Optical Constants of Cometary Ice Tholins

B. N. Khare, D. Cruikshank (NASA Ames), E. T. Arakawa (ORNL), G. D. McDonald (Cornell)

We have previously reported the optical constants from 0.06 to 40 tex2html_wrap_inline11 m of ice tholin I, produced by plasma irradiation of 1:6 ethane:water frost at 77K (Khare et al. 1993, Icarus 103, 290). Such ices containing water and small amounts of simple hydrocarbons either in the form of clathrates or intimate physical mixtures are thought to be common outer solar system condensates that were incorporated into comets and other bodies. For the imaginary part of the refractive index (k)< 0.001, an accurate measurement of k for solid films requires substantially thick optical quality films on calcium fluoride and cesium iodide substrates that are difficult to produce. We have remeasured the transmission through these films and find that the k values agree well with previously measured values for the entire wavelength region, except from 0.5 to 4 tex2html_wrap_inline11 m where k is small. The real part of the refractive index n is not affected by these changes since the k values are small.

Here we present our revised optical constants for ice tholin I, and compare with optical constants of ice tholin II, produced by plasma irradiation of ices made from a 80:16:3.2:0.8 water:methanol:carbon dioxide:ethane gas mixture (McDonald et al. 1996, Icarus 122, 107), a first order approximation to known and inferred cometary compositions. These additional data on the optical constants of ice tholin II will provide another choice for comparison with spectra of comets and other icy solar system bodies.