Does Titan Have an Ocean

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Does Titan Have an Ocean? Darren
Baird ESS-298 December 2, 2004
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Detection of Titans Atmosphere

• Gaseous methane atmosphere discovered by Kuiper
  in 1944
• Subsequently, additional hydrocarbons detected
• Presence of methane led to suggestions of surface
  layer of methane snow, ice, or liquid
• 4.5 B.Y. of methane photolysis could cover
  surface with 1 km of photochemical debris
• Others visualized the surface being coated with
  tar or gasoline

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Observations from Voyager I

• Defined atmospheric properties sufficiently to
  postulate existence of a deep hydrocarbon ocean
• Radio science occultation experiments fixed the
  temperature profile of the atmosphere
• Methane mole fraction (0.7) in nitrogen
  dominated surface
• Surface temperatures constrained to 92.5 - 101 K
• UV spectrometer identified nitrogen in upper
  atmosphere
• IRIS experiment identified a suite of
  hydrocarbons and nitrile emission features in
  stratosphere
• Methane, ethane, acetylene, and propane are
  abundant
• Supersaturation of latter 3 constituents at
  tropopause forms haze

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Salient Deductions from Voyager Data

• Ethane and propane are liquid at ambient surface
  temperature, while acetylene and others are solid
• Ethane flux is 5x greater than acetylene flux
• Solid acetylene more dense than liquid ethane, so
  surface deposit accumulated over age of solar
  system expected to be a liquid layer with
  sediments toward the bottom
• Methane lost by photolysis - must be a
  regeneration source
• Reservoir of pure methane at surface is a logical
  choice, but resulting saturated conditions in
  lower atmosphere inconsistent with Voyager I
  radio science occultation temperature profile
• Mixed methane-ethane ocean consistent with the
  temperature profiles
• Global ocean would have to be greater than 0.7 km
  on basis of accumulated amounts of ethane over
  the age of the solar system

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Effects of Tidal Dissipation

• Titans eccentricity is relatively high (0.03)
• Not maintained by resonances with other
  satellites
• Dissipative processes decrease eccentricity over
  time
• If k2 assumed to be 0.2 based on rigidity of
  water ice and t assumed to be age of solar system
• Q gt 200 for eccentricity to be maintained over
  age of solar system
• Requires a global ocean gt 400 m deep!
• Large basins can enhance tidal evolution
• Water-ammonia liquids could be present in the
  interior
• Interior could be highly dissipative

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Radiometry at cm Wavelengths

• Measure thermal flux and compare to inferred
  kinetic temperature
• Brightness temperature measured to be 80.4 0.6
  K, higher than all Galilean satellites except
  Callisto
• Emissivity of Titan measured to be 0.81 - 0.90
• For a global methane/ethane ocean, e 0.93 with
  dielectric constant of 1.6 - 1.8
• Observed e of 0.81 - 0.90 consistent with a
  dielectric constant of 2.3 - 3.5
• Emissivity has little or no variation with
  orbital phase
• Understanding low emissivities of Ganymede and
  Europa and how they compare with the high
  emissivities of Callisto and Titan requires
  consideration of radar data

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Radar Used to Understand Global Emissivities

• Large radar cross sections seen on Titan present
  a mystery
• A liquid hydrocarbon ocean would dissolve
  significant amounts of complex hydrocarbons and
  nitrile aerosols, but NOT enough to explain cross
  sections gt10.
• Hydrocarbon ocean containing bubbles of
  atmosphere (a frothy ocean) could have a radar
  cross section of 15, within error bars of
  observed radar cross sections.
• Potentially consistent with radio brightness
  temperature

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More Results/Mysteries from Radar and Radiometry
Data

• Ganymede, Europa, and Callisto exhibit large
  radar cross sections that reflect most of the
  signal in same sense of polarization as delivered
• Titan has opposite sense polarization except at
  bright spot
• Most of Titans surface NOT like Callisto despite
  similar cross section and e
• Most of the surface is consistent with a number
  of different materials (perhaps solid organic and
  nitrile polymers and maybe an ocean of polar
  aerosol polymers)
• --gt Titans surface inconsistent with global
  exposure of water ice and a pure global
  ethane-methane ocean
• Possibilities include dirty water ice, layers of
  solid organics and nitriles, or an ethane-methane
  ocean with reflective contaminants

