# Entrapment of CO in CO2 ice

## TL;DR

This study investigates whether CO2 ice can trap CO molecules effectively, finding that CO2 ice traps CO more efficiently than water ice under certain conditions, which impacts models of volatile distribution in planet formation.

## Contribution

It provides experimental evidence that CO2 ice can trap CO more efficiently than water ice, influencing our understanding of volatile partitioning in planet-forming disks.

## Key findings

- CO2 ice traps 40-60% of CO molecules during desorption experiments.
- CO2 ice traps CO more efficiently than H2O ice up to ~70K.
- Entrapment efficiency increases with ice thickness and CO dilution.

## Abstract

Planet atmosphere and hydrosphere compositions are fundamentally set by accretion of volatiles, and therefore by the division of volatiles between gas and solids in planet-forming disks. For hyper-volatiles such as CO, this division is regulated by a combination of binding energies, and by the ability of other ice components to entrap. Water ice is known for its ability to trap CO and other volatile species. In this study we explore whether another common interstellar and cometary ice component, CO2, is able to trap CO as well. We measure entrapment of CO molecules in CO2 ice through temperature programmed desorption (TPD) experiments on CO2:CO ice mixtures. We find that CO2 ice traps CO with a typical efficiency of 40-60% of the initially deposited CO molecules for a range of ice thicknesses between 7 and 50ML, and ice mixture ratios between 1:1 and 9:1. The entrapment efficiency increases with ice thickness and CO dilution. We also run analogous H2O:CO experiments and find that under comparable experimental conditions CO2 ice entraps CO more efficiently than H2O ice up to the onset of CO2 desorption at ~70K. We speculate that this may be due to different ice restructuring dynamics in H2O and CO2 ices around the CO desorption temperature. Importantly, the ability of CO2 to entrap CO may change the expected division between gas and solids for CO and other hyper-volatiles exterior to the CO2 snowline during planet formation.