Titan atmosphere Eric Chassefière Service d’Aéronomie/ IPSL/ Pôle de Planétologie CNRS & Université Pierre et Marie Curie Hunt for Molecules, 19-20 September.

Titan atmosphere

Eric Chassefière

Service d’Aéronomie/ IPSL/ Pôle de Planétologie

CNRS & Université Pierre et Marie Curie

Hunt for Molecules, 19-20 September 2005, Paris

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

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.

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

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)

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.

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

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)

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…

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

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)

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).

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

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.

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

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

Cloud tracking wind measurements

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

Discrete clouds (squares)

Streak clouds (diamonds)

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

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)

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.

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

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

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?