Origin of Oxygen Species in Titan

1
Origin of Oxygen Species in Titans Atmosphere

• Sarah M. Hörst, Véronqiue Vuitton, Roger V. Yelle
• Lunar and Planetary Laboratory, University of
  Arizona
• horst_at_lpl.arizona.edu

• Results
• Results shown are for Model 2

Introduction 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 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 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.
• 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 CH3
  ? 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 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.
• Previously suggested sources of oxygen-bearing
  species
• H2O from micrometeorite ablation (e.g. Yung et
  al. 1984, English et al. 1996)

Photodissociation Rates
Chemical Reaction Rates
Oxygen-bearing Species

• Discussion and Conclusions
• 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
  CO2 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.

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.

• 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 where ?
  0.9 po1.43x105 dyne
  cm-2 k83x107 cm2s-1
• flux given by
• adjusted OH and O(3P) fluxes to reproduce
  observed abundances of CO, CO2 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

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, doi428 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,
doi10.1016/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,
doi10.1016/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,
doi10.1126/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-,
doi10.1029/2005GL024817. Lara, L. M. et al.
(1996), Vertical distribution of Titans
atmospheric neutral constituents, J. Geophys.
Res., 101, 23,261-23,283, doi10.1029/96JE02036.
Lellouch, E. et al. (2003), Titans 5-µm window
observations with the Very Large Telescope,
Icarus, 162, 125-142, doi10.1016/S0019-1035(02)00
079- 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,
doi10.1006/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,
doi10.1038/nature04122. Owen, T. (1982), The
composition and origin of Titan?s atmosphere,
Planet. Space Sci., 30, 833-838,83
doi10.1016/0032- 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, doi10.1086/159289.
Toublanc, D. et al. (1995), Photochemical
modeling of Titans atmosphere, Icarus, 113,
2-26, doi10.1006/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,
doi10.1126/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-,
doi10.1029/2003JE002181. Wong, A.S. et al.
(2002), Evolution of CO on Titan, Icarus, 155,
382-392, doi10.1006/icar.2001.6720. Yelle, R.
V., J. Cui, and I. Muller-Wodarg (2007), Eddy
Diffusion and Methane Escape from Titans
Atmosphere, Submitted. Yung, Y. L., M. Allen, and
J. P. Pinto (1984), Photochemistry of the
atmosphere of Titan - Comparison between model
and observations, Astrophys. J., 55, 465-506,
doi10.1086/190963