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

## TL;DR

This study identifies a novel two-molecule combination band in CO/N$_2$ ice, observed both in laboratory spectra and on Triton, revealing intimate mixing of these volatile molecules crucial for understanding Triton's surface and atmospheric interactions.

## Contribution

The paper reports the first detection of a two-molecule combination band involving adjacent CO and N$_2$ molecules in ice, both in laboratory samples and on Triton, providing new insights into surface composition.

## Key findings

- Detection of a 4467.5 cm$^{-1}$ (2.239 μm) band in laboratory CO/N$_2$ ice.
- Observation of the same band in Triton's spectrum indicating mixed ice.
- Implication that CO and N$_2$ are intimately mixed on Triton's surface.

## Abstract

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