A New Two-Molecule Combination Band as Diagnostic of Carbon Monoxide Diluted in Nitrogen Ice On Triton.

A combination band due to a mechanism whereby a photon excites two or more vibrational modes ({\it 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 near-infrared spectra of laboratory CO/N$_2$ ice samples where we identify a band at 4467.5 cm$^{-1}$ (2.239 $\mu$m) 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 $\mu$m) 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. 1