Determination of the Complex Refractive Indices of Aerosol Analogs Formed at Low Temperatures with the NASA Ames Optical Constants Facility (OCF)

The NASA Ames COsmic SImulation Chamber (COSmIC) [1] is a unique experimental facility that allows: 1) cooling a gas mixture to low temperature (150 K) in a jet expansion before inducing chemistry by plasma; and 2) controlling the extent of the chemical reactions by employing a pulsed plasma discharge. This enables the study of the early stages of aerosol production, as well as specific chemical pathways in planetary environments (e.g. Titan's and Pluto's atmospheres). Both the gas and solid products can be studied. For a decade COSmIC has been used to simulate Titan's atmospheric chemistry at low, Titan-like temperature [2]. New developments on the COSmIC facility are investigating formation of aerosols in tenuous, or transitory, atmospheres of other icy bodies [3-5], as well as cool exoplanets atmospheres having a hydrocarbon component, that results in formation of hazes and/or surface deposits of refractory materials.

The new Ames Optical Constants Facility enables determination of the aerosol analogs' complex refractive indices, n and k, from 0.59 to 200 µm [2]. Here we report efforts of determining n and k from ex-situ transmission measurements of solid samples produced from binary N₂-CH₄ and Ar-CH₄, and tertiary N₂-CH₄-C₂H₂ and Ar-CH₄-C₂H₂ gas mixtures in COSmIC, and deposited onto various substrates. A computational technique [6] that addresses interference fringes observed in the laboratory transmission data, particularly at wavelengths < 3 μm, has been implemented and applied to determine n and k for the samples. At visible and near-infrared wavelengths (0.4-1.6 µm) the deposit thickness, and its variation, as well as n and k were determined by a commercial entity. These data provide the ability to compare results, from independent methods, in the region of overlap between the two approaches.

Acknowledgements

This research is supported by NASA's Science Mission Directorate SERA Directed Work Package. The authors acknowledge the outstanding technical support of R. Walker and E. Quigley.

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