Origin of Oxygen Species in Titan's Atmosphere

Origin of Oxygen Species in Titan's Atmosphere

Origin of Oxygen Species in Titans Atmosphere
Sarah M. Hrst, Vronqiue 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 Titans 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 Titans 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 N2CH4 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 Titans atmosphere are
connected to other sources of O in the Saturnian system.

Mole fractions listed are at 150 km

Model
2

Previous Work
CO has been observed in Titans 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.

Photodissociation Rates

Chemical Reaction Rates

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)
H2O 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 + CH 3 H2O + 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 CO 2 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:
O(3P)+CH3 HCHO+H (Peak ~1100, 200 km)
CO+H2+H (Peak ~1100, 200 km)
(Peak ~200 km)

O(3P)+CH3
CO2+h CO+O(1D)

CO2 forms via:
CO+OH CO2+H (Peak ~400 km)
H2O and OH :
H2O+h OH+H (Peak ~400 km)
H2O+CH2 (Peak ~1100 km)

OH+CH3

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
H2O 9.6 x 10-9 at 400 km

Previously suggested sources of oxygen-bearing species:
H2O 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
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
0.9
cm-2

where =
po=1.43x105 dyne
k=3x107 cm2s-1

flux given by

adjusted OH and O(3P) fluxes to reproduce observed abundances of CO, CO 2 and H2O for five
values of Ko
assumed observed abundances of CO, CO2 and H2O are constant in time and model them with
steady-state solutions to our chemistry/transport model

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 H2O 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 iikely 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 H2O 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 CO 2 photolyzes to produce O that react with
CH3 producing CO, so less input O is required
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 cm2s-1. Composition could change with time in response to changing
magnetospheric conditions, these changes would be difficult to detect.

References
Atreya, S. K., T. M. Donahue, and W. R. Kuhn (1978), Evolution of a nitrogen atmosphere on Titan,Science, 201, 611-613.
Atreya, S. K. et al. (2006), Titans methane cycle, Planet. Space. Sci., 54, 1177-1187, doi:428 10.1016/j.pss.2006.05.028.429
Baines, K. H. et al. (2006), On the discovery of CO nighttime emissions on Titan by Cassini/VIMS: Derived stratospheric abundances and geological implications, Planet. Sp. Sci., 54, 1552-1562, doi:10.1016/j.pss.2006.06.020.
Coustenis, A. et al. (1998), Evidence for water vapor in Titans atmosphere from ISO/SWS data, Astron. Astrophys., 336, L85-L89.
de Kok, R. et al. (2007), Oxygen compounds in Titans stratosphere as observed by Cassini CIRS, Icarus, 186, 354-363, doi:10.1016/j.icarus.2006.475 09.016.
English, M. A. et al. (1996), Ablation and chemistry of meteoric materials in the atmosphere of Titan, Advances in Space Research, 17, 157-160.
Hansen, C. J. et al. (2006), Enceladus Water Vapor Plume, Science, 311, 1422-1425, doi:10.1126/science.1121254.
Hartle, R. E. et al. (2006), Preliminary interpretation of Titan plasma interaction as observed by the Cassini Plasma Spectrometer: Comparisons with Voyager 1, Geophys. Res. Lett., 33, 8201-+, doi:10.1029/2005GL024817.
Lara, L. M. et al. (1996), Vertical distribution of Titans atmospheric neutral constituents, J. Geophys. Res., 101, 23,261-23,283, doi:10.1029/96JE02036.
Lellouch, E. et al. (2003), Titans 5-Hm window: observations with the Very Large Telescope, Icarus, 162, 125-142, doi:10.1016/S0019-1035(02)00079- 9.
Mousis, O. et al. (2002), An Evolutionary Turbulent Model of Saturns Subnebula: Implications for the Origin of the Atmosphere of Titan, Icarus, 156, 162-175, doi:10.1006/icar.2001.6782.
Niemann, H. B. et al. (2005), The abundances of constituents of Titan s atmosphere from the GCMS instrument on the Huygens probe, Nature, 438, 779-784, doi:10.1038/nature04122.
Owen, T. (1982), The composition and origin of Titan s atmosphere, Planet. Space Sci., 30, 833-838,83 doi:10.1016/0032- 0633(82)90115- 5.
Prinn, R. G., and B. Fegley, Jr. (1981), Kinetic inhibition of CO and N2 reduction in circumplanetary nebulae - Implications for satellite composition, Astrophys. J., 249, 308-317, doi:10.1086/159289.
Toublanc, D. et al. (1995), Photochemical modeling of Titans atmosphere, Icarus, 113, 2-26, doi:10.1006/icar.1995.1002.
Vuitton, V., R. V. Yelle, and J. Cui (2007), Benzene on Titan, Submitted.
Waite, J. H. et al. (2005), Ion Neutral Mass Spectrometer Results from the First Flyby of Titan, Science, 308, 982-986, doi:10.1126/science.1110652.
Wilson, E. H., and S. K. Atreya (2004), Current state of modeling the photochemistry of Titans mutually dependent atmosphere and ionosphere, Journal of Geophysical Research (Planets), 109, 6002-+, doi:10.1029/2003JE002181.
Wong, A.S. et al. (2002), Evolution of CO on Titan, Icarus, 155, 382-392, doi:10.1006/icar.2001.6720.
Yelle, R. V., J. Cui, and I. Muller-Wodarg (2007), Eddy Diffusion and Methane Escape from Titans Atmosphere, Submitted.
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