1. Titan’s Photochemical Model: Oxygen Species and Comparison with Triton and Pluto Vladimir Krasnopolsky Initial data: N 2 and CH 4 densities near the surface Initial data: N 2 and CH 4 densities near the surface Products: vertical profiles of 83 neutral species and 33 ions from 0 to 1600 km Products: vertical profiles of 83 neutral species and 33 ions from 0 to 1600 km

2. Main Features The only after-Cassini model of coupled neutral and ion chemistry The only after-Cassini model of coupled neutral and ion chemistry Hydrocarbon chemistry is extended to C 12 H 10 Hydrocarbon chemistry is extended to C 12 H 10 Radiative transfer using the Huygens data and a code for the aggregate particles Radiative transfer using the Huygens data and a code for the aggregate particles Ion chemistry is extended to C 10 H 11 + Ion chemistry is extended to C 10 H 11 + Ambipolar diffusion and escape of ions Ambipolar diffusion and escape of ions Involves effects of magnetospheric electrons, protons, and cosmic rays Involves effects of magnetospheric electrons, protons, and cosmic rays The number of reactions is reduced to 415 with column rates for each reaction The number of reactions is reduced to 415 with column rates for each reaction

3. Calculated extinction by haze using the Huygens data, refractive indices from Khare84, and a code for aggregate particles

4. Ionization by solar EUV, magnetospheric electrons, protons, and cosmic rays

5. Calculated absorption of solar EUV and UV on Titan (λ in nm)

7. Oxygen species formed by meteorite H 2 O and magnetospheric O +

8. Production of haze (100 m/Byr total)

9. Calculated and observed ionospheric profiles

11. Three bodies with N 2 -CH 4 atmospheres: Titan, Triton, and Pluto Titan 1.5 bar, Triton 40 μbar, Pluto 15 μbar. Why are they so different? Titan 1.5 bar, Triton 40 μbar, Pluto 15 μbar. Why are they so different? Titan at 10 AU, Triton at 30 AU, Pluto at 30-50 AU Titan at 10 AU, Triton at 30 AU, Pluto at 30-50 AU Titan T = 94 K, Triton T = 40 K, Pluto T = 38 -29 K Titan T = 94 K, Triton T = 40 K, Pluto T = 38 -29 K Titan N 2 is completely in the atmosphere, and N 2 is in surface ice on Triton and Pluto Titan N 2 is completely in the atmosphere, and N 2 is in surface ice on Triton and Pluto

13. Triton: mostly atomic composition (Krasnopolsky and Cruikshank 1995)

14. Pluto: molecular composition. [N] Triton /[N] Pluto ≈ 10 4 (Krasnopolsky and Cruikshank 1999)

15. Triton’s ionosphere: atomic ions, e max = 3x10 4 cm -3 (Krasnopolsky and Cruikshank 1995)

16. Pluto’s ionosphere: molecular ions, e max = 800 cm -3 (Krasnopolsky and Cruikshank 1999)

17. Why are Triton and Pluto so different? Conclusion: Triton still keeps Voyager-type chemistry