3D Modelling of theatmosphere of Pluto andTritonFrançois Forget, Mélanie Vangvichith,
Tanguy Bertrand, Jeremy Leconte
Laboratoire de Météorologie Dynamique (LMD), CNRS,Paris, France1

TritonAtmospheres in the solar system

Pluto•Terrestrial atmospheres
TitanMars
EarthMercuryVenus

PLUTO

Stellar Ocultation (Pluto• A surface covered by N2, CH4, CO
ices + “Tholins”• A strong seasonal cycle– 1 year pluto = 247.7 Earth year
– Obliquity = 122°– Perihelion = 29.6 AU/Aphelion=49.3 AU• A thin Nitrogen atmosphere.
Currently :– Ps ~2 – 5 Pa (TBC)– [CH4] ~0.5% (Lellouch et al. 2009)(R~1200 km)

New Horizons
478 kgDecembre 2005

New Horizons
Flyby
July 14, 2015

6

New Horizon Payload(atm. Observations)

2 x atmospherictemperature profiles

2 x solar
occulation profiles(Temperature, aerosols)

Airglow

Haze & surface
imaging

Map surface ice & surface
temperatures + hazes

The LMD Pluto Global climate model
1) 3D Hydrodynamical core-> to compute large scale atmosphere motions and transport- Grid point model  Typical resolution of 170 km- 25 vertical layers (10m, 20m, 40m,…160km)

2) Physical parametrizations-> to force the dynamic-> to compute the details of the local climate
Flux from thermosphere(negligible on Pluto)

With radiative transfer through CH4 and CO(with NLTE effects)

Possible
Atm N2 + CH4 condensation
Surface N2 + CH4
condensation/sublimation

T(z)Turbulence

Subsurface conduction (18 layers)

Thermal molecularconduction
Convection

Internal heat flux

“Classical” parametrizations• Thermal conduction in the high atmosphere :– Semi-implicite scheme– Flux assumed to negligible at the top• Subsurface conduction– Semi-implicite scheme– 22 layers to account for Pluto timescales heat waves• Turbulent mixing in the Boundary layer :– Turbulent closure scheme based on Mellor and Yamada 2.5
parameterisation :9Condensation/sublimation of Nitrogen• Surface condensation & sublimation- N2 condenses or sublime to conserve energy at N2 frost point…
- special parameterization in sigma coordinates : (Forget et al, 1998)→ Condensation:– the entire atmosphere is “pumped” into the ground.
– “warm” air must be cooled→ Sublimation : Cold and still air injected in the atmosphereCondensation

Bare Ground

Sublimation

10

Gravitational tides from Charon
Navier-Stokes equation :DvP  Cori olis  Diffusion  VtideDt

Vtide : gravitational potential caused by Charon's tides

7

3
1
1
2
22Vtide  V0  cos l cos(2f  nt)  cos l cos(2f  nt)   cos l   cosnt 482
8
2GM Charon  RPluto V0   3ea a a =distance Charon-Pluto,n = orbital angular velocitye = excentricity
l = latitude
f = longitude

Vwind < 1E-5 m/s
dps=2E-2Pa-> Gravitational tides are
assumed to be Negligible
11Modelling the Methane cycle??

Dilution by
subliming N2

microphysics ofN2-CH4 mixture
neglected

12

The key assumption: Initial map ofsurface deposits
We mostly assume the multi-terrain description of Pluto surface from
Lellouch et al. 2000 (consistent with visible imaging, lightcurves, and visible
and near-infrared spectroscopy).

•Diurnal Thermal
inertia set to 20 SI.

•Seasonal thermal
inertia set to 500 USI•

Emissivity = 0.9 for
all the three units•

Map with Buie et al,(1997) convention(Summer in thenorthern hemispherein the 21st century).

CH4 ice : Alb = 0.408
Tholin/H2O mix Alb=0.100
N2 ice: Alb=0.657
13

A challenge : longtimescales• Atm. Radiative timescale(10-20 years)• seasonal heat storage inthe subsurface
 We start our simulationin 1980 with a “realistic”
atmospheres, surface
and subsurface
temperatures, until 2015
 Converged results after~20 years (2008-2015) .

