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) T ~ 38  4 K (Voyager 2, 1989) d ~ 30 AU, a ~ 14 R Nep (icy) d ~ 30 AU, a ~ 14 R Nep (icy) e < , i ~ 159 , retro e < , i ~ 159 , retro R ~ 1353 km (Europa) R ~ 1353 km (Europa)  ~ 2 g/cm 3  ~ 2 g/cm 3 Varied terrain, cryovolcanoes Varied terrain, cryovolcanoes How was Triton formed? Still an open question… How was Triton formed? Still an open question… Similar to Pluto-Charon, KBOs. Similar to Pluto-Charon, KBOs. Understanding equation of state could yield important clues about the early solar system beyond the frost line. Understanding equation of state could yield important clues about the early solar system beyond the frost line.

Polar Regions: N 2 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). 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. 6 1-hr. integrations along latitudinal strips at ~ 99  longitude in H, K bands nm spectral resolution, s/n ~ 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 N 2. Compared with lab transmission spectra of ice crystals made in liquid phase in closed cryogenic cells  composition, T, , crystalline lattice phase of N 2. Created plane-parallel model of radiative transfer within these icy media (absorption, single and double scattering). 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. Use this to refine global model of observed spectrum  abundances and distribution of ices.

Ices Matching the Observed Spectra Ice templates used: C 2 H 2, C 2 H 4, C 2 H 6, C 3 H 8, NH 3, NO, NO 2, SO 2, CH 3 OH, CH 4, CO, CO 2, H 2 O, N 2. ? ??

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 % N 2 :CH 4 :CO, 45% H CO 2 with CH 4 ~ 0.11%, CO ~ 0.05% Two region model of the surface works best: 55 % N 2 :CH 4 :CO, 45% H CO 2 with CH 4 ~ 0.11%, CO ~ 0.05% N 2 ice embeds methane, carbon monoxide; T  35.6 K inferred since most of solid nitrogen observed is in β-phase (hexagonal). N 2 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. 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 CH 4 ice are unlikely: lead to inconsistent global spectral fits. Large concentrations of pure CH 4 ice are unlikely: lead to inconsistent global spectral fits. Still unsure of exact equation of state for the H CO 2 ice mixture: uncertainty in spectral fits. Still unsure of exact equation of state for the H CO 2 ice mixture: uncertainty in spectral fits.

Future Work Need more data to resolve the equation of state more clearly for the H CO 2 regions, to further constrain T, and to identify the new spectral features detected. Need more data to resolve the equation of state more clearly for the H CO 2 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  m range. Some of this can be accomplished nicely with future ground based observations with high signal-to-noise ratios and high spectral resolution in the  m range. More of Triton’s surface needs to be examined! 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. 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 Science 261, Cruikshank, D.P. et al Science 261, de Pater, I. and Lissauer, J. Planetary Sciences. de Pater, I. and Lissauer, J. Planetary Sciences. Quirico, E. et al Icarus 139, Quirico, E. et al Icarus 139,