An updated photochemical model of the atmosphere of Titan for astrobiology
Karen Willacy (JPL), Mark Allen (JPL/Caltech), Yuk Yung (Caltech), Run-Lie Shia (Caltech)
Introduction
A 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 ionmolecule reactions. Improvements to the code include
o the ability to model chemical equilibrium under the
appropriate temperature/pressure conditions
o low temperature chemistry
o extended hydrocarbon chemical network
o 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.
Sublimation and condensation
 Condensation of molecules onto solid particles is important
in many environments including the interstellar medium and
planetary atmospheres.
A stable method of modeling
condensation in Titan’s atmosphere
Condensation model results
 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:
 It alters gas phase composition and may allow new
molecules to form as a result of reactions on grain surfaces.
dn(X)/dt= Sx σ ng vx (n(X) - nsat(X) θ) molecules cm-3s-1
 Molecules can be returned to the gas via sublimation
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).
Molecules&
sublimate&
Gas&phase&&
molecules&
hit&surface&of&&
dust&grains&
Adsorbed&
molecules&can&
travel&across&
grain&surface&
 Condensation mainly important below 600 km.
Grain&surface&
Molecules&can&
react&forming&new&
species&
Fig 1: Schematic of interaction of gas molecules with grain
surface.
 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.
References
Allen, M., Yung, Y. & Waters, J. (1981) JGR, 86, 3617
Lavvas, P. et al. (2008) Planet. Space Sci., 56, 27
Wilson, E. & Atreya, S. (2004) GJR, 109, 6002
Yung, Y., Allen, M. & Pinto, J. P. (1984), ApJS, 55, 465
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%.
Summary
 We have implemented numerically stable, continuous method
of treating condensation on to grains in Titan’s atmosphere.
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.
 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.
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.