How the Radiolytic History of H2O-ice affects electron-induced sputtering: implications for Europa’s exosphere and potential plumes

When an icy surface, like Europa, is irradiated by energetic particles, surface material can be removed via sputtering and contribute to exospheres and/or potentially complicate the detection of plumes. While magnetospheric ions are important drivers of sputtering processes on icy surfaces, we recently determined that magnetospheric electrons are also significant contributors [1, 2]. While the majority of our laboratory measurements have been on fresh H₂O-ices, we have also observed that the irradiation history of the sample can cause a significant (up to a factor of six) increase in the measured total mass sputtering yield [3]. This enhancement is also observed in ion-irradiated samples, where the enhancement is hypothesized to be due to the build-up and release of O₂ [4, 5], however the mechanism for the enhancement appears to be different in our electron-irradiated ices. Here, we present our most recent laboratory results investigating how the radiolytic history of an ice can affect electron-induced sputtering yields under a variety of laboratory conditions (temperature, energy, fluence, etc.). These results will have implications for the icy Galilean, Saturnian, and Uranian moons, as they are constantly bombarded by electrons (and ions) of various energies and experience temperature variations. Understanding how sputtering yields change with ice history is critical to accurately predicting exosphere production and magnetospheric pick-up, as well as distinguishing between H₂O ejected from the surface via sputtering or potential plumes.

 

[1] Davis, M. R., Meier, R. M., Cooper, J. F., & Loeffler, M. J. (2021). The contribution of electrons to the sputter-produced O2 exosphere on Europa. The Astrophysical Journal Letters, 908(2), L53.

[2] Carmack, R. A., & Loeffler, M. J. (2024). Energy and Temperature Dependencies for Electron-induced Sputtering from H2O Ice: Implications for the Icy Galilean Moons. The Planetary Science Journal, 5(6), 146.

[3] Meier, R. M., & Loeffler, M. J. (2020). Sputtering of water ice by keV electrons at 60 K. Surface Science, 691, 121509.

[4] Teolis, B. D., Vidal, R. A., Shi, J., & Baragiola, R. A. (2005). Mechanisms of O2 sputtering from water ice by keV ions. Physical Review B—Condensed Matter and Materials Physics, 72(24), 245422.

[5] Teolis, B. D., Shi, J., & Baragiola, R. A. (2009). Formation, trapping, and ejection of radiolytic O2 from ion-irradiated water ice studied by sputter depth profiling. The Journal of chemical physics, 130(13).