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Does Titan Have an Ocean? Darren Baird ESS-298 December 2, 2004

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Does Titan Have an Ocean? Darren Baird ESS-298 December 2, 2004. Detection of Titan’s 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

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Does Titan Have an Ocean?

Darren Baird

ESS-298

December 2, 2004

detection of titan s atmosphere
Detection of Titan’s 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
observations from voyager i
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
salient deductions from voyager data
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
effects of tidal dissipation
Effects of Tidal Dissipation
  • Titan’s 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 > 200 for eccentricity to be maintained over age of solar system
      • Requires a global ocean > 400 m deep!
        • Large basins can enhance tidal evolution
      • Water-ammonia liquids could be present in the interior
        • Interior could be highly dissipative
radiometry at cm wavelengths
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
radar used to understand global emissivities
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 >10%.
    • 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
more results mysteries from radar and radiometry data
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 Titan’s 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)
  • --> Titan’s 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
cassini radar data
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

near infrared spectrophotometry
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 <0.02 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
estimating depth of hydrocarbon ocean
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

cassini infrared data from titan flyby
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

references
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 “Titan’s 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 Titan’s Surface.” Reviews of Geophysics 31, 2 (May 1993): 133-149.
  • Ori, Gian Gabriele, et al. “Fluid Dynamics of Liquids on Titan’s 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
backup slide
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
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