Laboratory H₂O:CO₂ ice desorption data: entrapment dependencies and its parameterization with an extended three-phase model

¹ Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: fayolle@strw.leidenuniv.nl
² Harvard-Smithsonian Center for Astrophysics, MS 42, 60 Garden Street, Cambridge, MA 02138, USA
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 10 November 2010
Accepted: 21 February 2011

Abstract

Context. Ice desorption affects the evolution of the gas-phase chemistry during the protostellar stage, and also determines the chemical composition of comets forming in circumstellar disks. From observations, most volatile species are found in H₂O-dominated ices.

Aims. The aim of this study is first to experimentally determine how entrapment of volatiles in H₂O ice depends on ice thickness, mixture ratio and heating rate, and second, to introduce an extended three-phase model (gas, ice surface and ice mantle) to describe ice mixture desorption with a minimum number of free parameters.

Methods. Thermal H₂O:CO₂ ice desorption is investigated in temperature programmed desorption experiments of thin (10–40 ML) ice mixtures under ultra-high vacuum conditions. Desorption is simultaneously monitored by mass spectrometry and reflection-absorption infrared spectroscopy. The H₂O:CO₂ experiments are complemented with selected H₂O:CO, and H₂O:CO₂:CO experiments. The results are modeled with rate equations that connect the gas, ice surface and ice mantle phases through surface desorption and mantle-surface diffusion.

Results. The fraction of trapped CO₂ increases with ice thickness (10–32 ML) and H₂O:CO₂ mixing ratio (5:1–10:1), but not with one order of magnitude different heating rates. The fraction of trapped CO₂ is 44–84% with respect to the initial CO₂ content for the investigated experimental conditions. This is reproduced quantitatively by the extended three-phase model that is introduced here. The H₂O:CO and H₂O:CO₂:CO experiments are consistent with the H₂O:CO₂ desorption trends, suggesting that the model can be used for other ice species found in the interstellar medium to significantly improve the parameterization of ice desorption.