Composition, Physical State and Distribution of Ices at the Surface of Triton

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Composition, Physical State and Distribution of
Ices at the Surface of Triton
Laura Brenneman ASTR688R Project, 12/9/04
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Triton Basics

• T 38 ? 4 K (Voyager 2, 1989)
• d 30 AU, a 14 RNep (icy)
• e lt 0.0005, i 159?, retro
• R 1353 km (Europa)
• ? 2 g/cm3
• Varied terrain, cryovolcanoes

• How was Triton formed? Still an open question
• Similar to Pluto-Charon, KBOs.
• Understanding equation of state could yield
  important clues about the early solar system
  beyond the frost line.

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Polar Regions N2 Frost
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Tritons Tenuous Atmosphere
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The work of Quirico et al. (1999)

• Attempted to discern physical state of Tritons
  surface via IR spectroscopy on UKIRT (Mauna Kea).
• 6 1-hr. integrations along latitudinal strips at
  99? longitude in H, K bands.
• 2.65 nm spectral resolution, s/n 300.
• Compared with lab transmission spectra of ice
  crystals made in liquid phase in closed cryogenic
  cells ? composition, T, ?, crystalline lattice
  phase of N2.
• Created plane-parallel model of radiative
  transfer within these icy media (absorption,
  single and double scattering).
• Use this to refine global model of observed
  spectrum ? abundances and distribution of ices.

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Ices Matching the Observed Spectra
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Ice templates used C2H2, C2H4, C2H6, C3H8, NH3,
NO, NO2, SO2, CH3OH, CH4, CO, CO2, H2O, N2.
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Methane Ice None in Pure Form??
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Carbon Dioxide Ice Pure Fraction Significant
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Best-Fit Model Two Regions
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Fitting to Temperature and Grain Size
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Conclusions

• Two region model of the surface works best 55
  N2CH4CO, 45 H20 CO2 with CH4 0.11, CO
  0.05
• N2 ice embeds methane, carbon monoxide T ?
  35.6 K inferred since most of solid nitrogen
  observed is in ß-phase (hexagonal).
• Radiative transfer model indicates grain size of
  10 cm too big for surface to be granular.
  Probably polycrystalline with large crystals.
• Large concentrations of pure CH4 ice are
  unlikely lead to inconsistent global spectral
  fits.
• Still unsure of exact equation of state for the
  H20 CO2 ice mixture uncertainty in spectral
  fits.

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Future Work

• Need more data to resolve the equation of state
  more clearly for the H20 CO2 regions, to
  further constrain T, and to identify the new
  spectral features detected.
• Some of this can be accomplished nicely with
  future ground based observations with high
  signal-to-noise ratios and high spectral
  resolution in the 1.0 - 2.5 ?m range.
• More of Tritons surface needs to be examined!
• It will also be helpful to have future missions
  for the specific purpose of observing the outer
  solar system with greater scrutiny, both in
  imaging and spectroscopy.

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References

• Cruikshank, D.P. et al. 1993 Science 261,
  742-745.
• de Pater, I. and Lissauer, J. Planetary
  Sciences.
• Quirico, E. et al. 1999 Icarus 139, 159-178.
• http//www.solarviews.com/eng/triton.htm