THERMAL AND CHEMICAL STRUCTURE VARIATIONS IN TITAN'S STRATOSPHERE DURING THECASSINIMISSION

Abstract

We have developed a line-by-line Atmospheric Radiative Transfer for Titan code that includes the most recent laboratory spectroscopic data and haze descriptions relative to Titan's stratosphere. We use this code to model Cassini Composite Infrared Spectrometer data taken during the numerous Titan flybys from 2006 to 2012 at surface-intercepting geometry in the 600-1500 cm −1 range for latitudes from 50 • S to 50 • N. We report variations in temperature and chemical composition in the stratosphere during the Cassini mission, before and after the Northern Spring Equinox (NSE). We find indication for a weakening of the temperature gradient with warming of the stratosphere and cooling of the lower mesosphere. In addition, we infer precise concentrations for the trace gases and their main isotopologues and find that the chemical composition in Titan's stratosphere varies significantly with latitude during the 6 years investigated here, with increased mixing ratios toward the northern latitudes. In particular, we monitor and quantify the amplitude of a maximum enhancement of several gases observed at northern latitudes up to 50 • N around mid-2009, at the time of the NSE. We find that this rise is followed by a rapid decrease in chemical inventory in 2010 probably due to a weakening north polar vortex with reduced lateral mixing across the vortex boundary.

Seasonal Evolution of Titan's Stratosphere Near the Poles

“…We use a monochromatic radiative transfer code adapted to Titan's atmosphere (ARTT; Coustenis et al 2010;Bampasidis et al 2012), and we extract the temperature profile from the best fit of the 7.7 μm methane band in FP4 (Figure 2 shows some examples of fitting the methane v 4 band in the 1250-1350 cm −1 region). Our model is based on a CH 4 abundance of 1.5% through the stratosphere and increasing below the cold trap to about 5% at the surface, in agreement with CIRS and with Huygens Gas Chromatograph Mass Spectrometer (GCMS) measurements (Flasar et al 2004;Niemann et al 2010).…”

“…The temperature profile is originally set to the one measured at altitudes 0-147 km by the Huygens Atmospheric Structure Instrument (HASI; Fulchignoni et al 2005). Adjustments were applied to the HASI profile to extend in altitude and to account for latitude dependences and seasonal variations in temperature as found by CIRS emission values in the 7.7 μm CH 4 band and from lower-resolution observations (Coustenis et al 2007(Coustenis et al , 2010(Coustenis et al , 2016Achterberg et al 2008Achterberg et al , 2011Bampasidis et al 2012). For more details on the method see the literature.…”

“…This assumption is essentially valid for all of the molecules considered here, due to their weak emission bands, except for C 2 H 2 whose abundance variations with height should be taken into account. Our method for inferring the acetylene mixing ratio values by making use of weighted averages over the P, Q, and R branches of the 729 cm −1 band and by testing vertical profiles is described in detail in Section 2 of Bampasidis et al (2012).…”

“…These investigations have shown that a large-scale reversal occurred in the single pole-to-pole circulation affecting the distribution of the atmospheric structure after the northern equinox in mid-2009, with the gases in the summer hemisphere showing upwelling while in the winter pole a corresponding subsidence was found (Achterberg et al 2008(Achterberg et al , 2011Bampasidis et al 2012;Teanby et al 2012). Whereas until 2011 a significant build-up of trace gases, haze, and condensates was still observed over the north pole (West et al 2011(West et al , 2016Jennings et al 2012aJennings et al , 2012bCoustenis et al 2013;Vinatier et al 2015), toward the end of that period, the vortex and hotspot reported in Jennings et al (2015) and Achterberg et al (2008) had practically disappeared.…”

Titan’s temporal evolution in stratospheric trace gases near the poles

“…The middle atmosphere, the stratosphere, showed evolution in temperature and chemical composition during the Cassini mission, especially in the north up to a couple of years ago (e.g. Achterberg et al, 2011;Teanby et al, 2010;Coustenis et al, 2010;Bampasidis et al, 2012;Coustenis et al, 2013). During the recent winter (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009), the northern pole remained in darkness, allowing for photochemical hazes forming in the upper atmosphere to migrate downwards and accumulate in the stratosphere, where a maximum enhancement was observed during the NSE (e.g.…”