The enigmatic CIRS-observed Titan's stratospheric ice clouds studied in the laboratory
Abstract
Stratospheric ice clouds have been repeatedly observed in Titan's atmosphere by the Cassini Composite InfraRed Spectrometer (CIRS) since the Cassini spacecraft entered into orbit around Saturn in 2004. Most of these stratospheric ice clouds form as a result of vapor condensation processes, composed of a combination of pure and mixed nitriles and hydrocarbons. So far, the crystalline cyanoacetylene (HC _{3}N) ν _{6} band at 506 cm ^{-1} (Anderson et al., 2010) and a co-condensed nitrile ice feature at 160 cm ^{-1}, dominated by a mixture of HCN and HC _{3}N ices (Anderson and Samuelson, 2011), have been identified in the CIRS limb spectra. However, the presence of other CIRS-observed stratospheric ice emission features, such as the ν _{8} band of dicyanoacetylene (C _{4}N _{2}) at 478 cm ^{-1} and the unidentified Haystack emission feature at 220 cm ^{-1}, are puzzling since they have no associated observed vapor emission features. As well, recently, a massive stratospheric ice cloud system, called the High-Altitude South Polar (HASP) cloud, was discovered in Titan's early southern winter stratosphere at high southern latitudes, with an emission feature peaking near 210 cm ^{-1} (Anderson et al., 2017). We are investigating in laboratory these perplexing observed stratospheric ices to better understand their formation mechanisms, identify their chemical compositions, and determine their optical properties. We have performed transmission spectroscopy of thin films of pure and mixed nitrile ices, as well as -CN ices combined with benzene, from the near- to far-infrared spectral region (50 cm ^{-1} to 11700 cm ^{-1}), using the SPECTRAL high-vacuum chamber. Their respective vapors were deposited at low temperatures from 30 K to 150 K and the resulting ices were analyzed at different times after deposition, from immediately after dosing to up to 24 hours post-dosing. Their spectral evolution with time and temperature were studied, the ice phase formation identified, and their optical constants computed. The first surprising yet significant result reveals that the libration mode of HCN (166 - 169 cm ^{-1}) is drastically altered by the surrounding molecules when mixing occurs in a co-condensed phase. For propionitrile ice, we observe peculiar temperature and time-driven ice phase transitions, revealed by significant spectral changes in the mid- and far-IR until a stable crystalline phase is achieved. Comparing our laboratory spectra to the CIRS data, we found that a HCN-C _{6}H _{6} mixed ice is a good match for the HASP cloud emission feature. We present a summary of our findings obtained so far.
- Publication:
-
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018
- Bibcode:
- 2018cosp...42E2457N