The three-micron spectral feature of the Saturnian haze: Implications for the haze composition and formation process

We retrieve a 3.4-μm spectral feature pertaining to the Saturnian haze from Cassini/VIMS Procyon occultation data obtained at 12 different altitudes from approximately 0.7 to 0.0018 mbar (Nicholson et al., 2006; Bellucci, 2008; Bellucci et al., 2009). The occultation occurred at 55°N latitude, at the edge of the “polar hood” region, where the extinction from stratospheric aerosols becomes strongest. The improved retrieval procedure eliminates the influence of strong adjacent CH4 absorption bands. We derive optical-depth spectra (hereafter called τ-spectra) of the haze on a relative scale and find that the τ-spectra exhibit a broad opacity peak at 3.36–3.41 μm. At the 0.7–0.025 mbar levels, the opacity is maximum at 3.41 μm, which approximately coincides with the 3.4-μm aliphatic C–H stretching bands typical of the solid state of hydrocarbons such as C5H12, C6H12, C6H14 and C7H14, but not limited to these molecules. At the 0.015–0.0018 mbar levels, however, maximum opacity occurs at 3.36 μm, similarly to the τ-spectra of Titanʹs haze (Kim et al., 2011), suggesting similar chemical composition and possibly a similar formation mechanism occurring in the upper stratospheres of Saturn and Titan. We also find that with mole fractions on the order of 1 ppb, C6H12, C6H14 and C7H14 can be saturated near the lower levels of the occultation, suggesting that condensation of these or other heavier hydrocarbons could contribute to the formation of the stratospheric haze particles at these altitudes on Saturn.