Reflectance spectra and chemical structure of Titanʹs tholins: Application to the analysis of Cassini–Huygens observations

Titanʹs tholins are used as analogs of Titanʹs aerosols and N-rich organic solids present on many icy surfaces. However, it is not clear whether or not they are relevant analogs, and which kind of tholins should be used among a wide set available in literature. This paper presents reflectance spectral data of two tholins selected as end-members of a series of samples covering a very wide range of continuous chemical and optical properties. These samples were formed under experimental conditions fairly consistent with Titanʹs stratosphere. A general framework for using these laboratory data to the analysis of spectral observation of Titanʹs surface or other objects is suggested. Furthermore, the study reports the first in situ unambiguous identification of aromatics compounds and evidences variations in the sp2 carbon structure, which controls the absorption properties in the visible/NIR. These results also point out it is very unlikely to derive quantitative chemical information (e.g., N content, sp2/sp3 ratio) from remote sensing reflectance data.