# The Effective Surface Area of Amorphous Solid Water Measured by the Infrared Absorption of Carbon Monoxide

## TL;DR

This study quantifies the pore surface area of amorphous solid water in interstellar ice analogs using infrared spectroscopy and modeling, revealing how pore structure varies with temperature and its implications for astrochemical processes.

## Contribution

It provides the first direct measurement and visualization of the pore surface area of amorphous solid water, linking morphology to temperature and surface chemistry in interstellar ice analogs.

## Key findings

- Pore surface area decreases linearly with temperature up to 120 K.
- Dangling OH bonds are only on pore surfaces.
- Most pores are connected to the vacuum interface, accessible for gas adsorption.

## Abstract

The need to characterize ices coating dust grains in dense interstellar clouds arises from the importance of ice morphology in facilitating the diffusion and storage of radicals and reaction products in ices, a well-known place for the formation of complex molecules. Yet, there is considerable uncertainty about the structure of ISM ices, their ability to store volatiles and under what conditions. We measured the infrared absorption spectra of CO on the pore surface of porous amorphous solid water (ASW), and quantified the effective pore surface area of ASW. Additionally, we present results obtained from a Monte Carlo model of ASW in which the morphology of the ice is directly visualized and quantified. We found that 200 ML of ASW annealed to 20 K has a total pore surface area that is equivalent to 46 ML. This surface area decreases linearly with temperature to about 120 K. We also found that (1) dangling OH bonds only exist on the surface of pores; (2) almost all of the pores in the ASW are connected to the vacuum--ice interface, and are accessible for adsorption of volatiles from the gas phase; there are few closed cavities inside ASW at least up to a thickness of 200 ML; (3) the total pore surface area is proportional to the total 3-coordinated water molecules in the ASW in the temperature range 60--120 K. We also discuss the implications on the structure of ASW and surface reactions in the ice mantle in dense clouds.