Summary We have implemented numerically stable, continuous method

of treating condensation on to grains in Titan’s atmosphere.

Our model can establish the partitioning of carbon in the material settling to the surface and the relative upper atmosphere/lower atmosphere source for this material.

Similar to kinetic regime (where mean free path is large compared to size of grains) used in Earth atmosphere models.

Future improvements will extend model to include continuum regime below 65 km.

An updated photochemical model of the atmosphere of Titan for astrobiology

Acknowledgments: Support was provided by the NASA Astrobiology Institute team ’Titan as a prebiotic chemical system’. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

Karen Willacy (JPL), Mark Allen (JPL/Caltech), Yuk Yung (Caltech), Run-Lie Shia (Caltech)

IntroductionA new comprehensive Titan model has been developed as part of the NASA Astrobiology Institute JPL-Titan team effort to understand the complexity of organic chemistry in the combined Titan atmosphere/surface system. The code is based on previous work (e.g., Allen et al., 1981; Yung et al., 1984; with updates) and has been improved to answer increasingly more complex questions on the chemical and dynamical processes responsible for the spatial and temporal distribution of chemical species in planetary atmospheres. The current reaction network consists of 1266 species and over 20,000 reactions including photolysis, neutral-neutral and ion-molecule reactions. Improvements to the code include

o the ability to model chemical equilibrium under the appropriate temperature/pressure conditions

o low temperature chemistryo extended hydrocarbon chemical networko condensation/sublimation processes

Here we present preliminary results from a model of Titan’s atmosphere, focusing on the condensation of organics and the nature of material settling to the surface.

A stable method of modeling condensation in Titan’s atmosphere We have implemented a continuous and numerically stable

method for treating condensation and sublimation that is easily incorporated into the chemical kinetics equations

o Condensation rate coefficient, kf, for molecule X determined from collision rates of molecules with grains

kf = SX σ ng vX

(SX = sticking coefficient, σ = surface area of grain

ng = number density of grains, vX = gas phase velocity)

o Sublimation rate coefficients, kd, determined from saturated vapor pressures (measured when condensation and sublimation are in equilibrium over pure ice)

kf nsat(X) = kd θ

For pure ice, surface coverage, θ = 1

o Similar to treatment of condensation in Earth’s atmosphere in kinetic regime (where mean free path is large compared to diameter of grain)

o Assuming kinetic regime everywhere in Titan’s atmosphere we overestimate the condensation rate below 65 km by a factor of ~ 1.6.

Net flow of molecules out of gas and onto grain:

dn(X)/dt = Sx σ ng vx (n(X) - nsat(X) θ) molecules cm-3s-1

where nsat(X) is the saturated number density for X and n(X) is its gas number density.

Method allows for reduction in sublimation rate due to mixed ices to be taken into account (where θ < 1) (Figure 2).

ReferencesAllen, M., Yung, Y. & Waters, J. (1981) JGR, 86, 3617Lavvas, P. et al. (2008) Planet. Space Sci., 56, 27Wilson, E. & Atreya, S. (2004) GJR, 109, 6002Yung, Y., Allen, M. & Pinto, J. P. (1984), ApJS, 55, 465

Sublimation and condensation Condensation of molecules onto solid particles is important

in many environments including the interstellar medium and planetary atmospheres.

It alters gas phase composition and may allow new molecules to form as a result of reactions on grain surfaces.

Molecules can be returned to the gas via sublimation

If partial pressure of a gas exceeds its saturation vapor pressure condensation will occur, otherwise sublimation can occur.

Previous Titan models, e.g. Yung et al. (1984), Wilson & Atreya (2004), Lavvas et al. (2008), switch condensation on or off depending on the partial pressure of the gas

o This can be numerically unstable, and cannot easily deal with mixtures of ices.

Condensation mainly important below 600 km.

o Some species not affected by condensation e.g. CH4

o Condensation reduces abundance of many hydrocarbons C2H2, C2H4, C2H6, CH3C2H, C3H6, C3H8,

Changes to abundance of a particular molecule driven not only by its condensation but also by the condensation of precursor molecules.

Gas phase abundances may be less than predicted from saturated vapor pressures because sublimation occurs from mixed rather than pure ices.

Condensation model results

Fig 2: Left panel shows rate of desorption from pure ice for blue molecules. In right panel equal amounts of blue and red molecules are mixed. The surface coverage of blue is therefore 50% of the pure ice case, and the sublimation rate is reduced by 50%.

Fig 1: Schematic of interaction of gas molecules with grain surface.