Titan’s Atmospheric Chemistry Emily Schaller GE/AY 132 March 2004

Titan Background Information  2 nd 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  N 2 (90-97%)  Methane (2-5%)  Argon (0-6%)  Assorted hydrocarbons & nitriles (C 2 H 2, C 2 H 6, HCN,…)  H 2 O (8 ppb), CO 2 (10 ppb)

Titan’s Atmosphere  Photodissociation of CH 4 and N 2  haze particle production  “Tholin” particles fall to the surface and methane is lost  Methane requires replenishment (outgassing?)  Temperature profile is similar in shape to Earth’s with an inversion layer.  Methane is near triple point on Titan – likely an active methane weather cycle

CO in Titan’s atmosphere  Reaction pathway for CO H 2 O  OH + H OH + CH 3  CO + 2H 2 OH + CO  CO 2 + H  10 ppm CO in atmosphere  Wong et al (2002) main reaction pathway for CO H 2 O  2H + O O + CH 3  H 2 CO + H H 2 CO  H 2 +CO  1.8 ppm CO in atmosphere

CO Infrared observations  CO first discovered from the 3-0 rotation-vibration band at 1.6  m (Lutz et al. 1983)  Mixing ratio 48 ppm ( )

CO Infrared Observations:  Absorbtion from the 1-0 vibrational band at 4.8  m. P10 to P19 lines are visible  Mixing ratio 32 +/- 10 ppm Lellouch et al. 2003

CO Millimeter Observations  Rotational Transition lines  12 CO (1 - 0), GHz  12 CO (2 - 1), GHz  12 CO (3 - 2), GHz Vo = GHz(Gurwell & Muhleman 2000)

CO Mixing Ratios Gurwell and Muhleman / um Lellouch et al 2003

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/H 2 O =.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

Cassini/VIMS Instrument  IR channel covers wavelength range  m over 256 spectral bands.  Goals: –Accessing the nature and distribution of atmospheric species and aerosols in Titan’s atmosphere.  It will map CO in Titan’s lower atmosphere.

Conclusions  CO is uniformly mixed in Titan’s 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!