Titan atmosphere

1
Titan atmosphere

• Eric Chassefière
• Service dAéronomie/ IPSL/ Pôle de Planétologie
• CNRS Université Pierre et Marie Curie

Hunt for Molecules, 19-20 September 2005, Paris
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First observations of Titans atmosphere

• Discovery in 1655 by Christiaan Huygens.
• Observation of center-to-limb darkening by José
  Comas Solà (1900s), suggesting an atmosphere.
• Confirmation in 1944 by Gerald Kuiper (CH4
  absorption).
• 1980 fly-by by Voyager 1, showing a uniform
  orange disk due to an ubiquitous photochemical
  haze.

3
Exobiology at Titan

• Titans atmosphere N2/CH4 irradiated by solar
  UV and Saturn electrons.
• Similarity with early Earth and Miller/Urey
  experiment CH4/NH3/H2/H2O vapor submitted to an
  electrical discharge during one week.
• Result dark brown deposit containing several
  aminoacids (glycine, alanine) and sugars.
• Titan natural laboratory of prebiotic
  chemistry.

Voyager image
Millers interview Urey gave a lecture in
October of 1951 when I first arrived at Chicago
and suggested that someone do these experiments.
So I went to him and said, "I'd like to do those
experiments... He said the problem was that it
was really a very risky experiment and probably
wouldn't work, So we agreed to give it six
months or a year As it turned out I got some
results in a matter of weeks.
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Main Titan characteristics

• Diameter 5150 km (40 Earth size, but
  gtMercury).
• Diurnal/annual cycles
• Titans day (period of orbit around Saturn)
  15.9 days.
• Titan seasonal cycle (Saturn orbital period)
  29.4 years.
• Obliquity 27.
• Distance to the Sun 9.5 AU. Low black-body
  temperature 90 K.
• Density (from both diameter and mass) 2 g
  cm-3.
• 1/2 rock-1/2 ice.

Fortes, 1999
Silicates 3 g cm-3 Ices 1 g cm-3
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Atmospheric vertical structure

• Surface temperature 94 K (low greenhouse effect
  of 4 K).
• Above troposphere stratosphere (like on
  Earth).
• Surface pressure 1.5 bars.
• Low g (1.35 m s-2)
• 10 times more massive atmosphere than on Earth.
• Larger vertical extension than on Earth
  (stratopause at 300 km altitude instead of 50 km)

Composite thermal profile (Voyager/Cassini-CIRS/me
sospheric models)
Flasar, Science, 2005
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Atmospheric composition

• Species derived by solar UV photons/ Saturn
  magnetospheric electrons chemistry
• H2, CO (per mil level), C2H6, C2H2, C2H4, C3H8,
  HCN, HC3N (ppm/ ppb level) etc
• Hazes of polymers formed from molecules.

• Main N2.
• Second most-abundant CH4 (2 at pole - 6 at
  equator)
• Possibly 40Ar.

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Tropospheric cycle of methane

• Precipitable amount of methane a few meters (to
  be compared to 5 cm H2O in Earth troposphere).
• Surface humidity level 0.1- 0.6 (McKay et al,
  1997) comparable to Earth troposphere (H2O)
  humidity.

• Lapse rate (Voyager radio-occultation) close to
  adiabatic, but smaller than dry lapse rate.

McKay et al, 1997
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Clouds on Titan

• Clouds observed by Cassini ISS imager (Porco et
  al, Nature, 2005), but only small coverage.

South Polar cloud field -1000 km wide- (over 4
hrs)
Discrete mid-latitude clouds

• South pole clouds already observed from Earth
  (Keck telescope, 2001, Brown et al)

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Tropospheric physics on Titan

• No extensive cloud systems observed outside South
  pole (southern summer solstice conditions).
• Why clouds at South pole ?
• more heating and vertical convection? Composition
  unknown (methane ethane?).
• Why no cloud (or little cloud) elsewhere?
• Combination of low humidity (like in Earths
  deserts)/ high supersaturation conditions (little
  number of available condensation nuclei).
• But it may (must) rain sometimes on Titan (like
  in deserts).

Drainage channels, as observed by DISR (Huygens
probe, Tomasko) probably due to rains
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Photo/ electron chemistry of Titans stratosphere
and mesosphere

• Modelled profiles of a few key species, as
  compared with Voyager/IRIS (squares) and Voyager
  UVS (horizontal lines).

