Modeling transmission windows in Titan’s lower troposphere: Implications for infrared spectrometers aboard future aerial and surface missions

Abstract

From orbit, the visibility of Titan's surface is limited to a handful of narrow spectral windows in the near-infrared (near-IR), primarily from the absorption of methane gas. This has limited the ability to identify specific compounds on the surface-to date Titan's bulk surface composition remains unknown. Further, understanding of the surface composition would provide insight into geologic processes, photochemical production and evolution, and the biological potential of Titan's surface. One approach to obtain wider spectral coverage with which to

Titan Stratospheric Haze Bands Observed in Cassini VIMS as Tracers of Meridional Circulation

“…VIMS consisted of two imaging spectrometers: a visual detector (VIMS-Vis) with 96 channels between 0.35 and 1.05 μm, and an infrared detector (VIMS-IR) with 256 spectroscopic channels between 0.89 and 5.13 μm (Brown et al 2004). The absorption and scattering by methane, nitrogen, and haze mean that some channels in VIMS can see the surface, while for other channels the atmosphere is completely opaque (Corlies et al 2021). For every pixel, we determined the viewing geometry and location information using the SPICE toolkit from NASA's Navigation and Ancillary Information Facility (Acton et al 2018).…”

“…Using superposition, we can infer the shape and altitude/depth of a feature by its appearance in the spectra. This technique has already been used extensively at Titan for measuring the height of clouds using observations from Cassini and ground-based telescopes (Brown et al 2002;Le Mouélic et al 2012;Ádámkovics et al 2016;Corlies et al 2021). As methane is the primary absorber in Titanʼs atmosphere at IR wavelengths, we used a methane opacity profile from Rannou et al (2016) to estimate the altitude of the EQA and the NPA.…”

Updated Radiative Transfer Model for Titan in the Near-infrared Wavelength Range: Validation against Huygens Atmospheric and Surface Measurements and Application to the Cassini/VIMS Observations of the Dragonfly Landing Area

“…We use the photometrically calibrated mosaic of Karkoschka & Schröder (2016) constituted of about 300 images taken by the Huygens/DISR instrument starting from 70 km altitude at 770 nm wavelength (Karkoschka et al 2007, and refined by Karkoschka et al 2012). Since the spatial resolution of the VIMS data cube does not allow us to distinguish as many small-scale surface features at the landing site, many attempts have been done to reconstruct the HLS spectrum with linear combinations of VIMS pixels around the HLS (e.g., Griffith et al 2012;Hirtzig et al 2013;Corlies et al 2021). But the method, the choice of VIMS pixels, and the calculation of the linear combination coefficients were not explicitly described and our tests with these different combinations showed that they were not accurate enough for a valid comparison.…”

“…In order to retrieve the surface albedo in the atmospheric windows, we apply a radiative transfer (RT) model to simulate the radiance factor (I/F) in the NIR channel of Cassini/VIMS observations of Titan. RT models in the NIR for Titan have been developed for decades (e.g., McKay et al 1989;Griffith et al 1991Griffith et al , 2003Griffith et al , 2012Rannou et al 1995Rannou et al , 2003Rannou et al , 2016Rannou et al , 2021Ádámkovics et al 2004, 2009Adriani et al 2005;Negrão et al 2006;Hirtzig et al 2013; Barnes et al 2018;Corlies et al 2021;Coutelier et al 2021), progressively upgrading the optical properties of atmospheric gases and photochemical aerosols and improving the robustness of Titan's radiative budget estimation and surface albedo retrievals. We developed in our team an RT model for Titan based on the model of Hirtzig et al (2013).…”