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)
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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 …