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

Observation of Pluton with a stellar occultation : I0I0 D M = (2  R p / H ) 1/2  r = susceptibility of molecules n(r) = concentration in molecules I = (1 - D M (  r /2) dn/dr) -1 I 0 e -   = extinction along the path

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

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

T=I/I 0 = (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 ] Uniform aerosol layer, distributed with a given scale height H ('planet' radius is R p )  ( ,z) =  ext (  ) n(z) (2  R p / H ) 1/2

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

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

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 : - F Pluton ≈ 0.11 F Titan -X CH4 = 0.1 to 2 times X CH4 on Titan (Strobel et al., 1996) -H PLUTO ≈ H TITAN ≈ km (T/g ≈ cst) - Same mean molecular mass ~ kg/mole - Pressure ≈ detached haze of Titan (0.1 to 1.0 Pa) (Sicardy et al., 2003) - How to estimate the aerosol production rate on Pluto ?

Pression (Pa) 1180 km 1230 km Pluto Surface Extinction layer - Production rate of aerosols (P)? P ≈ ( F o X CH4 ) q P PLUTO ≈ P TITAN [ (F o X CH4 ) PLUTO /(F o X CH4 ) TITAN ] q P PLUTO / P TITAN ≈ to 0.04 (q=2) or to 0.22 (q=1)

d C(r, z,t) = ∂ C(r,z,t)/ ∂ t sed. dt + ∂ C(r,z,t)/ ∂ t diff. dt + ∂ C(r,z,t)/ ∂ t coag. dt Coagulation Sedimentation and diffusion Particle radius Altitude Macromolecules produced in a narrow altitude zone (z 0 ± 20 km) with a production rate (mass / surface /time) P. Microphysical model of aerosol (Cabane et al., 1992, 1993)

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

Parameters for Pluto haze layer: Spherical particles / fractal particles Production rate noted Q, a factor of P 0 = kg/m 2 /s Production zone z0 between 100 and 600 km (Monomer radius function of z 0 - NOT A FREE PARAMETER) Coagulation with charged particles (  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= = 1215 km) (cf. Elliott et al., 2003, Sicardy et al., 2003)

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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

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  = K/km S=1 with T surf =36 K  = K/km

Cloud model of Pluton (based on Mars and Titan model): Two species CO and N 2 Nucleation/condensation + collapse of the main atmosphere Temperature profile with a troposphere (  = K/km), a surface temperature ~ 36 K and a stratosphere at 110 K Wettability of CO ice and N 2 ice on aerosols > 0.9 Drop as large as 1  m are possible Work under progress …