Haze and cloud in Pluto atmosphere

Pascal Rannou, Franck MontmessinService d'Aéronomie/IPSL, Université Versailles-St-Quentin

Observation of Pluton with a stellar occultation :

I0

DM = (2 R

p / H )1/2

r = susceptibility of molecules

n(r) = concentration in molecules

I = (1 - D M (r /2) dn/dr)-1I

0 e-

= extinction along the path

Sicardy et al. (2003)

Elliott et al. (2003)

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

Refraction

E

xtinction

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]

Uniform aerosol layer, distributed with a given scale height H ('planet' radius is R

p)

(,z) = ext

() n(z) (2 Rp / H )1/2

+

+ + +

+

+

+Tholins Khare et al. (1984)

eff = 0.3

Tholins Khare et al. (1984)

eff = 0.3

o

oo 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

O @ Pluton ≈ 0.11 F

O @ 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 ?

0.3

1

100

103

105

10

Pre

ssio

n (

Pa)

1180 km

1230 km

Pluto Surface

Extinction layer

- Production rate of aerosols (P)?

P ≈ (Fo X

CH4) q

PPLUTO

≈ PTITAN

[(Fo X

CH4)

PLUTO /(F

o X

CH4)

TITAN] q

PPLUTO

/ PTITAN

≈1.2 10-4 to 0.04 (q=2)

or 0.011 to 0.22 (q=1)

d C(r, z,t) = ∂ C(r,z,t)/∂t sed.

dt + ∂ C(r,z,t)/∂tdiff.

dt + ∂ C(r,z,t)/∂tcoag.

dt

Coagulation

Sedimentation and diffusion

Particle radius

Alt

itu

de

Macromolecules produced in a narrow altitude zone (z0± 20 km) with a

production rate (mass / surface /time) P.

Microphysical model of aerosol (Cabane et al., 1992, 1993)

2

Cabane et al. (1993)

lmfp >> r

Spherical (Df=3)

Fractal (Df=2)

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 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=1180 +35 = 1215 km)(cf. Elliott et al., 2003, Sicardy et al., 2003)

o

o o oo

o

o

o o oo

o

o

o o oo

o

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 = -0.56098 K/km

S=1 with Tsurf

=36 K = -0.083 K/km

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 m are possible

Work under progress …