Seasonal variations in Titan’s middle atmosphere during the northern spring derived from Cassini/CIRS observations

Abstract

International audienceWe analyzed spectra acquired at the limb of Titan in the 2006–2013 period by the Cassini/Composite Infrared Spectrometer (CIRS) in order to monitor the seasonal evolution of the thermal, gas composition and aerosol spatial distributions. We are primarily interested here in the seasonal changes after the northern spring equinox and interpret our results in term of global circulation seasonal changes. Data cover the 600–1500 cm−1 spectral range at a resolution of 0.5 or 15.5 cm−1 and probe the 150–500 km vertical range with a vertical resolution of about 30 km. Retrievals of the limb spectra acquired at 15.5 cm−1 resolution allowed us to derive eight global maps of temperature, aerosols and C2H2, C2H6 and HCN molecular mixing ratios between July 2009 and May 2013. In order to have a better understanding of the global changes taking place after the northern spring equinox, we analyzed 0.5 cm−1 resolution limb spectra to infer the mixing ratio profiles of 10 molecules for some latitudes. These profiles are compared with CIRS observations performed during the northern winter. Our observations are compatible with the coexistence of two circulation cells upwelling at mid-latitudes and downwelling at both poles from at last January 2010 to at least June 2010. One year later, in June 2011, there are indications that the global circulation had reversed compared to the winter situation, with a single pole-to-pole cell upwelling at the north pole and downwelling at the south pole. Our observations show that in December 2011, this new pole-to-pole cell has settled with a downward velocity of 4.4 mm/s at 450 km above the south pole. Therefore, in about two years after the equinox, the global circulation observed during the northern winter has totally reversed, which is in agreement with the predictions of general circulation models. We observe a sudden unexpected temperature decrease above the south pole in February 2012, which is probably related to the strong enhancement of molecular gas in this region, acting as radiative coolers. In July and November 2012, we observe a detached haze layer located around 320–330 km, which is comparable to the altitude of the detached haze layer observed by the Cassini Imaging Science Subsystem (ISS) in the UV

Mapping Vinyl Cyanide and Other Nitriles in Titan’s Atmosphere Using ALMA

“…It shows relatively little spatial variation compared with other species (e.g. Vinatier et al 2015), so we approximate the C 3 H 8 distribution in our radiative transfer model using a vertical profile with a con- stant abundance above the saturation altitude of 60 km. Pressure broadening coefficients of Γ = 0.12 cm −1 bar −1 , α = 0.5 were adopted based on the typical values for rovibrational transitions of C 3 H 8 (see Gordon et al 2017).…”

“…By virtue of the rapid production in the upper atmosphere and relatively short chemical lifetimes of some nitriles (Wilson & Atreya 2004), these molecules can be used as powerful probes of the seasonally variable chemistry and dynamics in Titan's upper atmosphere (e.g. Teanby et al 2012;Cordiner et al 2014;Vinatier et al 2015).…”

Seasonal Evolution of Titan's Stratosphere Near the Poles

“…We do not have any high-resolution nadir data at 70°N before 2014, so we cannot infer the evolution of the abundances of these species prior to the past year, but they do not seem to have changed much since their values reported in Vinatier et al (2015) from limb data in 2011 (their Figure 8). With that caveat in mind, our radiative transfer results show that the high northern latitudes (at about 20°from the pole) only became significantly depleted in trace gases after 2015 ( Figure 5).…”

“…This argues for a fast and strong build-up of the gases in the southern pole as it goes deeper into the shadow during the 2013-2014 onset of winter, as predicted by models. The limb spectra also show variations in time with altitude, with, in 1 particular, an area covering the whole northern hemisphere at altitudes between 400 and 500 km strongly depleted in gas and haze in early 2016, indicative of a complex dynamical effect yet to be identified (Vinatier et al 2015;Teanby et al 2017).…”

“…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. In its place, similar features started to appear near the south pole, which have evolved to what we see today.In the absence of sunlight, gases that subsidize have accumulated since 2012 and become enhanced (e.g., Coustenis et al 2016 and references within;Teanby et al 2017), and a subset of those have been suggested to be causing the haze decrease found in the north and its subsequent increase in the south (Jennings et al 2012b(Jennings et al , 2015.…”