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           Ion irradiated CH₄ ice

Outer solar system bodies show a great variety of spectral colors. This can be explained by both compositional and space weathering effects, in competition with rejuvenating processes, such as impacts or internal activity.

The space weathering processes induce color variations that can reproduce the observed spectral variety of Centaurs and TNOs (Trans-Neptunian Objects). The hydrocarbon refractories, in particular, are generally characterized by low albedo and red spectral slope. These components are thought to be present on the majority of outer solar system bodies.

We estimated optical constants of ion irradiated methane ice (ICH4) at 16 K, in the 0.69-2.55 �m spectral range. The icy film was irradiated with H+ and Ar+ ions (energy of 200 keV) to a dose of 280 +/- 50 eV per 16 amu. Errors in the estimated optical constants are of about 5% in n (real index) and 20% in k (imaginary index). We stress that ICH4 is not a simple ice. In fact, there is little, if any, ice left after irradiation, and the remaining residue is a complex hydrocarbon.

The optical properties of carbonaceous refractory materials strongly depend on the initial composition, synthesis conditions, and energy involved. Comparing our optical constants of ICH4 to those of carbonaceous compounds created by various energetic processes, in particular Titan and ice tholins, and different types of amorphous carbon, we note that ICH4 (at the dose we used) has real and imaginary indices that lie in between those of tholins and amorphous carbon. This indicates that ICH4 has not yet evolved towards a black carbon.

Data are from: Brunetto, R., and Roush, T.L., 2008, Impact of irradiated methane ice crusts on compositional interpretations of TNOs, A&A 481, 879-882. More details can be found in the paper.

Please make reference to Brunetto & Roush 2008, A&A 481, 879, when using these optical constants.

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Optical constants of ion irradiated CH₄