First Optical Constants of Laboratory-generated Organic Refractory Materials (Tholins) Produced in the NASA Ames COSmIC Facility from the Visible to the Near Infrared (0.4–1.6 μm): Application to Titan’s Aerosols

Abstract

We have measured the complex refractive indices, from 0.4 to 1.6 μm, of five laboratory-generated organic refractory materials (tholins) produced at low temperature (150 K) using plasma chemistry in the stream of a supersonic expansion in NASA Ames’ COsmic SImulation Chamber (COSmIC) facility. Three samples were produced from N2:CH4 gas precursors (with different voltages inducing different degrees of ionization in the plasma), one sample was produced from N2:CH4:C2H2, and one sample was produced from Ar:CH4 in order to produce a purely carbonaceous sample. The optical constants, n and k, of the samples were determined using spectral reflectance measurements. We observe that both n and k appear to be correlated with the nitrogen content in the solid sample, with samples containing more nitrogen having higher n and k. Comparisons to previous laboratory studies and Titan aerosol optical constants derived from observations show that the COSmIC tholins with a higher nitrogen content (higher n and k) are closer analogs of Titan aerosols. We also present a new analysis of Cassini Visible Infrared Mapping Spectrometer observations of Titan’s atmosphere in the visible to near infrared using the COSmIC tholin optical constants in a radiative transfer model. The COSmIC tholin sample produced from N2:CH4 with the lowest energy level has a spectral behavior that appears well suited to reproduce the observed Titan aerosol properties. This study has therefore demonstrated that this COSmIC tholin sample has valuable and promising optical properties for the analysis of Cassini’s Titan atmospheric observations.