Cassini Results on Changes of Titan's Atmospheric Properties Near the Poles Since the Northern Equinox
Résumé
Since 2010, we observe the onset and enhancement at Titan's south pole of several trace species such as HC3N and C6H6 previously observed only at high northern latitudes. This is due to the transition of Titan's seasons from northern winter in 2002 to (late) northern spring in 2012 and the advent of winter in the south pole. During this time, the reversal of Titan's main atmospheric circulation cell is expected to channel fresh photochemical products from mid latitudes towards the south pole. Meanwhile, gases transported north during the previous season remain concentrated around the north pole, undergoing slow photochemical destruction. Following the northern winter, species with longer chemical lifetimes should remain in the north for a little longer while those with shorter lifetimes disappear, reappearing in the south. We will present an analysis of spectra acquired by Cassini/CIRS at high resolution from 2010 onwards, both in nadir and limb mode [1]. Subsidence gases that accumulate in the absence of ultraviolet sunlight, increased quickly since 2012 and some of them may be responsible for the haze decrease in the north and its appearance in the south at the same time [2]. For some of the most abundant and longest-lived hydrocarbons the evolution in the past years at a given latitude is not significant up to mid-2013 [1]. But in more recent dates these molecules show a dramatic trend for increase in the south. The 70°S and mid-latitudes show different behavior indicative of different dynamical processes in and out of the polar vortex region. While the 70°N data show generally no change with a trend rather to a small decrease for most species within 2014 the 70°S results indicate a strong enhancement in trace stratospheric gases after 2012. This is a strong indication of the rapid and sudden buildup of the gaseous inventory in the southern stratosphere during 2013-2014, as expected as the pole moves deeper into winter shadow. The limb spectra also show variations in time with altitude, with in particular, in early 2016, a zone depleted in molecular gas and aerosol in the entire northern hemisphere between 400 and 500 km, suggesting a complex unknown dynamical effect [3]. References:[1] Coustenis, et al., Icarus 207, 461, 2010 ; Icarus, 270, 409, 2016 ; [2] Jennings et al., ApJ 804, L34, 5, 2015; [3] Vinatier et al. Icarus 250, 95, 2015.