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An Estimate of the Importance of Electron Sputtering at Mercury

Presentation #116.03 in the session Mercury (Poster + Lightning Talk)

Published onOct 23, 2023
An Estimate of the Importance of Electron Sputtering at Mercury

We have reinvestigated the importance for Mercury’s exosphere and ionosphere of electron stimulated desorption (ESD). Here, we use the ESD cross section measurements of McLain et al. (2011) for Na-bearing silicate glass desorbing Na+(their work was not sensitive to neutral Na emission). For the electron flux and energy in the cusp region, we use the estimates from Lindsay et al. (2011) who found fluxes on the order of 109 electrons cm-2 s-1, with energies of hundreds of eV. Their estimated electron fluxes and energies, and footprints of electron precipitation are based on X-ray fluxes seen by the X-Ray Spectrometer, XRS, onboard the MESSENGER spacecraft. Our result shows that the emission of Na+ from ESD is approximately 3 to 4 orders of magnitude less than the photoionization loss rate from the average neutral Na exosphere reported by Jasinski et al. (2021). We also compare the total ESD Na+ yields to the revised ion-sputtering yields for neutral Na from Morrissey et al. (2022). We find that our estimate of Na+ from ESD, including both the dayside cusps and nightside horns, is comparable to the sputtering rate of neutral Na by solar wind ion impact onto Na-bearing feldspars. If ESD produces only Na+, then the Na+ would become caught up in Mercury’s magnetic field and then entrained in the solar wind and quickly escapes the planet. Much of the neutral Na from ion sputtering also quickly escapes due to its generally high velocity. Comparing our results to neutral Na produced by photon stimulated desorption and micrometeorite impact vaporization, we conclude that neither ESD nor solar wind ion sputtering is an important source for exospheric sodium at Mercury.

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