Variability in Titan's Mesospheric HCN and Temperature Structure as Observed by ALMA

Abstract

The temperature structure of Titan's upper atmosphere exhibits large variability resulting from numerous spatially and temporally irregular external energy sources, seasonal changes, and the influence of molecular species produced via photochemistry. In particular, Titan's relatively abundant HCN is thought to provide substantial cooling to the upper atmosphere through rotational emission, balancing UV/EUV heating and thermal conduction. Here, we present the analysis of ALMA observations of Titan from 2012, 2014, 2015, and 2017, corresponding to planetocentric solar longitudes of ~34-89${}^{\circ }$, including vertical HCN and temperature profiles retrieved from the lower mesosphere through the thermosphere (~350-1200 km; $3\times 10^{-2}$-$2\times 10^{-8}$ mbar). Throughout the atmosphere, temperature profiles differ by 10 to 30 K between observations approximately one Earth year apart, particularly from 600-900 km. We find evidence for a large imbalance in Titan's upper atmospheric energy budget between 2014 and 2015, where the mesospheric thermal structure changes significantly and marks the transition between a mesopause located at ~600 km ($2\times 10^{-4}$ mbar) and at ~800 km ($3\times 10^{-6}$ mbar). The retrieved HCN abundances vary dramatically during the 2012 to 2017 time period as well, showing close to 2 orders of magnitude difference in abundance at 1000 km. However, the change in HCN abundance does not appear to fully account for the variation in mesospheric temperatures over the $L_S\sim$34-89${}^{\circ }$ period. These measurements provide additional insight into the variability of Titan's mesospheric composition and thermal structure following its 2009 vernal equinox, and motivate continued investigation of the origins of such rapid changes in Titan's atmosphere throughout its seasonal cycle.

arXiv commentPublished in the Planetary Science Journal. 25 pages, 11 figures, 1 table

References in corpus (16)

• Titan's Atmosphere and Climate
• Photolytically generated aerosols in the mesosphere and thermosphere of Titan
• Submillimeter Observations of Titan: Global Measures of Stratospheric Temperature, CO, HCN, HC3N, and the Isotopic Ratios 12C/13C and 14N/15N
• A new astrobiological model of the atmosphere of Titan
• ALMA Observations of HCN and its Isotopologues on Titan
• Isotopic Ratios of Carbon and Oxygen in Titan's CO using ALMA
• Abundance Measurements of Titan's Stratospheric HCN, HC${}_3$N, C${}_3$H${}_4$, and CH${}_3$CN from ALMA Observations
• Titan interaction with the supersonic solar wind
• Seasonal changes in the middle atmosphere of Titan from Cassini/CIRS observations: temperature and trace species abundance profiles from 2004 to 2017
• The seasonal cycle of Titan's detached haze
• Spatial Variations in Titan's Atmospheric Temperature: ALMA and Cassini Comparisons from 2012 to 2015
• Seasonal evolution of temperatures in Titan's lower stratosphere
• Detection of Dynamical Instability in Titan's Thermospheric Jet
• Haze seasonal variations of Titan's upper atmosphere during the Cassini Mission
• Aerosols optical properties in Titan's Detached Haze Layer before the equinox
• Ground-based HCN submillimetre measurements in Titan's atmosphere: an intercomparison with Herschel observations

Cited by in corpus (3)

• The Composition and Chemistry of Titan's Atmosphere
• The Atmosphere of Titan in Late Northern Summer from JWST and Keck Observations
• Hydrocarbon complexity and photochemical shielding of prebiotic feedstock molecules in exoplanet atmospheres