Laboratory evidence for efficient water formation in interstellar ices

TL;DR

This study provides laboratory evidence that water forms efficiently in interstellar ices through hydrogenation of O2, with a process that is surprisingly temperature-independent and more efficient than previously thought.

Contribution

The paper experimentally demonstrates that O2 hydrogenation is a highly efficient water formation route in interstellar ices, challenging prior assumptions.

Findings

Water formation via O2 hydrogenation is highly efficient.

The process is largely temperature-independent between 12-28 K.

Astrochemical models show this route's significance in space environments.

Abstract

Even though water is the main constituent in interstellar icy mantles, its chemical origin is not well understood. Three different formation routes have been proposed following hydrogenation of O, O2, or O3, but experimental evidence is largely lacking. We present a solid state astrochemical laboratory study in which one of these routes is tested. For this purpose O2 ice is bombarded by H- or D-atoms under ultra high vacuum conditions at astronomically relevant temperatures ranging from 12 to 28 K. The use of reflection absorption infrared spectroscopy (RAIRS) permits derivation of reaction rates and shows efficient formation of H2O (D2O) with a rate that is surprisingly independent of temperature. This formation route converts O2 into H2O via H2O2 and is found to be orders of magnitude more efficient than previously assumed. It should therefore be considered as an important channel for interstellar water ice formation as illustrated by astrochemical model calculations.