Titan: an overview

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Titan an overview

• Basic facts
• Motivation
• Radiative transfer
• Photochemistry
• Dynamics
• What do we see, and can we explain it?
• Titans possible future

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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) gt always has same side to Saturn
• 674 Titan days per Titan year gt 29.5 Earth
  years per Titan year
• Inclined at 26.7 to the Sun (cf Earths 23) gt
  Titan experiences seasons

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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 doesnt
  suffer rapid Jeans escape.

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Basic Titan facts II

• 95 N2 (cf Earth)
• Psurf 1.5 Bar ( 1.5 x Earth)
• Tsurf 90K (Earth 288K)
• 4CH4 close to saturation, possibly
  supersaturated gt 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
  Earths stratosphere) and creates
  anti-greenhouse effect

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Titans atmosphere

• NB 1D radiative transfer codes are able to
  produce matching temperature profiles by
  including what we know about Titans composition

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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 Titans atmosphere January
  14th 2005.

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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 Titans atmosphere January
  14th 2005.

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The Cassini mission

• Cassinis Saturn tour involves 44 close flybys of
  Titan
• Instruments used to examine Titans atmosphere
  and surface include cameras ir, vis and uv
  mappers radio science and radar

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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

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Titans atmospheric structure

• Present understanding comes largely from Voyager
  observations
• Cassinis 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

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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 (lt400nm) Rayleigh scattering plus haze
  absorption
• VISIBLE (400-700nm) haze absorption (hides
  surface from human eyes)

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Radiative transfer on Titan II

• IR (gt750nm lt13,000cm¹) 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)

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The Yung et al. photochemical model
CH4 methane C2H2 acetylene C2H6 ethane
C2H4 ethylene C3H8 propane C4H2
diacetylene CH3C2H methylacetylene
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Photochemistry, Titans 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 gt oily/solid
  substances which wont re-evaporate gt 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

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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 v0, radiative
  equilibrium regime allowed at higher latitudes
• Held and Hous model gives the latitude at which
  the solutions intersect (the latitude to which
  the Hadley cell extends)
• fH (5/3 x g H
  ?H)½ / Oa,
• (where Htropopause height, ?Hfractional
  drop in potential temperature
  between equator and poles, Orotation rate and
  aradius)
• gt as Oa decreases, Hadley cells extend further
  polewards
• gt a nearly pole to pole Hadley cell exists
  around solstice

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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
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Limb brightening and the smile
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Titans 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

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Features strongly linked to dynamics

• North-south albedo asymmetry due to transport of
  haze to winter hemisphere by Hadley circulation.
  gt 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

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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

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The possible future of Titan

• If CH4 did eventually run out, then the
  greenhouse effect would be reduced (gt Tsurf?)
• But CH4 is the basic ingredient required for the
  haze, hence the anti-greenhouse effect would
  also be reduced (gt Tsurf?)
• However, less haze would also mean less heating
  in the stratosphere (gt Tstrat?)
• Plus no CH4 would mean no more H2 to balance that
  escaping to space, and H2 is also an important
  greenhouse gas (gt Tsurf?)
• Lower temperatures overall would eventually lead
  to N2 condensation, gt Psurf ? gt atmospheric
  collapse