The Seasonal Evolution of Titan's Stratospheric Ice Clouds as observed by Cassini CIRS
Abstract
Over the course of Cassini's 13-year mission, the Composite InfraRed Spectrometer (CIRS) observed numerous ice clouds in Titan's stratosphere. This includes the ν_{6} band of HC_{3}N near 506 cm^{-1} [Anderson et. al. (2010) Icarus, 207, 914-922], as well as the ν_{8} band of C_{4}N_{2} at 478 cm^{-1}, in which solid-state photochemistry was invoked as a plausible formation mechanism [Anderson et. al. (2016) Geophys. Res. Lett., 43, 3088-3094]. CIRS also observed geometrically thin ice clouds spanning latitudes 85°N to 60°S during northern winter, which we call {the 160 cm^{-1} ice clouds}, with broad spectral dependences peaking at 160 cm^{-1} [Anderson and Samuelson (2011) Icarus, 212, 762-778]. Laboratory experiments indicate that these ice clouds are spectrally compatible with co-condensed mixtures of HCN and HC_{3}N. As Titan transitioned into northern spring/southern fall, these ice clouds vanished, and eventually, a massive cloud system developed in Titan's mid stratosphere at high southern latitudes. This is the ice cloud we have named the High-Altitude South Polar (HASP) cloud, which is at least an order of magnitude more intense than the 160 cm^{-1} ice cloud system, with a different spectral signature. Laboratory experiments show the HASP cloud is spectrally compatible with an HCN-C_{6}H_{6} co-condensed ice mixture. CIRS also observed a spectrally broad and intense unidentified emission feature in Titan's stratosphere that spectrally peaks at 221 cm^{-1}, which we call {the Haystack}. The Haystack was observed to persevere during northern winter at latitudes poleward of ∼50°N, with diminishing emission at high northern latitudes as Titan transitioned out of northern winter. During Titan's mid southern fall season, CIRS then observed the Haystack's reappearance at high southern polar latitudes [Jennngs et. al. (2012), ApJ Letters, 761:L15]. In this presentation, we will review the numerous time varying physical and chemical characteristics of Titan's CIRS-observed stratospheric ice clouds. A companion paper (21314) by Nna-Mvondo et al. will detail our dedicated laboratory efforts to identify their chemical compositions, structural characteristics, and formation mechanisms.
- Publication:
-
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018
- Bibcode:
- 2018cosp...42E..88A