Haze and cloud in Pluto atmosphere

1. Haze and cloud in Pluto atmosphere Pascal Rannou, Franck Montmessin Service d'Aéronomie/IPSL, Université Versailles-St-Quentin

2. e- = extinction along the path Observation of Pluton with a stellar occultation : I0 I = (1 - D M (r /2) dn/dr)-1I0 M= (2  Rp / H )1/2 r = susceptibility of molecules n(r) = concentration in molecules D

3. Elliott et al. (2003) Sicardy et al. (2003)

4. Sicardy et al. (2003) Elliott et al. (2003) Refraction Extinction

5. Uniform aerosol layer, distributed with a given scale height H ('planet' radius is Rp) (,z) = ext () n(z) (2  Rp / H )1/2 T=I/I0 = (1 - D M (r /2) dn/dr)-1 e- The refraction can be estimated to about ~0.1, then the extinction by the layer is inferred using: ( ) = - ln [(1 - D M (r /2) dn/dr) T]

6. Tholins Khare et al. (1984) eff = 0.3 + + + + + + +

7. Tholins Khare et al. (1984) eff = 0.3 o o o o o o

8. Is it possible to have such large aerosols and such a thick haze on Pluto ? And what if Pluto aerosols are fractal ? Comparison with Titan : - FO @ Pluton ≈ 0.11 FO @ Titan - XCH4 = 0.1 to 2 times XCH4 on Titan (Strobel et al., 1996) - HPLUTO≈ HTITAN ≈ 40 - 50 km (T/g≈ cst) - Same mean molecular mass ~28 10-3 kg/mole - Pressure ≈ pressure @ detached haze of Titan (0.1 to 1.0 Pa) (Sicardy et al., 2003) - How to estimate the aerosol production rate on Pluto ?

9. 1230 km 0.3 Extinction layer 1 1180 km Pluto Surface 10 Pression (Pa) 100 - Production rate of aerosols (P)? P ≈(Fo XCH4)q PPLUTO≈PTITAN [(Fo XCH4)PLUTO /(Fo XCH4)TITAN]q PPLUTO / PTITAN ≈ 1.2 10-4 to 0.04 (q=2) or 0.011 to 0.22 (q=1) 103 105

10. Microphysical model of aerosol (Cabane et al., 1992, 1993) Macromolecules produced in a narrow altitude zone (z0± 20 km) with a production rate (mass / surface /time) P. d C(r, z,t) = ∂ C(r,z,t)/∂tsed. dt +∂ C(r,z,t)/∂tdiff. dt +∂ C(r,z,t)/∂tcoag. dt Sedimentation and diffusion Altitude Coagulation Particle radius

11. 2 Cabane et al. (1993) lmfp >> r Spherical (Df=3) Fractal (Df=2)

12. Parameters for Pluto haze layer: Spherical particles / fractal particles Production rate noted Q, a factor of P0 = 3.5 10-13 kg/m2/s Production zone z0 between100 and 600 km (Monomer radius function of z0- NOT A FREE PARAMETER) Coagulation with charged particles (c in e-/m) Molecular mass = 28 a.m.u Condition T (z)= 100 K, P (z=35 km) = 0.5 Pa (cf. Sicardy et al., 2003) Tan. opacity (( )) for z=35 km (that is R=1180 +35 = 1215 km) (cf. Elliott et al., 2003, Sicardy et al., 2003)

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17. Haze on Pluto: The extinction layer due to aerosols, even fractal, is unlikely We would need aerosol production rate ~ 10 times larger than those used for Titan !! Hypothesis : the extinction layer could be rather due to clouds

18. Clouds on Pluto: Temperature profile unknown below about 30 km ! Surface temperature at about 40K Unknow composition (Nitrogen + minor species as carbon monoxide and methane) Temperature gradient in troposphere Dry adiabatic = -0.56098 K/km S=1 with Tsurf=36 K = -0.083 K/km

21. Cloud model of Pluton (based on Mars and Titan model): Two species CO and N2 Nucleation/condensation + collapse of the main atmosphere Temperature profile with a troposphere ( = -0.083 K/km), a surface temperature ~ 36 K and a stratosphere at 110 K Wettability of CO ice and N2 ice on aerosols > 0.9 Drop as large as 1 mm are possible Work under progress …