Composition, Physical State and Distribution
of Ices at the Surface of Triton
Laura Brenneman
ASTR688R Project, 12/9/04
Triton Basics
• T ~ 38  4 K (Voyager 2, 1989)
• d ~ 30 AU, a ~ 14 RNep (icy)
• e < 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.
Polar Regions: N2 Frost
Triton’s Tenuous Atmosphere
The work of Quirico et al. (1999)
• Attempted to discern physical state of Triton’s 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.
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.
Methane Ice: None in Pure Form??
Carbon Dioxide Ice:
Pure Fraction Significant
Best-Fit Model: Two Regions
Fitting to Temperature and Grain Size
Conclusions
• Two region model of the surface works best: 55 % N2:CH4:CO,
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 β-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.
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 Triton’s 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.
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