Titan haze distribution and optical properties retrieved from recent observations

The VIMS instrument onboard Cassini observed the north polar region of Titan at 113° phase angle, 28 December 2006. On this spectral image, a vast polar cloud can be seen northward to 62°N, and elsewhere, the haze appears as the dominant source of scattering. Because the surface does not appear in the wavelength range between 0.3 and , this spectro-image is ideal to study airborn scatterers both in methane bands and windows. In this work, we study this image, along with another image taken at 13° phase angle. This image probe both the atmosphere and the surface from pole to pole. First, we characterise the spatial distribution of the haze layer above 100 km between 80°S and 70°N. We find a north south asymmetry with a haze opacity increasing by a factor 3 from the south pole to the equator, then a constant value up to about 30°N and a decrease of a factor 2 between 30°N and about 60°N. Beyond 60°N, we can see the influence of the north polar cloud, even in the band, but no polar haze accumulation. The fact that the north polar region is still in the polar night is a possible explanation. No haze accumulation is observed in the southern polar region. Secondly, we partly identify the origin of spectral features in the 2.8-μm methane window, which are found to be due to deuterated methane (CH₃D). This allows the analyse of this window and to retrieve the opacity of scatterers layer below 80 km (presumably made of aerosols and condensate droplets) between 35°N and 60°N. Finally, we constrained the values and the spectral behaviour of the imaginary part of the aerosol refractive index in the range between 0.3 and . To do so, we studied the 2.8-μm window with the image taken at 113° phase angle. To complete the analysis, we studied the transmission through the haze layer in the 3.4-μm band observed in solar occultation mode with VIMS, and we analysed the single scattering albedo retrieved with DISR instrument between 0.4 and . The imaginary part of the refractive index that we find for Titan aerosols follows Khare et al. (Khare, B.N. et al. 1984]. Icarus 60, 127-137) optical constant up to and becomes constant beyond this wavelength at least up to . It also has a prominent peak at and a secondary peak at , which indicates material rich in C-H bonds, with much less N-H bonds than in Khare et al. (1984) tholins.