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

TL;DR

This study models how lowering observation altitude on Titan can significantly widen spectral windows, enabling better surface composition analysis and detection of compounds not observable from orbit.

Contribution

It provides detailed radiative transfer modeling of Titan's atmosphere to assess how lower-altitude observations can expand spectral windows for surface studies.

Findings

Transmission windows can increase by up to 317% at lower altitudes.

Wider windows enable detection of compounds like nitriles and amino acid precursors.

Onboard illumination is necessary for appreciable window widening.

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 study Titan's surface is by decreasing the integrated column of absorbers (primarily methane) and scatterers between the observer and the surface. This is only possible if future missions operate at lower altitudes in Titan's atmosphere. Herein, we use a radiative transfer model to measure in detail the absorption through Titan's atmosphere from different mission altitudes, and consider the impacts this would have for interpreting reflectance measurements of Titan's surface. Over our modeled spectral range of 0.4 - 10 micron, we find that increases in the width of the transmission windows as large as 317% can be obtained for missions performing remote observations at the surface. However, any appreciable widening of the windows requires onboard illumination. Further, we make note of possible surface compounds that are not currently observable from orbit, but could be identified using the wider windows at low altitudes. These range from simple nitriles such as cyanoacetylene, to building blocks of amino acids such as urea. Finally, we discuss the implications that the identifications of these compounds would have for Titan science.