Seasonal evolution of Titan's stratosphere during the Cassini mission.

N A Teanby,S Vinatier,C A Nixon,M Sylvestre,P G J Irwin,J Sharkey

doi:10.1029/2018gl081401

Abstract

Titan's stratosphere exhibits significant seasonal changes, including break-up and formation of polar vortices. Here we present the first analysis of mid-infrared mapping observations from Cassini's Composite InfraRed Spectrometer (CIRS) to cover the entire mission (Ls=293-93°, 2004-2017) - mid-northern winter to northern summer solstice. The north-polar winter vortex persisted well after equinox, starting break-up around Ls∼60°, and fully dissipating by Ls∼90°. Absence of enriched polar air spreading to lower latitudes suggests large-scale circulation changes and photochemistry control chemical evolution during vortex break-up. South-polar vortex formation commenced soon after equinox and by Ls∼60° was more enriched in trace gases than the northern mid-winter vortex and had temperatures ∼20 K colder. This suggests early-winter and mid-winter vortices are dominated by different processes - radiative cooling and subsidence-induced adiabatic heating respectively. By the end of the mission (Ls=93°) south-polar conditions were approaching those observed in the north at Ls=293°, implying seasonal symmetry in Titan's vortices.

Highlights

Saturn's largest moon Titan has a thick atmosphere comprising ∼98% nitrogen and ∼2% methane with ∼1.5 bar surface pressure (Fulchignoni et al, 2005)
South polar vortex formation commenced soon after equinox and by Ls ∼ 60◦ was more enriched in trace gases than the northern middle-winter vortex and had temperatures ∼20 K colder
Seasonal Temperature Variations The midstratosphere 1-mbar temperatures shown in Figure 3 are warmest at the equator, with peak temperatures occurring at Ls ∼ 30◦

Summary

Introduction

Saturn's largest moon Titan has a thick atmosphere comprising ∼98% nitrogen and ∼2% methane with ∼1.5 bar surface pressure (Fulchignoni et al, 2005). The south polar stratosphere achieved extremely cold temperatures, which created ice clouds of HCN (de Kok et al, 2014) and benzene (Vinatier et al, 2018) at ∼300 km These cold temperatures were not observed in the north and could be caused by extreme trace gas enrichments acting as infrared coolers, combined with slow initial subsidence producing only modest levels of adiabatic heating (Teanby et al, 2017). Cassini's Imaging Science Subsystem observations of Titan's detached haze layer at 350–500 km imply upwelling speeds greater than haze particle free-fall velocity are required to dynamically clear the lower mesosphere of haze (West et al, 2018) These Visual and Infrared Mapping Spectrometer and Imaging Science Subsystem observations further confirm the meridional circulation inferred from Cassini CIRS and GCMs. Here we present the first analysis of all CIRS midinfrared mapping sequences from the entire Cassini mission, spanning 2004–2017 (Ls = 293–93◦, ΔLs = 160◦), almost half a Titan year. We use our analysis to answer three key questions: (1) are thermal and chemical behaviors of Titan's north and south polar vortices comparable? (2) what are the main factors controlling chemistry during polar vortex breakup? (3) how long does the winter polar vortex breakup take? These questions are essential for understanding Titan's atmospheric chemistry and dynamics, in addition to constraining future GCMs and photochemical models

Radiative Transfer Analysis

Results

Discussion

References from Supporting Information