Convection behind the Humidification of Titan’s Stratosphere

Abstract

On Titan, methane is responsible for the complex prebiotic chemistry, the global haze, most of the cloud cover, and the rainfall that models the landscape. Its sources are located in liquid reservoirs at and below the surface, and its sink is the photodissociation at high altitude. Titan’s present and past climates strongly depend on the connection between the surface sources and the atmosphere upper layers. Despite its importance, very little information is available on this topic. In this work, we reanalyze two solar occultations made by Cassini before the northern spring equinox. We find a layer rich in methane at 165 km and at 70°S (mixing ratio 1.62% ± 0.1%) and a dryer background stratosphere (1.1%–1.2%). In the absence of local production, this reveals an intrusion of methane transported into the stratosphere by convective circulation. On the other hand, methane transport through the tropopause at a global scale appears quite inhibited. Leaking through the tropopause is an important bottleneck of Titan’s methane cycle at all timescales. As such, it affects the long-term evolution of Titan’s atmosphere and the exchange fluxes with the surface and subsurface reservoirs in a complex way. Global climate models accounting for cloud physics, thermodynamical feedbacks, and convection are needed to understand the methane cycle, and specifically the humidification of the stratosphere, at the present time, and its evolution under changing conditions at a geological timescale.

Analysis of four solar occultations by Titan’s atmosphere with the infrared channel of the VIMS instrument: Haze, CH4, CH3D, and CO vertical profiles

“…With the T 10E observation, the methane profile found at 70 o S has a sharp, methane-rich layer at 165 km, with a mixing ratio around 1.45 ± 0.05% in a relatively dry background atmosphere. We note that this differs slightly from the previous evaluation by Rannou et al (2021), that is, 1.65 ± 0.05%, after a spurious value in data was corrected (Figs. 11 and 12).…”

“…We also note that a strong variation in the spectral slope, but not in the extinction coefficient, occurs in the low stratosphere below 150 km, which indicates a specific process strongly altering the properties of the haze layer in some way. This feature appears correlated with the humid layer reported by Rannou et al (2021) and by us in the next section; it is attributed to an intrusion of air from the tropical region. We note that these results for the haze layer obtained with T 10E can be compared to those retrieved by Bellucci et al (2009).…”

“…We also plot the methane mole fraction retrieved with the GCMS onboard Huygens (Niemann et al (2010)) and with DIRS (Bézard (2014)) and CISR (Lellouch et al (2014)). The green dashed profile in the upper left graph shows the evaluation made by Rannou et al (2021).…”

“…With the methane bands between 0.88 and 2.00 µm, we find four valuable methane mixing ratio profiles from about 100 km up to 350 or 400 km. The profiles derived before the northern spring equinox (that is, T 10E and T 53E) were already analysed in Rannou et al (2021), and we simply remind the reader here of the main conclusions of their studies. We also discuss the retrieval made with T 78E and T 78I observations.…”

“…At this step, this seems to imply that the methane mixing ratio decreases from 5% to 1.4% due to cloud activity and then further decreases from 1.4% to reach a low value of about 1.15% around 200 − 220 km. Rannou et al (2021) compared the atmospheric circulation pattern, the vertical profiles of the methane mixing ratio obtained with T 10E and T 53E and mixing ratios in the stratosphere and at the cold trap derived from CIRS observations (Lellouch et al (2014)). They came to the conclusion that the methane-rich layer at 70 o S and at 165 km (T 10E) has a mixing ratio much larger than the that at the cold trap just below.…”