Biology
Scientific paper
Oct 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.3615l&link_type=abstract
American Astronomical Society, DPS meeting #42, #36.15; Bulletin of the American Astronomical Society, Vol. 42, p.1067
Biology
1
Scientific paper
Titan's aerosols result from the complex atmospheric photochemistry. The particles in the stratospheric layer have an aggregate structure with quasi-spherical monomers of 50 nm radius. Thus, a transition region is expected between a spherical growth region and a region of aggregate growth.
Usually, aerosol models assume that once particles are formed from the background photochemistry they cease to interact with the gas phase molecules, and coagulation controls any further growth. Yet, recent investigations show that chemistry at the surface of particles can significantly affect their size and structure. Titan's neutral chemistry is mainly driven by radical reactions. We investigate here the role of some of these radicals in the growth of aerosols.
We use a stochastic approach to simulate both the coagulation and surface chemistry of particles. This allows to track the evolution of a particle assemble from the initial chemical species to the final aggregate structures. The simulation starts with the benzene molecules detected in the thermosphere, which grow to PAHs through reaction with radicals. The PAHs collide and produce primary particles, which keep growing through reaction with the background gas species, and through collisions with other PAHs. The interaction of two primary particles provides an aerosol. The chemistry continues to act on the surface of aerosols controlling their shape and size.
Our results demonstrate that the evolution of aerosols in terms of size, shape and density is a result of competing processes between surface growth, coagulation and sedimentation. Particles have a spherical growth region in the upper atmosphere followed by a transition to an aggregate growth region below. The transition altitude ranges between 500 and 600 km based on the parameters of the simulation.
This work has been supported through NASA grants NNX09AP14G and NNX09AB58G and NASA's Astrobiology Initiative through JPL subcontract 1372177 to the University of Arizona.
Imanaka Hiroshi
Kraft Markus
Lavvas Panayotis
Sander Michael
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