V14

Pluto’s Diurnal cycle in 2015
T(K)

15

Thermal structure

16Mean Temperature profile:
Comparison with observations and other models

Radius (km)GCM
1D model(Lellouch et al. 2009)

Zalucha et al.
2011

Temperature (K)17Mean Meridional Circulation
Mass streamfunction (107 kg m-2 s-1)

18

Mean Meridional Circulation IN THE LOWER ATMOSPHERE
Mass streamfunction (107 kg m-2 s-1)

Mean Meridional Circulation
Mass streamfunction (107 kg m-2 s-1)
Thermal cell

Condensation flow

20

m/s
21

Mean zonal wind in 2015

22

Propagation of Thermal tides to the upper atmosphere(Signature detetected by stellar occulations (Toigo et al. 2010)
Temperature anomaly at 0N 0E (K)23Propagation of Thermal tides to the upper atmosphere(Signature detetected by stellar occulations (Toigo et al. 2010)
Zonal zind anomaly at 0N 0E (m/s)

Time (Pluto days)

24

Will New Horizons see Hazes
and clouds ?
• Nitrogen Ice clouds? Probably not• Methane ice clouds ?
• Photochemical organic Hazes ?

25Modelling the Methane cycle??

Dilution bysubliming N2

microphysics of N2-CH4
mixture neglected26

CH4 Column averaged volume mixing ratio

27

2015

Cloud condensation28

If one assume that CH4 ice clouds can nuclei on Pluto…
(Assuming 100 Cloud condensation nuclei per kg of air)
Map of clouds (mostly in the low, cold atmosphere)

2015 Methane ice column mean mixing ratio
Ice mixing ratio
mol/mol %29

2010 Mean Methane ice column mean mixing ratio
Ice mixing ratio
mol/mol %~ Maximum
Visible Optical
depth

30

Ice mixing ratio
mol/mol %CH4 ice Clouds could be
detected at the limb
near CO2 frost surfaces

This is sensitive toinitial surface
deposits
31

Cassini UV(false color)

Organic Hazes, like on Titan

See Rannou and Durry, 200932

Titan-likeaerosols
formations• Pathway to aerosols not very well known: [Lebonnois et al. (2002),
Wilson and Atreya (2003) and Lavvas et al (2008)]• Hypothesis for Pluto: Aerosols generated by Lyman-α
photolysis of methane (CH3, CH2 , CH  + N …)
 This provide an excellent estimation of Titan’s total aerosol
production (but not Triton?)•Scheme for Pluto GCM1) Radiative transfer of Ly-α (Solar + Interplanetary: Gladstone et ql.
2015)  precursor molecules in the upper atmosphere  Transport2) Precursor convert to aerosols with characteristic decay time scale τ (afraction of C and N is added with C/N = 2).
 Transport & Sedimentation (monomers ~40 nm, ρ = 800 kg m-3)

34

Direct conversion CH4+ hν  aerosols

35Direct conversion CH4+ hν  aerosols

ZOOM

36

CH4+ hν  precursors (τ=1.E7s)  aerosols

Polar night37

CH4+ hν  precursors (τ=1.E7s)  aerosols

38CH4+ hν  precursors (τ=1.E7s)  aerosols

Cell strongly driven by N2condensation

39

However: the winter polar cap may not yet becondensing (Olkin et al. 2014, Young 2013)
L. Young’s model• Observations— ModelTo explain the surface pressure increase detected by stellar occultations, it isnecessary to account for a high seasonal thermal inertia, which prevent nitrogen to
condense around the rotational south pole before 2050
40

Haze on a Young & Olkin high thermal inertia Pluto• High seasonal thermal inertia (2000 SI below 3 cm, 20 SI above)• Low initial subsurface temperature (37K)• no condensation in the rotational south Pressure (strongly) increases ain the recent years expected41

Organic Haze on a Young & Olkin high thermal inertia
Pluto

42

A fast volatile surface model derived
from the Global Climate Model
The atmospheric dynamics is replaced by an
instantaneous mixing
Dilution bysubliming N2??

