2013 Annual Science Report
NASA Jet Propulsion Laboratory - Titan Reporting | SEP 2012 – AUG 2013
Task 3.2: Longer Wavelength Photochemistry of Condensates and Aerosols in Titan’s Lower Atmosphere and on the Surface.
This study focuses on the condensed phase photochemistry on Titan. In particular, we focus on understanding longer wavelength photochemistry of solid hydrocarbons to simulate photochemistry that could occur based on the UV penetration through the atmosphere and on the evolution of complex organic species in astrobiologically significant regions on Titan’s surface. Here we investigate the oxygenation chemistry involving the condensed Titan’s organic aerosols with water-ice on Titan’s surface – induced by high energy photons simulating the cosmic ray induced chemistry on Titan’s surface.
We published the work on longer wavelength photochemistry of condensed organic ices in Titan’s lower atmosphere in Nature Communications this year, based on our past years work, and continued more studies on the photochemistry of acetylene trapped in dicyanoacetylene as well as acetylene trapped cyanodiacetylene. The laboratory work in 2013 was mainly focused on surface photochemistry simulations of Titan’s condensed aerosols on the surface using laboratory analogs (tholins) on which abundant molecules such as acetylene were condensed in order to simulate surface solid photochemistry involving most abundant unsaturated molecule (acetylene) on Titan. Our results in longer wavelength photochemistry of C4N2 ice prompted reexamination of UV-VIS light reaching Titan’s surface, measured by Huygens probe, showing that the flux of >350 nm light reaching Titan’s surface is not insignificant. Based on these ideas, we focused our studies more towards understanding longer wavelength photochemical activity of Titan’s surface organic simulants.
We started a collaborative work this year with Dr. Nathalie Carrasco and her student Benjamin Fleury, who brought discharge tholin samples with various degree of methane in N2 atmosphere, simulating the Titan’s atmospheric aerosols. These materials were prepared on a sapphire window, which could be directly mounted on a cryogenic system and cooled to desired cryogenic temperatures under vacuum. To begin with, we tested pure acetylene ice at 50 K by laser irradiation at 355 nm. Depletion over a long period of time (several hours) was negligible. Then we simulated accretion of acetylene (Titan’s abundant volatile) on aerosol analogs (tholins prepared under various conditions). We found that when deposited over tholins, acetylene depletion upon laser irradiation is more significant.
Further systematic studies are underway, using various laboratory-generated Titan’s aerosol analogs (which also represent Titan’s surface organic solids), now produced by Dr. Mark Smith’s lab (University of Houston) as well. We are presently analyzing the data for several publications. Our results indicate that while VUV irradiation resulted in extensive photochemical processing, longer wavelength photochemistry is slow, but not negligible in Titan’s lower atmosphere and on the solid surface. These new photochemical channels could contribute to the formation of complex organics as well as prebiotic molecules in regions of water abundance on Titan.
PROJECT MEMBERS:Murthy Gudipati
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