New views on Titan’s surface as probed by corrected VIMS/Cassini spectra.

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 particles also prevent the surface from being seen clearly, except in methane spectral windows that provide narrow wavelength keyholes through which surface can be probed. Titan has been observed by many means over time and with many instruments. Here we use VIMS/Cassini for our studies. However, recent comparisons between VIMS/Cassini, STIS/HST, NIRSpec/JWST and ISS/Cassini clearly reveal that VIMS spectra must be substantially corrected to be consistent with the other instruments. In this work, we explore the effect of this correction on the retrieved spectra of Titan’s surface. With VIMS observations, retrieved surface spectra that have a peak around 1.1 or 1.2 μm significantly differ from the ground truth observation made with DISR onboard Huygens. However, when a correction is applied to VIMS intensities, the retrieved surface albedos are in better agreement with expected levels from in-situ observations (Figure 1). 

With this model, we are able to map regions of interest on Titan and retrieve surface reflectivity consistent with the only existing ground truth from Huygens DISR. We then further use as indicators the ice index (as already used) or color index, to produce regional maps, allowing us to better comprehend differences of nature of various terrains beyond their brightnesses. 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: Left top: The region of the Huygens Landing Site and its surrounding as observed with VIMS (1732874866_1) during the flyby T88 on 29 Nov 2012 at 2 μm. Surface reflectivity retrieved with the model corresponding to Coutelier et al. (2021). Left bottom: With data as available publicly and the wavelength shift given by ”RC19” (label ”INITIAL”), with the sucessive implementation of the wavelength shift corrections (label ”SHIFT1” and ”SHIFT2”) and with intensity correction (label ”SHIFT2 + CORRECTED INTENSITY”). These synthetic spectra are compared to those retrieved with DISR onboard Huygens (Karkoschka et al. (2012); Karkoschka and Schröder (2016)). Right top: Retrieval of the surface reflectivity in the region of the Huygens Landing Site, with pixels including the HLS and with two bright pixels. Right bottom: Retrieval of the surface reflectivity in the region of the Selk crater (Image not shown here), with two dark pixels inside the crater and with two bright pixels on the ejecta outside the crater.