Haze, Methane, and Carbon Monoxide in Titan’s North Polar Atmosphere from a Cassini/VIMS Specular Transmission Spectrum on the T104 Flyby

Abstract We analyze a specular reflection of the Sun off of Titan’s largest sea, Kraken Mare, observed by the Cassini Visual and Infrared Mapping Spectrometer on the T104 flyby (2014 August 21). We use the specularly reflected I / F signal to derive a transmission spectrum. Owing to the low incidence and emission angles (51°) of the reflection, the specular signal is visible down to the 1.3 and 1.6 μ m windows, as well as the 2.0, 2.7/2.8, and 5 μ m windows as had been seen previously on T85. We confirm the T85 result that Titan’s atmosphere absorbs more at 2.7 μ m than it does at 2.8 μ m and that the “notch” between those windows results from absorption in the atmosphere and not from the surface. We compare our derived T104 spectrum to numerically integrated optical depths calculated using Huygens-derived haze properties and correlated- k gas absorption coefficients. We fit the T104 observation using 63% ± 3% greater haze abundance than derived from Huygens and a wavelength exponent of 2.50 ± 0.03, steeper than the 2.0 derived from Huygens. We measure a surface methane fraction of 0.043 ± 0.001 over Kraken Mare at 68°N latitude, slightly lower than the Huygens measurement of 0.0539 ± 0.0014 in Titan’s tropics. The comparison shows the utility of specular reflection transmission spectra for quantitative determination of Titan’s atmospheric properties. Fitting of carbon monoxide (CO) absorption places a constraint of 22.6 ± 0.3 ppm on the fraction of CO in Titan’s atmosphere. Mismatches between our model and these specular transmission observations may allow quantitative estimation of the effects of unknown composition gaseous absorbers throughout Titan’s atmosphere.