A New Two-molecule Combination Band as a Diagnostic of Carbon Monoxide Diluted in Nitrogen Ice on Triton

Abstract

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.

A Geoscientific Review on CO and CO2 Ices in the Outer Solar System

“…A major question is whether the CO molecules are diluted in N 2 ice or if the CO exists separately as a pure ice [144]. It is not surprising, however, for CO and N 2 mix as the compounds have similar volatility and are fully miscible in one another, in both liquids and solids.…”

“…It is not surprising, however, for CO and N 2 mix as the compounds have similar volatility and are fully miscible in one another, in both liquids and solids. The two species could separate through a solid-state distillation process [144]. While the geysers on Triton are dominantly N 2 outgassing, CO ices could still potentially play a role in the structure and sublimation nature of the geysers (Figure 9b).…”

Phase Diagram for the Methane–Ethane System and Its Implications for Titan’s Lakes

“…Experiments were conducted at the Northern Arizona University Astrophysical Materials Laboratory. A detailed description of this facility (Figure 1a) is given by Tegler et al (2010Tegler et al ( , 2012, Grundy et al (2011), Roe and Grundy (2012) and the Raman set-up (Figure 1b) is described by Tegler et al (2019). The cylindrical sample cell is made of aluminum alloy with sapphire windows.…”

“…Due to the significance of methane and ethane on Titan, we mapped the binary phase diagram at low temperatures using the Astrophysical Materials Laboratory at Northern Arizona University (Grundy et al, 2011;Tegler et al, 2010Tegler et al, , 2019. This included identifying the eutectic point and liquidus and solidus curves using visual inspection and Raman spectroscopy.…”

Ice Giant Systems: The scientific potential of orbital missions to Uranus and Neptune

“…Intense tidal heating following its capture could have easily melted its icy interior (McKinnon et al 1995 (Quirico et al, 1999;Cruikshank et al, 2000;Tegler et al, 2012). A 2.239 µm absorption feature suggests 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 (Tegler et al, 2019). Mapping the spatial variation of this absorption feature will constrain how the surface-atmosphere interaction affects the surface composition, and more generally its climate and geologic evolution.…”