titan atmosphere
DESCRIPTION
Titan atmosphere. Eric Chassefière Service d’Aéronomie/ IPSL/ P ôle de Planétologie CNRS & Université Pierre et Marie Curie. Hunt for Molecules, 19-20 September 2005, Paris. Voyager image. First observations of Titan’s atmosphere. Discovery in 1655 by Christiaan Huygens. - PowerPoint PPT PresentationTRANSCRIPT
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Titan atmosphere
Eric Chassefière
Service d’Aéronomie/ IPSL/ Pôle de PlanétologieCNRS & Université Pierre et Marie Curie
Hunt for Molecules, 19-20 September 2005, Paris
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First observations of Titan’s atmosphere
• Discovery in 1655 by Christiaan Huygens.
• Observation of center-to-limb darkening by José Comas Solà (1900’s), 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. Voyager image
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Exobiology at Titan
Voyager image
• Titan’s 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.
Miller’s 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 >Mercury).
• Diurnal/annual cycles :– Titan’s 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) Flasar, Science, 2005
Composite thermal profile (Voyager/Cassini-CIRS/mesospheric models)
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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
Earth’s 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 Titan’s 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 600°C, then MS.
m/z = 27 : Hydrogen cyanide HCN
m/z = 17 : Ammonia NH3
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Titan’s 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)
summer winter
2001
2004
winter0.2 mbars
summer
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Doppler measurements
• During Titan’s 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)
Discrete clouds (squares)
Streak clouds (diamonds)
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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).
summerwinter
1989
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Latitudinal gradients of chemical species
Hourdin et al, 2004Flasar et al, 2005
VOYAGER
CASSINI
• Chemical species are also enhanced at winter pole due to :– Meridional circulation.– Presence of polar vortex (low temperatures,
dynamical isolation like on Earth)
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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.
Bright regionsHigh-standing (a few 100 m)Contaminated water ice (?)
Dark regionsLow-standingPrecipitated hydrocarbons (?)
Porco et al, 2005
Elachi et al, 2005
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Radar scatterometry/ radiometry comparison
• 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)
Brightness temperature from radar radiometry (reversed gray-scale)
Backscatter cross-section from radar scatterometry
SAR-brightSAR-dark
ColdWarm
Huygens landing site
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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?