Coloration and darkening of methane clathrate and other ices by charged particle irradiation: Applications to the outer solar system

Methane clathrate is expected to be an important carbon-containing ice in the outer solar system. We investigate the effect of electron irradiation by coronal discharge on several simple hydrocarbons enclathrated in or mixed with H₂O or H₂O+NH₃ in simulation of the effects of the solar wind, planetary magnetospheric particles, and cosmic rays on surfaces containing these ices in the outer solar system and interstellar space. H₂O+CH₄ clathrate, H₂O+C₂H₆, H₂O+CH₄+NH₃, H₂O+C₂H₆+NH₃, and H₂O+C₂H₂ are all initially white ices, and all produce yellowish to brownish organic products upon charged particle irradiation. Significant coloration occurs with doses of 10⁹ erg cm^-2, corresponding to short interplanetary irradiation times.

Uranian magnetospheric electrons penetrate to ~1 mm depth and deposit this dose in 8, 30, 65, 200, and 500 years into the surfaces of Miranda, Ariel, Umbriel, Titania, and Oberon, respectively. Further irradiation of the laboratory ice surface results in a progressive darkening and a more subdued color. For a conversion efficiency to solids G≃1 molecule keV^-1, the upper limit for the time for total destruction of CH₄ and other simple hydrocarbons in the upper 1 mm is 5×10⁴ years (Miranda) to 3×10⁶ years (Oberon). Remote detection of CH₄ is possible only when its replenishment rate exceeds the destruction rate at the depth probed by spectroscopy.

Reflection spectroscopy of irradiated H₂O+CH₄ frost is compared with the spectra of several outer solar system objects and to other relevant organic and inorganic materials. Ultraviolet-visible and infrared transmission spectroscopy of the postirradiation residues is presented. Persistence of color and of CH₄ ice bands on Triton and Pluto suggests ongoing surface activity and/or atmospheric haze. Over 4×10⁹ year time scales, >=10 m of satellite and cometary surface material is processed by cosmic rays to a radiation-hardened ice-tholin mixture devoid of CH₄. Preaccretional chemistry, exogenous materials, and endogenous organic chemistry all contribute to the spectral properties of icy satellites which accreted simple CH(O) molecules. Radiation darkening traces the deposition of mobilized or impact-exposed carbon-bearing volatiles on these satellites. More exhaustive experiments are necessary to work out the detailed relationships between initial composition, exposure age, and color/albedo.