Wilson and Atreya, JGR, 2004
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Haze layers

• Polymerization of hydrocarbons/ nitriles through
  UV photons/ electrons.
• Small monomers form, then settle and grow by
  coagulation (fluffy, fractal micron-sized
  particles, see Cabane et al, 1997).
• ? Why layers?

ISS image (Cassini, Porco et al, 2005)
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Dynamical simulation of detached haze layer

• Particles are formed at high altitude, then
  transported by meridional circulation from summer
  (left) to winter (right) hemisphere, where the
  detached haze merged into main haze (Rannou et
  al, 2003).

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Composition of aerosols

• ACP (Aerosol Collector Pyrolyzor), coupled to
  GCMS (Israel et al, Nature, 2005).
• Two samplings (130-135 km, 20-25 km).
• Pyrolysis at 600C, then MS.

m/z 27 Hydrogen cyanide HCN m/z 17
Ammonia NH3
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Titans superrotation

• The whole Titan atmosphere rotates in the
  prograde direction faster than the planet winds
  of 100-200 m/s at 300 km altitude.
• Observed and/or inferred by different techniques
  
• Direct Doppler measurements at IR (C2H6, Kostiuk
  et al, 2001) and microwave (HC3N, CH3CN, Moreno
  et al, 2005). 100-500 km.
• Stellar occultation (central flash, giving the
  meridional shape of isodensity levels -yielding
  zonal wind-, Hubbard et al, 1993, Bouchez et al,
  2003). 200-300 km altitude.
• Tracking of tropospheric clouds (Porco et al,
  2005). 0-20 km.
• Inference from temperature field (assuming
  cyclostrophic equilibrium) (Flasar et al, 1981
  -Voyager-, 2005 -Cassini-). 100-250 km.

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Occultation measurements/ thermal winds
Observed wind profiles are compared to the
coupled dynamics-microphysics model of Rannou et
al (2004).
Thermal wind from Cassini-CIRS temperature data
(Flasar et al, 2005)
0.2 mbars
summer
winter
winter
summer
2004
2001
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Doppler measurements

• During Titans Southern summer
• 160 m/s at 300 km altitude.
• 60 m/s at 450 km altitude (first measurement).

Moreno et al, 2005
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Cloud tracking wind measurements

• Low-middle troposphere super-rotation of 20
  m/s (Porco et al, 2005)

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Why a superrotation?

• The self-rotation rate of Titan is low (period
  16 days).
• Hadley cells may develop without breaking up to
  polar regions, transporting
• Angular momentum (resulting in super-rotation,
  latitudinally smoothed by barotropic planetary
  waves).
• Chemical species and haze.

• Enhancement of the cooling rate at winter pole
  stronger meridional wind, with enhanced
  super-rotation (Rannou et al, 2004).

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Latitudinal gradients of chemical species

• Chemical species are also enhanced at winter pole
  due to
• Meridional circulation.
• Presence of polar vortex (low temperatures,
  dynamical isolation like on Earth)

VOYAGER
CASSINI
Flasar et al, 2005
Hourdin et al, 2004
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Long-term methane cycle

• Methane is converted to aerosols, which settle
  and deposit at the surface (dark regions?).
• Non-reversible cycling of methane, arising two
  major questions
• Deposited aerosols few 100 meters layer (at
  present conversion rate). Is the layer of
  sedimented organics observed?
• CH4 lifetime 107 years. What is the source of
  methane? No ocean, nor any proof of any liquid
  standing body of methane.

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Surface imaging

• Titan image at 938 nm (best window in CH4
  absorption bands).
• Resolution from 10 to 180 km.

Elachi et al, 2005
Porco et al, 2005
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Radar scatterometry/ radiometry comparison
Huygens landing site
Backscatter cross-section from radar scatterometry
SAR-bright
SAR-dark
Brightness temperature from radar radiometry
(reversed gray-scale)
Cold
Warm

• Possible explanation
• Bright/ cold areas have a high volume scattering
  (fractured and/or porous ice)
• Dark/ warm areas have a low dielectric constant
  (precipitated hydrocarbons and/or porous water
  ice)

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A possible source of methane cryovolcanism

• No ocean, neither lakes of methane at the
  surface.
• Bright circular feature, diameter 30 km (Sotin et
  al, Nature, 2005), resembling Earth volcanic
  edifices with lobate flows (the 2 wings extending
  westward).
• Release of methane by volcanoes, with subsequent
  methane rains?