43

surface N2 ice layer“Volatile Surface
Model simulation”(Example)
MEDIUM thermal inertia:

I =400 SI
AN2 = 0.65
ACH4 = 0.45
ε = 0.9Time (Terrestrial years)

surface CH4 ice layer

Initially 50 kg m-2 of both ices
everywhere

Time (Terrestrial years)

44

“Volatile Surface
Model simulation”(Example)
Evolution ofmodeled surfaceices(CH4 ice , N2 ice)

~Observations in~2000 using thesame plotting
system45

Some resultswith Triton’ GCM

Triton seen by Voyager 2, 1989

TRITON ATMOSPHERE• Atmosphere: N2 + 0.02 % CH4• Surface pressure (1989) ~1.4 Pa, rising• Some things we know about
Triton in 1989
– Surface dark streaks direction :eastward surface wind in thesouthern hemisphere– Geyser like Plumes• Westward wing ad ~8 km in thesouthern hemisphere• Tropopause around ~8 km• What we don’t know well:– Surface frost distribution (N2, CO,CH4, …)

New calculation of
Triton seasonalvariations required
to compute cap
evolution…
(Forget et al. 2000)

Triton free atmosphere processes : hypothesis• No radiative transfer below 40 km (Yelle et al.
• Conduction : Temperature below 40 km insensitive to thermosphere variations(400 km)Constant flux from thermosphere
Example of temperature profiles
400 kmTminTmean

Diurnal cycle

60 km
40 km

Tmax

Triton General Circulation Model1) Hydrodynamical code to compute large scale atmospheric motions
and transport- Grid point model- Horizontal resolution : ~200 km (32x24)- 15 vertical layers (5m, 20m, … 50 km)

2) Physical parameterizations
 to force the dynamic
 to compute the details of
the local climate

Flux fromthermosphereT(z)Thermalconduction

Atm N2 condensation
Convection
Surface N2 condensation

TurbulenceInternal heatflux
Subsurface conduction (13 layers)

Triton : atmosphere andsurface

Case 1 : Triton totally covered by N2 ice

Mean temperature profiles for three different latitudes :0°,45°N and45°S.

thermosphere• Bolometric albedo :
0.8
0.6
90°S

30°S

20°N

90°N

• Emissivity = 0.74
• Thermal inertia =50 J m-2 s-1/2 K-1• Flux géothermique nul• Ice Surface temperature = 38K• Initial temperature profile = isotherm

M. Vangvichith (LMD/ EDX)

troposphere

Surfacetemperature
inversion

Case 1 : Triton totally covered by N2 ice
Wintercondensation
Angular momentum +Streamfunction (107 kg m-2 s-1)

Retrograde winds

Summersublimation

Case 1 : Triton totally covered by N2 ice

Observations
Wind streaks Surface→retrograde
Plumeswinds
Plume (z~6-8km)( ~8km) →prograde
winds
60°S

40°S
20°S

No progradewinds(contraryRetrograde
tosurfaceobservations) winds

Not consistent with Voyager 2 winds observations
Slide 27

Triton : atmosphere and surface

AtmosphereCasesouthernhemisphereCase22: :FrostFrostfree
freesouthernhemisphere

Frost free southernhemisphere (Spenceret al. 1990)(“Dark cap model”,Hansen and Paige
1992)• Ice bolometric albedo : 0.6(dark ice)
• Ground bolometric albedo = 0.8• Emissivity = 0.5• Thermal inertia =50 J m-2 s-1/2 K-1• Flux géothermique nul• Ice Surface temperature = 38K• Initial temperature profile = isotherm

Not consistent with Voyager 2 winds observations

Triton : tmosphere and surface

Case 3 : Triton with an unfrosted equatorial band
Initialisation of the band's temperature :a serie of 1D simulations over more than 2000 terrestrial
years for different values of thermal inertia

Ingersoll et al. 1990
SURFA A• Bolometric albedo :0.8
0.690°S

30°S

20°N

90°N

• Emissivity = 0.74
• Flux géothermique nul• Ice Surface temperature = 38K• Initial temperature profile = isotherm• Initialisation of the band'stemperature ?

C T
E
1T 5
E °
M N
P
ERA
T
UR
E

IT=
50uSI

IT=1000uSI

40 K

42 K

39.7 K
38 K

IT (uSI)50

2005001000

T in 1989 (K)

38

39.7

4042

Triton : atmosphere and surface

Atmosphere

Case 3 : Triton with an unfrosted equatorial band
Case with IT=1000 uSI

Origin of prograde winds
Mean zonal winds at 7 km (centered at40°S)

Section of temperature in1989

Temperature
gradient
between theequator andsouthernlatitudes

Progradewinds
M. Vangvichith (LMD/ EDX)

Modeling of atmospheres/ices on Triton/Pluto

May 31th, 2013

Slide 31• Thank you !