Titan atmosphere Eric Chassefière Service d Aéronomie IPSL Pôle

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 (CH 4 absorption). • 1980 : fly-by by Voyager 1, showing a uniform orange disk due to an ubiquitous photochemical haze. Voyager image

Exobiology at Titan • Titan’s atmosphere : N 2/CH 4 irradiated by solar UV and Saturn electrons. • Similarity with early Earth and Miller/Urey experiment : CH 4/NH 3/H 2 O 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 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. Silicates : ≈ 3 g cm-3 1/2 rock-1/2 ice. Ices : ≈ 1 g cm-3 Fortes, 1999

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/mesospheric models) Flasar, Science, 2005

Atmospheric composition • Main : N 2. • Second most-abundant : CH 4 (2% at pole - 6% at equator); • Possibly 40 Ar. • Species derived by solar UV photons/ Saturn magnetospheric electrons chemistry : – H 2, CO (per mil level), C 2 H 6, C 2 H 2, C 2 H 4, C 3 H 8, HCN, HC 3 N (ppm/ ppb level) etc… – Hazes of polymers formed from molecules.

Tropospheric cycle of methane • Precipitable amount of methane : a few meters (to be compared to ≈5 cm H 2 O in Earth troposphere). • Surface humidity level : ≈0. 1 - 0. 6 (Mc. Kay et al, 1997) : comparable to Earth troposphere (H 2 O) humidity. • Lapse rate (Voyager radiooccultation) close to adiabatic, but smaller than dry lapse rate. Mc. Kay 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, 2025 km). • Pyrolysis at 600°C, then MS. m/z = 27 : Hydrogen cyanide HCN m/z = 17 : Ammonia NH 3

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 (C 2 H 6, Kostiuk et al, 2001) and microwave (HC 3 N, CH 3 CN, 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). summer Thermal wind from Cassini. CIRS temperature data (Flasar et al, 2005) 0. 2 mbars winter summer 2004 2001

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

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 superrotation (Rannou et al, 2004). winter summer 1989

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 Hourdin et al, 2004 Flasar et al, 2005 CASSINI

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? • CH 4 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 CH 4 absorption bands). • Resolution : from 10 to 180 km. Elachi et al, 2005 Bright regions High-standing (a few 100 m) Contaminated water ice (? ) Dark regions Low-standing Precipitated hydrocarbons (? ) Porco et al, 2005

Radar scatterometry/ radiometry comparison Backscatter cross-section from radar scatterometry Brightness temperature from radar radiometry (reversed grayscale) Huygens landing site SAR-dark Warm SAR-bright Cold • 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)

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?