Titan: an overview

1. Titan: an overview • Basic facts • Motivation • Radiative transfer • Photochemistry • Dynamics • What do we see, and can we explain it? • Titan’s possible future

3. Basic Titan facts I • Moon of Saturn, and second largest moon in solar system (size is between Mercury and Mars) • Slowly rotating (cf Venus), 1day ~ 16 Earth days • Is also orbit time since Titan is tidally locked (cf our moon) => always has same side to Saturn • ~ 674 Titan days per Titan year => ~ 29.5 Earth years per Titan year • Inclined at 26.7° to the Sun (cf Earth’s ~23°) => Titan experiences seasons

4. Large moons and small planets Titan is the only one to have a significant atmosphere – probably it was big and cold enough to retain ammonia when the solar system formed (as ammonia hydrate ices). Titan is sufficiently cold that the nitrogen released to form the present atmosphere doesn’t suffer rapid Jeans escape.

5. Basic Titan facts II • ~ 95% N2 (cf Earth) • Psurf ~ 1.5 Bar (~ 1.5 x Earth) • Tsurf ~ 90K (Earth ~ 288K) • ~ 4%CH4 – close to saturation, possibly supersaturated => CH4 ‘hydrological’ cycle (cf H2O on Earth) • Photochemistry is important (cf Earth, Venus) • CH4 is a ‘greenhouse gas’ (cf Earth) • Stratospheric haze absorbs solar energy (cf O3in Earth’s stratosphere) and creates ‘anti-greenhouse’ effect

6. Titan’s atmosphere NB – 1D radiative transfer codes are able to produce matching temperature profiles by including what we know about Titan’s composition

7. Why the interest? • All the similarities and parallels with Earth • Link into planetary evolution • Cassini/Huygens mission Cassini should reach Saturn on July 1st 2004, Huygens due to be released December 25th this 2004, entering Titan’s atmosphere January 14th 2005.

8. Why the interest? • All the similarities and parallels with Earth • Link into planetary evolution • Cassini/Huygens mission Cassini should reach Saturn on July 1st 2004, Huygens due to be released December 25th this 2004, entering Titan’s atmosphere January 14th 2005.

9. The Cassini mission • Cassini’s Saturn tour involves 44 close flybys of Titan • Instruments used to examine Titan’s atmosphere and surface include cameras; ir, vis and uv mappers; radio science; and radar

10. The Huygens probe • Huygens will take 2½ hours to descend through atmosphere • Instruments include those to measure atmospheric structure during descent; surface imagers; spectral radiometers; solar sensors (giving aerosol data); in situ composition analysers; surface science package

11. Titan’s atmospheric structure • Present understanding comes largely from Voyager observations • Cassini’s 4 year + mission will only cover one Titan season, but will still greatly increase temporal and spatial coverage • Voyager and Earth-based spectra indicate composition, important for explaining atmospheric T structure and past evolution

12. Radiative transfer on Titan I • In lower atmosphere, ‘greenhouse’ effect due to collision-induced absorption of thermal radiation (H2-H2, N2-CH4, etc.) and absorption in vibration-rotation bands of gases with permanent dipole moments (e.g. CH4) • In upper atmosphere, ‘anti-greenhouse’ effect due to absorption of incoming solar radiation by haze particles • UV (<400nm): Rayleigh scattering plus haze absorption • VISIBLE (400-700nm): haze absorption (hides surface from human eyes)

13. Radiative transfer on Titan II • IR (>750nm; <13,000cm¯¹): haze scattering plus strong CH4 absorption bands with windows to the surface between them • Also see many emission features (see above) from species present in stratosphere (where T increases with height)

14. The Yung et al. photochemical model CH4 = methane C2H2 = acetylene C2H6 = ethane C2H4 = ethylene C3H8 = propane C4H2 =diacetylene CH3C2H = methylacetylene

15. Photochemistry, Titan’s haze and CH4 loss • Photodissociation products of N2 & CH4 recombine, form larger molecules which condense to form haze • Sufficiently large particles will fall out • May act as nucleation sites for CH4 condensate • Some will be ‘refractory’ => oily/solid substances which won’t re-evaporate => net loss of CH4 • Requires mechanism to replace CH4, or total removal estimated in tens of millions of years • This is significant, as the haze and most trace species are derived from CH4 • Surface oceans of C2H6-CH4 suggested as source and sink of CH4 cycle, but incompatible with high radar reflectivity and evidence of surface features • Alternatives include outgassing from interior or methane clathrates

16. The meridional circulation • A solution with no meridional flow, and radiative equilibrium surface temperatures everywhere, exists for frictionless flow • However, friction requires a meridional flow (a ‘Hadley’ cell or cells) to exist within some region about the equator, with the v=0, radiative equilibrium regime allowed at higher latitudes • Held and Hou’s model gives the latitude at which the solutions intersect (the latitude to which the Hadley cell extends): φH = (5/3 x g H ΔH)½ / Ωa, (where H=tropopause height, ΔH=fractional drop in potential temperature between equator and poles, Ω=rotation rate and a=radius) • => as Ωa decreases, Hadley cells extend further polewards • => a nearly pole to pole Hadley cell exists around solstice

17. Equatorial superrotation • (wind speeds faster than surface speed) expected away • from equator when conserving angular momentum (e.g. • zonal jet in winter hemisphere) • Superrotation at equator requires mechanism to • deposit momentum here • Gierasch mechanism found to be plausible in General • Circulation Models NP EQ

18. Limb brightening and the ‘smile’

19. Titan’s surface • The bright features (seen in gaps between near IR CH4 absorption bands) are thought to be regions of high IR albedo on the surface • The dark regions may correspond to hydrocarbon oceans

20. Features strongly linked to dynamics • North-south albedo asymmetry: due to transport of haze to winter hemisphere by Hadley circulation. => darker in UV and visible (more haze absorption),brighter in IR (little absorption; mostly scattering). As expected, is observed to reverse every ~15 years • Polar hood: during polar night, chemical species normally destroyed by photolysis build up, and temperatures fall, encouraging these and other species to condense • The detached haze layer: this has recently been produced in general circulation models:

21. Simulation of the detached haze layer From Rannou, Hourdin and McKay, Nature 2002 • Haze production occurs at the highest altitudes shown • Away from equinox, the Hadley circulation transports haze down in altitude over the winter pole (here the north • Haze is then spread out at this altitude and below, producing the main haze layer

22. The possible future of Titan • If CH4 did eventually run out, then the ‘greenhouse’ effect would be reduced (=> Tsurf↓) • But CH4 is the basic ingredient required for the haze, hence the ‘anti-greenhouse’ effect would also be reduced (=> Tsurf↑) • However, less haze would also mean less heating in the stratosphere (=> Tstrat↓) • Plus no CH4 would mean no more H2 to balance that escaping to space, and H2 is also an important greenhouse gas (=> Tsurf↓) • Lower temperatures overall would eventually lead to N2 condensation, => Psurf ↓ => atmospheric collapse