Electron-induced chemistry and sputtering of volatile species from amorphous and crystalline water ice

TL;DR

This study investigates how electron irradiation causes sputtering and chemical changes in amorphous and crystalline water ice, relevant for understanding surface processes on icy celestial bodies.

Contribution

It provides experimental measurements of electron-induced sputtering yields for various gaseous species from water ice, highlighting differences between amorphous and crystalline forms.

Findings

Crystalline ice exhibits lower sputtering yields than amorphous ice.

Sputtering yields decrease with increasing electron energy from 0.5 keV to 2 keV.

Results align with previous data for amorphous ice and expand understanding of electron interactions with water ice.

Abstract

Electron irradiation of water-rich ices plays a significant role in initiating the chemical and physical processes on the surface of airless icy bodies in radiation environments such as Europa and Enceladus, as well as on the Moon, comets, and asteroids interacting with the solar wind. The sputtering process by electrons and ions leads to chemical modification and outgassing of their icy surfaces and the subsequent formation of a tenuous atmosphere. Though electron-sputtering yields are known to be lower compared to ion-sputtering yields, one needs to also account for their differential fluxes. In our experiments, the electron-induced sputtering yields of all the gaseous species H2, O, OH, H2O, and O2 are investigated for electron energies lower than 2 keV in terms of partial pressure vs. time of irradiation. The effective averaged change in partial pressure of the desorbed species is converted to the number of sputtered atoms or molecules per second per cm2 from the ice, and then to the sputtering yields (number of species sputtered per electron). Our data agrees well with the previously reported data for the sputtering of O2 and H2O yield for the amorphous ice. We also find that crystalline ice shows significantly lower sputtering yields when compared to amorphous ice, in agreement with the observation of similar trends in the literature. Our work indicates that sputtering yields per keV of O, OH, O2, and H2O drop with increasing electron energy from 0.5 keV to 2 keV.