The Role Of Irradiating Electrons In Icy Moon Exosphere Creation

The exospheres of Jupiter's icy galilean moons all contain O2. Since Europa is not massive enough to retain its exosphere long term, the persistence of Europa's O2 exosphere suggests it is constantly being replenished via sputtering, or the removal of surface material by irradiating particles. While Jupiter's irradiating magnetospheric particles consist of both ions and electrons, electrons make up ~90% of the total particles striking Europa. However, only a handful of experiments have studied electron-induced sputtering. Here, we investigate the importance of electrons in O2 exosphere creation on icy bodies by measuring their O2 sputtering yield. Water ice samples are irradiated with 0.75 to 10 keV electrons at temperatures relevant to icy body surfaces (14 - 125 K) and monitored by a combination of microbalance gravimetry and mass spectrometry. We use our measured O2 yields to adapt a widely used ion-induced sputtering model for irradiating electrons. Applying our model to Europa, we find the globally averaged sputtering rate of O2 from impacting electrons to be the same or possibly more than the O2 sputtering rate for all ion types combined. In contrast, the O2 in Ganymede's and Callisto's atmospheres appears to be produced mainly by irradiating ions, though electrons still likely contribute a nonnegligible amount. Regardless, our results indicate that electrons need to be considered in future exospheric models