INFRARED-SPECTROSCOPY OF TRITON AND PLUTO ICE ANALOGS - THE CASE FOR SATURATED-HYDROCARBONS

The infrared transmission spectra and photochemical behavior of variou s organic compounds isolated in solid N2 ices, appropriate for applica tions to Triton and Pluto, are presented. It is shown that excess abso rption in the surface spectra of Triton and Pluto, i.e., absorption no t explained by present models incorporating molecules already identifi ed on these bodies (N2, CH4, CO, and CO2), that starts near 4450 cm-1 (2.25 mum) and extends to lower frequencies, may be due to alkanes (C( n)H2n+2) and related molecules frozen in the nitrogen. Branched and li near alkanes may be responsible. Experiments in which the photochemist ry of N2:CH4 and N2:CH4:CO ices was explored demonstrate that the surf ace ices of Triton and Pluto may contain a wide variety of additional species containing H, C, O, and N. Of these, the reactive molecule dia zomethane, CH2N2, is particularly important since it may be largely re sponsible for the synthesis of larger alkanes from CH4 and other small alkanes. Diazomethane would also be expected to drive chemical reacti ons involving organics in the surface ices of Triton and Pluto toward saturation, i.e., to reduce multiple CC bonds. The positions and intri nsic strengths (A values) of many of the infrared absorption bands of N2 matrix-isolated molecules of relevance to Triton and Pluto have als o been determined. These can be used to aid in their search and to pla ce constraints on their abundances. For example, using these A values the abundance ratios CH4/N2 almost-equal-to 1.3 x 10(-3), C2H4/N2 less -than-or-equal-to 9.5 x 10(-7), and H2CO/N2 less-than-or-equal-to 7.8 x 10(-7) are deduced for Triton and CH4/N2 almost-equal-to 3.1 x 10(-3 ), C2H4/N2 less-than-or-equal-to 4.1 x 10(-6), and H2CO/N2 less-than-o r-equal-to 5.2 x 10(-6) deduced for Pluto. The small amounts of C2H4 a nd H2CO in the surface ices of these bodies are in disagreement with t he large abundances expected from many theoretical models.