First Optical Constants from 0.4 to 1.6 µm of Titan Aerosol Analogs Produced in the NASA Ames COSmIC Facility and Their Use in a New Analysis of Cassini VIMS Observations

<p>We have determined the real and imaginary refractive indices (<em>n</em> and <em>k</em>, respectively), from the visible to the near infrared (0.4 to 1.6 &#181;m), of five laboratory-generated organic refractory materials produced from gas-phase chemistry with the NASA Ames COSmIC facility. The solid samples were produced using a plasma discharge in the stream of a 200-K supersonic jet-cooled expansion of different gas mixtures to study the impact of the molecular precursors on the solid sample optical properties. Three samples were produced from N₂:CH₄ (95:5) gas mixtures using three different high voltages (700V, 800V and 1000V) to vary the energy in the plasma discharge. One sample was produced from a N₂:CH₄:C₂H₂ (94:5:5:0.5) gas mixture, with a high voltage of 1000 V. The fifth sample was produced in an Ar:CH₄ (95:5) gas mixture with a high voltage of 1000 V to produce a nitrogen-free hydrocarbon sample. The optical constants, <em>n</em> and <em>k</em>, of these five samples were determined using spectral reflectance measurements. They appear to be positively correlated with the nitrogen content in the solid sample, <em>i.e.</em>, a sample with larger nitrogen content exhibits higher <em>n</em> and <em>k</em> values.</p> <p>We have used these refractive indices as input parameters in a radiative transfer model to analyze Cassini Visible Infrared Mapping Spectrometer (VIMS) observations of Titan&#8217;s atmosphere. The results show that using the tholin samples with higher <em>n</em> and <em>k</em> values (higher nitrogen content) provides a better fit to the observational data than using the samples with lower <em>n</em> and <em>k</em> values (lower nitrogen content). The Titan tholins with higher nitrogen content therefore appear to be more representative of the Titan aerosols observed by VIMS.</p>