Origin of Oxygen Species in Titan’s Atmosphere
Sarah M. Hörst, Véronique Vuitton, Roger V. Yelle
Lunar and Planetary Laboratory, University of Arizona
[email protected]
Introduction
Results
The Saturnian system is oxygen rich. Sources include the rings and satellites, especially
Enceladus [Hansen et al. 2006]. Recently, the Cassini Plasma Spectrometer (CAPS) detected O+
precipitating into Titan’s atmosphere [Hartle et al. 2006]. Other recent Cassini results have made
it clear that surprisingly complex molecules are synthesized in Titan's upper atmosphere [Vuitton
et al. 2007]. The possibility that oxygen could be incorporated into organic molecules of this
complexity through natural atmospheric processes is quite exciting. Additionally, the origin of
the CO, CO2, and H2O observed in Titan’s atmosphere is unknown. Thermochemical
considerations imply that the main nitrogen- and carbon-bearing species in the primordial solar
nebula were either N2 and CO or NH3 and CH4 [Prinn and Fegley 1981]. The existence of an N2 -CH4
atmosphere on Titan is thus unexpected. CO plays an important role in most hypothesized
explanations. If the origin of CO on Titan could be determined, it would represent a significant
constraint on physical conditions early in the history of the solar system.
O+ Deposition Altitude
Here we investigate the fate of the observed O+ and explore the
possibility that O-bearing species in Titan’s atmosphere are
connected to other sources of O in the Saturnian system.
Mole fractions listed are at 150 km
Model
2
Previous Work
Photodissociation Rates
Chemical Reaction Rates
CO has been observed in Titan’s atmosphere using numerous telescopes and Cassini CIRS and
VIMS. Though early discrepancies in the observations indicated that the CO abundance varies
with altitude, more recent observations, including those by Cassini, indicate that the CO mole
fraction is constant with altitude at 5 x 10-5. Observations from Voyager 1 & 2, ISO and CIRS
indicate that the CO2 abundance is roughly constant from equator to pole and constant with
altitude above the condensation level with a value of 1.5 x 10-8. The globally averaged abundance
of H2O was determined by ISO, 8 x 10-9 at 400 km. It was not detected by CIRS.
Summary of observations:
• CO 5 x 10-5 at 150 km (e.g. de Kok et al. 2007)
• CO2 1.5 x 10-8 at 150 km (e.g. de Kok et al. 2007)
• H2 O 8 x 10-9 at 400 km, globally averaged (Coustenis et al. 1998)
Previous photochemical models have been unable to simultaneously reproduce the observed
abundances of CO, CO2 and H2O. Their difficulties were complicated by the use of a reaction
whose products were poorly understood. Wong et al. [2002] reviewed laboratory experiments and
concluded that the reaction between CH3 and OH does not produce CO as assumed by all
previous models. Instead the reaction proceeds as OH + CH3 → H2 O + CH2 , recycling the water
that was destroyed by photolysis instead of forming CO [Wong et al. 2002]. The realization that
OH from micrometeorite ablation is not the source of CO and CO2 led later models to use another
source of CO or to fix the CO abundance to observations. The previous photochemical models
are summarized in Table 2.
Results shown are for Model 2
Oxygen-bearing Species
•CO forms via:
3
O( P)+CH3 → HCHO+H (Peak ~1100, 200 km)
O(3P)+CH3 → CO+H2+H (Peak ~1100, 200 km)
CO2+hν → CO+O(1D) (Peak ~200 km)
• CO2 forms via:
CO+OH → CO2+H (Peak ~400 km)
• H2 O and OH :
H2O+hν → OH+H (Peak ~400 km)
OH+CH3 → H2O+CH2 (Peak ~1100 km)
Calculated abundances from Model 2:
• CO 5 x 10-5 at 150 km, constant because efficiently redistributed and does not condense
• CO2 1.5 x 10-8 at 150 km, condenses at low altitudes, diffusively separated at high altitudes
• H 2 O 9.6 x 10-9 at 400 km
Previously suggested sources of oxygen-bearing species:
• H2 O from micrometeorite ablation (e.g. Yung et al. 1984, English et al. 1996)
• CO from the surface (volcanic outgassing, ocean source) (Lara et al. 1996, Baines et al. 2006)
• CO from micrometeorite ablation (Lara et al. 1996)
• CO from episodic resupply by cometary impacts (Lellouch et al. 2003)
• CO is primordial (Wong et al. 2002, Wilson and Atreya 2004)
The Model
• hydrocarbon network- 40 species, ~130 neutral-neutral reactions and ~40 photodissociations
(reaction list from Vuitton et al. 2007)
• 10 oxygen species, 32 neutral-neutral reactions
• calculated oxygen ion deposition altitude using theoretical stopping cross sections
Discussion and Conclusions
The observed densities of CO, CO2 and H2O in Titan's atmosphere can be explained by a
combination of O and OH or H2 O input to the upper atmosphere. Given the detection of
O+ precipitation into Titan's upper atmosphere, it is no longer necessary to invoke
outgassing from Titan's interior as the source for atmospheric CO. Instead, a more likely
source is Enceladus.
• Input of O alone produces only CO which lacks an effective loss process thus steady-state
solutions are not possible. Input of OH or H2 O alone does not produce CO and only produces
CO2 if CO is already present
• Larger values of K require larger fluxes because the molecules formed in the upper atmosphere
are transported to the loss region in the lower atmosphere more quickly
• Necessary ratio of OH flux to O flux decreases with increasing eddy coefficient. The best
example is Model 1 where significantly less O flux is required and the ratio of OH flux to O flux is
much larger than the other models. This occurs because sluggish eddy mixing builds up large
CO2 densities in the lower atmosphere. The CO2 photolyzes to produce O that react with CH3
producing CO, so less input O is required
• 1 keV oxygen deposited at ~1100 km
• assume neutral once deposited, final charge state is O(3P)
• temperature profile based on HASI, GCMS, CIRS, INMS and interpolation of Yelle et al. 2007
• eddy diffusion profile
where γ = 0.9
po=1.43x105 dyne cm-2
k =3x107 cm2s-1
• Unlikely that the input fluxes are constant with time, vertical transport time for minor
constituents in Titan's atmosphere approximately by Ha2/Ko, which has a value of 1000 years for
Ha= 30 km and Ko= 200 cm2 s-1. Composition could change with time in response to changing
magnetospheric conditions, these changes would be difficult to detect.
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