Mapping Titan’s haze and mist with the near infra-red spectrometer onboard the James Webb space telescope

Observations of Titan were performed on the 4 November 2022 with the Near Infra- Red Spectrometer (NIRSpec) onboard the James Webb Space Telescope (JWST). The observation is a spectro-image of Titan taken about two and a half years before the northern autumnal equinox. This spectro-image at low phase angle covers a spectral range between 0.95 to 5 . 27 μ m and has a spectral resolving power R= λ / Δ λ =2700. Titan was resolved with about eight pixels across the disk, corresponding to a spatial resolution of 643 km at the centre of Titan. However, the data was resampled to obtain a final spectro-image with thirty pixels across the disk. 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 and altitude. Due to the high NIRSpec spectral resolution, we can distinguish five mist sublayers. We found a haze distribution with extinctions increasing from the north to the south, consistent with the direction of stratospheric meridional winds. The distribution of the mist layer depends on both the circulation and the thermodynamic conditions relating to cloud formation. We found an upper mist layer, above 40 km, having a latitudinal distribution similar to the haze with an increase from north to south. The lower mist, below 40 km, has a minimum around 10 ° N and increases toward the north and the south. The retrieved mist layer also presents a significant increase in extinction that emerges from the background at an altitude of around 50 to 60 km in the southern hemisphere. Finally, we retrieved the surface reflectivity that varies with latitude but is always spectrally consistent with the reflectivity of water ice grains with radius 100 μ m . These results yield important constraints for climate models and, in return, climate models such as the Titan Planetary Climate Model (PCM) will help to fully understand the meaning of our results.