A Transmission Spectrum of Titan's North Polar Atmosphere From a Specular Reflection of the Sun

Abstract

Cassini/VIMS T85 observations of a solar specular reflection off of Kivu Lacus (87. • 4N 241. • 1E) provide an empirical transmission spectrum of Titan's atmosphere. Because this observation was acquired from short range (33,000 km), its intensity makes it visible within the 2.0, 2.7, and 2.8 μm atmospheric windows in addition to the 5 μm window where all previous specular reflections have been seen. The resulting measurement of the total one-way normal atmospheric optical depth (corresponding to haze scattering plus haze and gas absorption) provides strong empirical constraints on radiative transfer models. Using those models, we find that the total haze column abundance in our observation is 20% higher than the Huygens equatorial value. Ours is the first measurement in the 2-5 μm wavelength range that probes all the way to the surface in Titan's arctic, where the vast majority of surface liquids are located. The specular technique complements other probes of atmospheric properties such as solar occultations and the direct measurements from Huygens. In breaking the degeneracy between surface and atmospheric absorptions, our measured optical depths will help to drive future calculations of deconvolved surface albedo spectra.

Cassini/VIMS observes rough surfaces on Titan’s Punga Mare in specular reflection

“…Differences in the real index of refraction caused by composition differences between Kivu Lacus and Punga Mare cannot account for the offset, as at this phase angle, the first-surface reflectance of pure methane and that of pure ethane differ by only ∼ 20% [21]. We note, however, that this new value of I/F max is closer to the theoretical value as derived using the Soderblom et al [21] relations of ∼ 70 for methane and ∼ 90 for ethane than that from the Kivu Lacus observation [20]. …”

“…It is not clear why the best-fit value for I/F max is discrepant from the Barnes et al [20] value (I/F = 32.4) by a factor of 3. The main specular pixel is saturated, so the error could be a result of the Barnes et al 2013 reconstruction of the saturated values.…”

“…the first pixel is pixel number zero). For both pixel (27,21) and (26,20) we used the average of (26,19) and (27,22). And for the pixel (24,14) at the south end of Punga Mare we used the average of pixels (23,13) and (25,15).…”

“…It shows a very bright specular reflection (in white in Figure 1 off a lake at 87.5°N called Kivu Lacus, which was previously used to derive an atmospheric transmission spectrum [20]). This cube was acquired from a range to Titan closer than any other specular reflection observation to date (30000 km).…”

“…The specular reflection appears extended at 5 μm because of forwardscattering of specularly-reflected sunlight from haze in the atmosphere above the lake [20]. The central specular pixel and its neighbor are saturated at 5 μm and 2.8 μm.…”