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Cassini Radar Data

• Dark regions may represent areas that are smooth,
  made of radar-absorbing materials, or are sloped
  away from the direction of illumination.
• Lower (southern) edges of the features are
  brighter, consistent with the structure being
  raised above the relatively featureless darker
  background
• Possibly a cryovolcanic flow, where water-rich
  liquid has welled up from Titan's warm interior
• Area in image 150 km square, centered at 45
  north, in area not yet imaged optically

Image courtesy of NASA/JPL
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Near Infrared Spectrophotometry

• Windows exist in the near IR, between methane
  bands, through which data on surface can be
  obtained
• Surface albedo varies inversely with wavelength
• Pure liquid ethane/methane layer would produce
  albedo lt0.02 across all bands
• Water ice experiences increasing albedo with
  decreasing l, but it is too bright to match the
  data
• Surface layer of mixed water ice and hydrocarbons
  match data
• A pure, global hydrocarbon ocean can be ruled out
  safely

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Estimating Depth of Hydrocarbon Ocean

• Radar and spectrophotometry data do NOT rule out
  possibility of seas of ethane/methane
• Depth estimates driven by estimate of accumulated
  amount of liquid ethane over age of solar system
• Photochemical models attempt to reproduce
  stratospheric ethane column abundance from
  Voyager I IRIS data
• Ocean depth as low as 300 m can be contemplated,
  which would be restricted to low-lying basins

• Subsurface hydrocarbon oceans proposed
• Contained in porous ice regolith or cracks
• 1-km deep regolith sufficient to store 200
  m deep global ocean
• Clathrate model could store large amounts of
  methane in aquifer
• How could ethane move to subsurface?
• Impact stirring
• Circulation of fluids in porous media near surface

Lunine 1993
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Cassini Infrared Data from Titan Flyby
2.0 ?m
2.8 ?m
5.0 ?m

• Change in albedo at various wavelengths can be
  caused by absorption by gases, variations in haze
  or cloud opacity, or because of a change in
  surface albedo

Image courtesy of NASA/JPL/ University of
Arizona/ USGS
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Summary of Possible Titan Surface Models and
Constraints
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References

• Campbell, Donald B. Radar Evidence for Liquid
  Surfaces on Titan. Science 302 (2003) 431-434.
• De Pater, Imke. Introduction to Special Section
  Titan Pre-Cassini view. Geophysical Research
  Letters 31, (2004).
• Dermott, Stanley F., and C. Sagan. Tidal Effects
  of Disconnected Hydrocarbon Seas on Titan.
  Nature 374 (1995) 238-240.
• Flasar, F.M. Oceans on Titan? Science 221 (July
  1983) 55-57.
• Griffith, Caitlin A., Tobias Owen, and Richard
  Wagener. Titan Surface and troposphere,
  Investigated with Ground-Based Near-Infrared
  Observations. Icarus 93 (1991) 362-378.
• Lorenz, Ralph D. , and J. Lunine Titans Surface
  Reviewed the Nature of Bright and Dark Terrain.
  Planetary Space Science 45, 8 (1997) 981-992
• Lunine, Jonathan I. Does Titan Have an Ocean? A
  Review of the Current Understanding of Titans
  Surface. Reviews of Geophysics 31, 2 (May 1993)
  133-149.
• Ori, Gian Gabriele, et al. Fluid Dynamics of
  Liquids on Titans Surface. Planetary Space
  Science 46, 9/10 (1998) 1417-1421.
• Sears, William D. Tidal Dissipation in Oceans on
  Titan. Icarus 113 (1995) 39-56.
• http//photojournal.jpl.nasa.gov/catalog/PIA06993
• http//photojournal.jpl.nasa.gov/catalog/PIA06405

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Backup Slide

• Alternate sources of methane
• Volcanism from interior
• Upper mantle may contain large amounts of methane
• Stored as clathrate hydrate or in crustal
  aquifer-like system
• If surface reservoir exists, it is mixed with
  more ethane over time
• Exogenic supply by impacts
• Not enough methane to be sole re-generation
  mechanism
• Freezing point of ethane and methane is 91 K,
  close to surface temperature
• Eutectic minimum melting point at
  72K for 0.7 mole
  fraction of methane
• Dissolved nitrogen lowers freezing
  point well below
  surface temperatures
  even for methane-rich models