Returning to Titan: Assessing Titan's Transmission for Future Missions

We present in this work an analysis of the transmission of Titan's atmosphere as a function of altitude modelled using radiative transfer. We find that unless close to the surface the spectral shape of Titan's atmosphere changes very little. Despite a decade's worth of observations and discoveries with Cassini, the composition of Titan's surface still remains elusive. This is because there are only a few narrow "windows" that are able to see through Titan's thick atmosphere to the surface, due to methane absorption and scattering off of Titan's complex hazes. Despite these limitations, there are still some constraints for Titan's surface composition, suggesting complex hydrocarbons, and CO2 and H2O ices, although the limited spectral coverage makes species identification difficult. An increased range in the spectral regions sensitive to the surface would not only vastly improve our knowledge of composition, but would inform studies of surface processes and photochemical production in the atmosphere (and subsequent deposition onto the surface). Further, detailed studies of the surface would constrain astrobiologically relevant chemistry in the atmosphere and on the surface (e.g. water-organic interactions).We model Titan's atmosphere using the radiative transfer code, PyDISORT. This code uses profiles measured by the Huygens probe to simulate Titan's atmosphere and contains contributions from multiple scattering off of hazes, collision-induced absorptions, and variable compositions with altitude. To model Titan's methane absorption, we use the k-coefficients derived from laboratory measurements and direct observations of Titan's atmosphere. This allows for the most complete spectral coverage to even shorter wavelengths where current absorption databases are incomplete. The code permits any number of species in the atmosphere, but for this work we remain limited to methane only.To best align with potential future missions to Titan, we model the transmission of Titan's atmosphere at three different characteristic altitudes: from orbit, a balloon, or for a lander/aircraft, at 1500 km, 10 km, and 10 m respectively. We find significant broadening in the spectral windows with decreasing altitude, but only at altitudes less than 10 km. This is a result of the increased atmospheric density and methane abundance close to the surface. However, because incident solar radiation is absorbed quickly in Titan's upper atmosphere, an onboard lamp is required to access these additional spectral regions. However, even a modest lamp, comparable to a common 100-W incandescent lamp, is sufficient to provide an observable signal over all the broadened spectral regions at a distance of 10m, thus greatly increasing the spectral regions accessible to future surface missions.