Titan haze and surface observed with the NIRSpec/JWST, VIMS/Cassini and STIS/HST

 The photochemical haze layer in the stratosphere and the condensation haze (hereafter, mist) in the lower stratosphere and troposphere completely cover Titan and play a dominant role in its climate. These hazes determine the atmospheric radiative balance and are also good tracers of the atmospheric circulation. These particles also prevent from seeing clearly the surface except in methane windows that provide narrow keyholes through which surface can be perceived. Titan has been observed by many means over time and in this work we used the results of three instruments; the STIS/HST, NIRSpec/JWST and VIMS/Cassini. Our goal is to compare the spectra that were obtained by each and to use these data to gain information on the haze and mist properties and about the surface reflectivity. Recently, the James Webb Space Telescope (JWST) has provided observations of very high spectral quality (in resolution and uncertainties). These images and spectra provide information on the latitudinal distribution of haze, from the lower stratosphere (100-150 km) down into the troposphere with good vertical resolution. With this dataset, we are able to describe the mist layer in five distinct sublayers: the deepest layer between 0 and 40 km and four layers of 10 km thickness between 40 and 80 km. The retrievals performed allow us to reconstruct a map of the haze and mist layer on the date of observation (5 November 2022) and thus highlight haze distributions related to circulation. Besides knowledge about the haze itself, a good haze retrieval allows access to the surface properties (i.e., reflectivities), in general, with uncertainty levels that allow us to detect differences between different terrains. With NIRSpec/JWST, we obtain information with relatively low spatial resolution. These surface reflectivities differ from previous analysis made with VIMS. With the same model adapted to VIMS/Cassini observation, we also analyzed a selected part of the VIMS/Cassini dataset collected from 2004 to 2017. We could monitor, albeit with a lower vertical resolution, how these hazes evolve over season and we retrieve surface albedo too. These results allow us to put in context the results obtained with the NIRSpec/JWST and also offer an interesting comparison to the results obtained with NIRSpec/JWST since Cassini observed the opposite season, in 2007. In our work we found that VIMS/Cassini and NIRSpec/JWST do not produce exactly the same retrieval and the difference is beyond what is expected from a seasonal difference. Concerning the surface, retrieved spectra differ substantially. To clarify this, we used STIS/HST data as a third constraint to better understand the causes of the differences. We consequently propose a new way to analyze VIMS observations and especially for retrieving the surface albedo more in line with expectations based on in-situ observations (Figure 1). With the upcoming very large telescopes (Extremely Large Telescope, Thirty Meter Telescope,...) with high sensitivity and spectral resolution, this work shows it is important to fully understand past observations and to obtain as much information as possible from them. Finally, fully characterizing Titan’s atmosphere and surface with models to obtain the most accurate analysis and results from them is a major present-day objective to prepare for future missions to Titan such as Dragonfly.

 

Figure 1 : At Left : Selk crater and its surrounding as observed with VIMS (1578263500 1) during the flyby T40 on 5 Jan 2008 with a RGB composition. The red line shows the parallel at 6.95°N along which we perform analysis. The pixel A and B, located at 6.94°N,160.24°E and 6.94°N,165.50°E are used for testing purpose in our work. The other letters along the parallel 6.95°N indicate the various types of terrain, as reported by Bonnefoy et al. (The Planetary Science Journal, 3(8):201, 2022), where different units are identified as plains (”p”), crater rim (”r”), dune field (”d”), crater ejecta (”e”) and hummocky (”h”). At right : Proxy of haze (Fh) and mist (Fm1 and Fm2) opacities (top), outgoing radiance factor I/F at 2.20 μm (middle) and retrieved surface reflectivity along the parallel at 6.95°N (bottom).