A combination band due to a mechanism whereby a photon excites two or more vibrational modes (e.g. a bend and a stretch) of an individual molecule is commonly seen in laboratory and astronomical spectroscopy. Here, we present evidence of a much less commonly seen combination band − one where a photon simultaneously excites two adjacent molecules in an ice. In particular, we present nearinfrared spectra of laboratory CO/N 2 ice samples where we identify a band at 4467.5 cm −1 (2.239 µm) that results from single photons exciting adjacent pairs of CO and N 2 molecules. We also present a near-infrared spectrum of Neptune's largest satellite Triton taken with the Gemini-South 8.1 meter telescope and the Immersion Grating Infrared Spectrograph (IGRINS) that shows this 4467.5 cm −1 (2.239 µm) CO-N 2 combination band. The existence of the band in a spectrum of Triton indicates that CO and N 2 molecules are intimately mixed in the ice rather than existing as separate regions of pure CO and pure N 2 deposits. Our finding is important because CO and N 2 are the most volatile species on Triton and so dominate seasonal volatile transport across its surface. Our result will place constraints on the interaction between the surface and atmosphere of Triton.
On Titan, methane (CH4) and ethane (C2H6) are the dominant species found in the lakes and seas. In this study, we have combined laboratory work and modeling to refine the methane–ethane binary phase diagram at low temperatures and probe how the molecules interact at these conditions. We used visual inspection for the liquidus and Raman spectroscopy for the solidus. Through these methods, we determined a eutectic point of 71.15 ± 0.5 K at a composition of 0.644 ± 0.018 methane–0.356 ± 0.018 ethane mole fraction from the liquidus data. Using the solidus data, we found a eutectic isotherm temperature of 72.2 K with a standard deviation of 0.4 K. In addition to mapping the binary system, we looked at the solid–solid transitions of pure ethane and found that, when cooling, the transition of solid I–III occurred at 89.45 ± 0.2 K. The warming sequence showed transitions of solid III–II occurring at 89.85 ± 0.2 K and solid II–I at 89.65 ± 0.2 K. Ideal predictions were compared with molecular dynamics simulations to reveal that the methane–ethane system behaves almost ideally, and the largest deviations occur as the mixing ratio approaches the eutectic composition.
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