2009 Annual Science Report
NASA Jet Propulsion Laboratory - Titan Reporting | JUL 2008 – AUG 2009
Task 2.1.3 Aerosol Nucleation and Growth
Organic macromolecular aerosols in the Titan atmosphere may contribute to the orange haze seen in the visible spectrum and can serve as the initial stage of prebiotic chemistry on Titan.
Co-Investigator Roger Yelle and his research group have performed detailed simulations of the aerosol and cloud evolution at the Huygens landing site, validated against the optical properties retrieved by the Huygens DISR instrument. Our results provide a good fit to the observed particle size, density, and phase functions, which put further constraints on the aerosol mass production rate, the atmospheric mixing, the monomer radius and the aerosol charge density. The aerosol mass flux required to match the observations is 3.0 × 10-14 g cm2s-1, the charge density of the particles is 15 e/μm, while the atmospheric mixing necessary to reproduced the observed aerosol extinction profiles is larger than the profile retrieved for the gas species in the lower atmosphere. Furthermore, we have retrieved an adjusted imaginary refractive index (k) for the aerosol particles that is able to reproduce the observed wavelength dependence of the single scattering albedo.
The pure aerosol simulation provides a good match to the observations above 80 km, while for lower altitudes the inclusion of cloud formation is necessary in order to reproduce the vertical extinction profiles and the wavelength dependence of the opacity. We include in the aerosol simulation detailed description of nucleation and condensation/evaporation processes that are also coupled with the evolution of the condensing/evaporating gases. We consider three condensates: HCN, C2H6 and CH4, which are able to reproduce the main observed characteristics. Between 80 and 30 km, the HCN-nucleated aerosols are the main opacity source, while below 30 km the contribution of CH4 clouds is becoming more important. We consider both ice methane and liquid CH4-N2 clouds in the calculations with the latter reaching to a sub-millimeter size. Ethane clouds have a minor contribution to the opacity and their size is of a few microns.
PROJECT INVESTIGATORS:Roger Yelle
RELATED OBJECTIVES:Objective 1.1
Formation and evolution of habitable planets.
Outer Solar System exploration
Sources of prebiotic materials and catalysts
Origins and evolution of functional biomolecules
Origins of energy transduction