Titan

1
Titans Atmospheric Chemistry

• Emily Schaller
• GE/AY 132
• March 2004

2
Titan Background Information

• 2nd largest satellite in Solar System (2575 km
  radius)
• Larger than Mercury (radius 2439 km)
• Orbits Saturn at a distance of 21 Saturn radii
• Rotation is tidally locked to Saturn period
  16 earth days
• Thick atmosphere with a surface pressure of 1.5
  bar.
• Main atmospheric constituents
• N2 (90-97)
• Methane (2-5)
• Argon (0-6)
• Assorted hydrocarbons nitriles (C2H2, C2H6,
  HCN,)
• H2O (8 ppb), CO2 (10 ppb)

3
Titans Atmosphere

• Photodissociation of CH4 and N2 ? haze particle
  production
• Tholin particles fall to the surface and
  methane is lost
• Methane requires replenishment (outgassing?)
• Temperature profile is similar in shape to
  Earths with an inversion layer.
• Methane is near triple point on Titan likely an
  active methane weather cycle

4
CO in Titans atmosphere

• Reaction pathway for CO
• H2O ? OH H
• OH CH3 ? CO 2H2
• OH CO ? CO2 H
• ?10 ppm CO in atmosphere
• Wong et al (2002) main reaction pathway for CO
• H2O ? 2H O
• O CH3 ? H2CO H
• H2CO ? H2 CO
• ?1.8 ppm CO in atmosphere

5
CO Infrared observations

• CO first discovered from the 3-0
  rotation-vibration band at 1.6 mm (Lutz et al.
  1983)
• Mixing ratio
• 48 ppm (100 -32)

6
CO Infrared Observations

• Absorbtion from the 1-0 vibrational band at 4.8
  mm. P10 to P19 lines are visible
• Mixing ratio 32 /- 10 ppm

Lellouch et al. 2003
7
CO Millimeter Observations

• Rotational Transition lines
• 12CO (1 - 0), 115.271 GHz
• 12CO (2 - 1), 230.538 GHz
• 12CO (3 - 2), 345.796 GHz

Vo 230.5380 GHz
(Gurwell Muhleman 2000)
8
CO Mixing Ratios
Gurwell and Muhleman 2000
32 /- 10 4.8 um Lellouch et al 2003
9
What does this mean?

• Consensus is CO has about four times the mixing
  ratio predicted by chemical modeling.
• CO is not in equilibrium and must be being
  delivered by some other process
• Meteorites (need CO/H2O .13) (Lara et al 1996)
• Outgassing from interior of planet
• Comets (one event every 100,000 years would do
  it) (Lellouch et al. 2003)
• Combination of all processes

10
Cassini/VIMS Instrument

• IR channel covers wavelength range 0.85-5.1 mm
  over 256 spectral bands.
• Goals
• Accessing the nature and distribution of
  atmospheric species and aerosols in Titans
  atmosphere.
• It will map CO in Titans lower atmosphere.

11
Conclusions

• CO is uniformly mixed in Titans atmosphere
• CO Mixing ratio is 40 ppm
• Chemical modeling predicts an equilibrium of only
  10 ppm or 1.8 ppm
• Indicates that CO is not in steady state and is
  constantly being replenished
• Replenishment could come from Meteors, Comets,
  Outgassing or combination
• Cassini/Huygens will tell us much more!