The Surface Composition of Titan

Mapping the surface composition of Titan with the Cassini Visual and Infrared Mapping Spectrometer (VIMS) requires knowledge of the atmospheric absorption in the windows through which VIMS can observe the surface as well as the spectral properties of candidate materials. Recent atmospheric models are refining that knowledge. Titan's surface, in the VIMS spectral range, is seen in only a few spectral windows, near 0.94, 1.1, 1.3, 1.6, 2.0, 2.68-2.78, and 4.9-5.1 microns. Atmospheric models fail to fit the the observed spectra on the long wavelength side of the 2-micron window without invoking surface absorption at 2.1 microns. This new knowledge, along with the spectral shapes of the 2.68-2.78-micron, and 5-micron windows provide powerful constraints on Titan's surface composition. Water ice is incompatible with the observed 2.78/2.68 micron I/F ratio but likely exists below the surface. Many organic compounds have absorptions that are not seen in spectral windows of Titan, eliminating them as possible major components at the surface, including many polycyclic aromatic hydrocarbons (PAH). We find that some ring compounds and compounds with single N-H bonds have a close match to Titan's overall spectrum and can explain the relative intensities observed in the spectral windows, including the 2.68 and 2.78-micron double window, the low 3-5 micron reflectance, and increased absorption near 2.1-microns. Glycine is the only NH2 compound we have found that is also compatible. Combinations of coronene (C24H12), phenanthene (C14H12), pentacene (C22H14), indole (C8H7N), uracil (C4H4N2O2), and glycine (NH2CH2C00H) match the overall spectral structure of Titan spectra. We are searching for additional compounds that are also compatible. Indole, cytosine, and uracil, have 1.5-micron bands that are similar to the feature observed in Huygens DISR spectra of Titan's surface. These compounds, if present, can also help explain the pyrolysis results from the Huygens probe.