3Basic Titan facts IMoon of Saturn, and second largest moon in solar system (size is between Mercury and Mars)Slowly rotating (cf Venus), 1day ~ 16 Earth daysIs 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 yearInclined at 26.7° to the Sun (cf Earth’s ~23°) => Titan experiences seasons
4Large 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.
5Basic 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 O3 in Earth’s stratosphere) and creates ‘anti-greenhouse’ effect
6Titan’s atmosphereNB – 1D radiative transfer codes are able to produce matching temperature profiles by including what we know about Titan’s composition
7Why the interest? All the similarities and parallels with Earth Link into planetary evolutionCassini/Huygens missionCassini should reach Saturn on July 1st 2004, Huygens due to be released December 25th this 2004, entering Titan’s atmosphere January 14th 2005.
8Why the interest? All the similarities and parallels with Earth Link into planetary evolutionCassini/Huygens missionCassini should reach Saturn on July 1st 2004, Huygens due to be released December 25th this 2004, entering Titan’s atmosphere January 14th 2005.
9The Cassini missionCassini’s Saturn tour involves 44 close flybys of TitanInstruments used to examine Titan’s atmosphere and surface include cameras; ir, vis and uv mappers; radio science; and radar
10The Huygens probeHuygens will take 2½ hours to descend through atmosphereInstruments include those to measure atmospheric structure during descent; surface imagers; spectral radiometers; solar sensors (giving aerosol data); in situ composition analysers; surface science package
11Titan’s atmospheric structure Present understanding comes largely from Voyager observationsCassini’s 4 year + mission will only cover one Titan season, but will still greatly increase temporal and spatial coverageVoyager and Earth-based spectra indicate composition, important for explaining atmospheric T structure and past evolution
12Radiative 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 particlesUV (<400nm): Rayleigh scattering plus haze absorptionVISIBLE ( nm): haze absorption (hides surface from human eyes)
13Radiative transfer on Titan II IR (>750nm; <13,000cm¯¹): haze scattering plus strong CH4 absorption bands with windows to the surface between themAlso see many emission features (see above) from species present in stratosphere (where T increases with height)
15Photochemistry, Titan’s haze and CH4 loss Photodissociation products of N2 & CH4 recombine, form larger molecules which condense to form hazeSufficiently large particles will fall outMay act as nucleation sites for CH4 condensateSome will be ‘refractory’ => oily/solid substances which won’t re-evaporate => net loss of CH4Requires mechanism to replace CH4, or total removal estimated in tens of millions of yearsThis is significant, as the haze and most trace species are derived from CH4Surface oceans of C2H6-CH4 suggested as source and sink of CH4 cycle, but incompatible with high radar reflectivity and evidence of surface featuresAlternatives include outgassing from interior or methane clathrates
16The meridional circulation A solution with no meridional flow, and radiative equilibrium surface temperatures everywhere, exists for frictionless flowHowever, 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 latitudesHeld 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
17Equatorial superrotation (wind speeds faster than surface speed) expected awayfrom equator when conserving angular momentum (e.g.zonal jet in winter hemisphere)Superrotation at equator requires mechanism todeposit momentum hereGierasch mechanism found to be plausible in GeneralCirculation ModelsNPEQ
19Titan’s surfaceThe bright features (seen in gaps between near IR CH4 absorption bands) are thought to be regions of high IR albedo on the surfaceThe dark regions may correspond to hydrocarbon oceans
20Features 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 yearsPolar hood: during polar night, chemical species normally destroyed by photolysis build up, and temperatures fall, encouraging these and other species to condenseThe detached haze layer: this has recently been produced in general circulation models:
21Simulation of the detached haze layer From Rannou, Hourdin and McKay, Nature 2002Haze production occurs at the highest altitudes shownAway from equinox, the Hadley circulation transports haze down in altitude over the winter pole (here the northHaze is then spread out at this altitude and below, producing the main haze layer
22The 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