto and Baratta, 2003). We have used FTIR spectroscopy of the water OH-stretch band
Conclusions
Sputtering, electron-energy-loss spectroscopy and infrared spectroscopy, which are rarely considered in the radiolysis literature, can be used in laboratory to obtain insight on radiation processes in ice and to help the interpretation of astronomical observations of icy surfaces in the outer solar system. In the future, it is envisioned that these techniques will produce a wealth of information from the detailed analysis of the spectra, once it becomes possible to couple the measurements with
Acknowledgements
We acknowledge financial support from the NASA Office of Space Science, the NSF Astronomy Division, and the Cassini CAPS program through JPL contract #1210586. MJL acknowledges support from the Virginia Space Grant Consortium.
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