Rebecca A. Carmack | Searcharxiv

arXiv2026

A Possible Complication of Using the 4.14 μm band to Assess D/H Ratios on Icy Bodies

Determining isotopic ratios on the surfaces of extraterrestrial objects can provide insight into their formation conditions and evolution. Recently, remote-sensing detections of the 4.14 μm HDO absorption band have enabled astronomers to estimate the D/H ratio on several icy objects in the outer solar system. Here, we quantify how ice phase and radiation processing affect the 4.14 μm absorption band. While the band is visible in H2O-ice samples with a crystalline component, we cannot detect it in amorphous H2O-ice. Furthermore, we find that radiation processing, using 10 keV electrons as a proxy, quickly makes this absorption feature undetectable at a rate consistent with the amorphization of an initially crystalline sample. Interestingly, we also find that recrystallizing the irradiated sample causes this band to reappear nearly to its original band depth. For the Saturnian satellite Mimas, we estimate that the HDO feature will decrease by a factor of two within ~1 x 10^4 yrs at typical depths probed by remote sensing spectroscopy. While we suspect that existing methods could still determine the D/H ratio from a surface composed entirely of crystalline H2O-ice, deriving it for surfaces with a significant amorphous fraction may lead to severe underestimation of the true D/H ratio. However, given the direct correlation between the amorphous fraction of the sample and the HDO band depth, we propose that one could still estimate the D/H ratio of a surface with mixed phases by determining the crystalline fraction of the surface H2O-ice using other absorption features.

astro-ph.EP↗

arXiv2024

Energy and temperature dependencies for electron-induced sputtering from H$_2$O-ice: Implications for the icy Galilean moons

To better assess the role that electrons play in exosphere production on icy-rich bodies, we measured the total and O$_2$ sputtering yields from H$_2$O-ice for electrons with energies between 0.75 and 10 keV and temperatures between 15 and 124.5 K. We find that both total and O$_2$ yields increase with decreasing energy over our studied range, increase rapidly at temperatures above 60 K, and that the relative amount of H$_2$O in the sputtered flux decreases quickly with increasing energy. Combining our data with other electron data in literature, we show that the accuracy of a widely used sputtering model can be improved significantly for electrons by adjusting some of the intrinsic parameter values. Applying our results to Europa, we estimate that electrons contribute to the production of the O$_2$ exosphere equally to all ion types combined. In contrast, sputtering of O$_2$ from Ganymede and Callisto appears to be dominated by irradiating ions, though electrons still likely contribute a non-negligible amount. While our estimates could be further refined by examining the importance of spatial variations in electron flux, we conclude that, at the very least, electrons seem to be important for exosphere production on icy surfaces and should be included in future modeling efforts.

astro-ph.EP↗