Seasonal variations of Titan’s haze and mist layers monitored by VIMS-IR onboard Cassini

The Cassini orbiter around Saturn monitored Titan with multiple instruments 13 years, between 2004 to 2017. This is about half of a Titan year and this period included a major seasonal change at the North Spring Equinox (NSE) in 2009 that could be observed. The Visual and Infrared Mapping Spectrometer (VIMS) onboard Cassini produced a large amount of observations of Titan generally presented under the form of spectro-images. The observations with the IR part of VIMS detector are taken in a spectral range between 0.88 to 5 . 12 μ m and with a resolving power between R= λ / Δ λ = 120 and 180. The spatial resolution of the images depends on the observation and is few hundreds of meters at the best. In this study, we retrieved the distribution of the photochemical haze (above 80 km) and the condensate mist layer (below 80 km) as a function of latitude, altitude, and tile throughout the Cassini era. We found a haze latitudinal distribution with an extinction increasing from the south to the north at the beginning of the Cassini mission. The distribution evolved around the North Spring Equinox and a turnover was completely achieved at the end of the Cassini mission. This evolution is linked to the stratospheric circulation that blows from the summer hemisphere to the winter polar region. The latitudinal distribution of the mist layer evolves differently than the haze distribution because it depends on both the circulation pattern in the low atmosphere and on the conditions of condensation for several species. The distribution of the mist layer is also modulated with the seasons, but always increases from the equator and inter-tropical latitude band to the poles. These results yield a quantitative description of the aerosol layer on Titan and its seasonal evolution. This has been compared with previous works and with the output of the Titan Planetary Climate Model. Producing such results is quite important to constrain climate models. In return, we expect from these climate models to be helpful in fully understanding the meaning of our results and more generally in characterizing Titan’s climate.