The Phase Composition of Triton's Polar Caps

How obliquity has differently shaped Pluto's and Triton's landscapes and climates

Triton and Pluto are believed to share a common origin, both forming initially in the Kuiper Belt but Triton being later captured by Neptune. Both objects display similar sizes, densities, and atmospheric and surface ice composition, with the presence of volatile ices N2, CH4, and CO. Yet their appearance, including their surface albedo and ice distribution strongly differ. What can explain these different appearances? A first disparity is that Triton is experiencing significant tidal heating due to its orbit around Neptune, with subsequent resurfacing and a relatively flat surface, while Pluto is not tidally activated and displays a pronounced topography. Here we present long-term volatile transport simulations of Pluto and Triton, using the same initial conditions and volatile inventory, but with the known orbit and rotation of each object. The model reproduces, to first order, the observed volatile ice surface distribution on Pluto and Triton. Our results unambiguously demonstrate that obliquity is the main driver of the differences in surface appearance and in climate properties on Pluto and Triton, and give further support to the hypothesis that both objects had a common origin followed by a different dynamical history.

Inference of a "Hot Ice" Layer in Nitrogen-Rich Planets: Demixing the Phase Diagram and Phase Composition for Variable Concentration Helium-Nitrogen Mixtures Based on Isothermal Compression

Van der Waals (vdW) chemistry in simple molecular systems may be important for understanding the structure and properties of the interiors of the outer planets and their satellites, where pressures are high and such components may be abundant. In the current study, Raman spectra and visual observation are employed to investigate the phase separation and composition determination for helium-nitrogen mixtures with helium concentrations from 20 to 95% along the 295 K isothermal compression. Fluid-fluid-solid triple-phase equilibrium and several equilibria of two phases including fluid-fluid and fluid-solid have been observed in different helium-nitrogen mixtures upon loading or unloading pressure. The homogeneous fluid in helium-nitrogen mixtures separates into a helium-rich fluid (F₁) and a nitrogen-rich fluid (F₂) with increasing pressure. The triple-phase point occurs at 295 K and 8.8 GPa for a solid-phase (N₂)₁₁He vdW compound, fluid F₁ with around 50% helium, and fluid F₂ with 95% helium. Helium concentrations of F₁ coexisted with the (N₂)₁₁He vdW compound or δ-N₂ in helium-nitrogen mixtures with different helium concentrations between 40 and 50% and between 20 and 40%, respectively. In addition, the helium concentration of F₂ is the same in helium-nitrogen mixtures with different helium concentrations and decreases upon loading pressure. Pressure-induced nitrogen molecule ordering at 32.6 GPa and a structural phase transition at 110 GPa are observed in (N₂)₁₁He. In addition, at 187 GPa, a pressure-induced transition to an amorphous state is identified.

Highly repulsive interaction in novel inclusion D2-N2 compound at high pressure: Raman and x-ray evidence

We present spectral and structural evidences for the formation of a homogeneous cubic δ-N(2)-like, noncrystalline solid and an incommensuratelike hexagonal (P6(3)22) inclusion compound (N(2))(12)D(2), formed by compressing a nitrogen-rich mixture to 5.5 and 10 GPa, respectively. A strong repulsive coupling in (N(2))(12)D(2) is evident from a blue shift, discontinuous changes, and the absence of turnover of the D(2) vibron to 70 GPa--all in sharp contrast to both pure D(2) and other inclusion compounds. This repulsive interaction is responsible to the observed incommensuratelike structure and large internal pressure.

Spectroscopic Determination of the Phase Composition and Temperature of Nitrogen Ice on Triton

Laboratory spectra of the first overtone band (2.1480 micrometers, 4655.4 reciprocal centimeters) of solid nitrogen show additional structure at 2.1618 micrometers (4625.8 reciprocal centimeters) over a limited temperature range. The spectrum of Neptune's satellite Triton shows the nitrogen overtone band as well as the temperature-sensitive component. The temperature dependence of this band may be used in conjunction with ground-based observations of Triton as an independent means of determining the temperature of surface deposits of nitrogen ice. The surface temperature of Triton is found to be 38.0(+2.0)(-1.0) K, in agreement with previous temperature estimates and measurements. There is no spectral evidenceforthe presence of alpha-nitrogen on Triton's surface, indicating thatthere is less than 10 percent carbon monoxide in solid solution with the nitrogen on the surface.