Abstract | Each species in Mercury’s exosphere exhibits a different, but seasonally repeatable pattern. Episodic events that augment this behavior have been challenging to isolate within the UVVS data. Here we identify and simulate three such events: one dayside and two night side. All are consistent with ~10 cm size impactors that ejected just a few kg of measureable vapor. Fast decay times of these vapor plumes are poorly reproduced by simulations where the atoms bounce across the surface, as expected for cold night-side surface interactions. Spectra of multiple species were obtained for one event showing Na and Mg in a ratio of 3, stoichiometric with their local ratio in the topmost soil. Both gases were enhanced several times above the nominal background, but Ca showed no deviation from the steady state exosphere. This non-response may reflect that it condenses into a molecular form within impact ejecta (e.g., CaO or CaS) since photolysis of these products is absent in shadow. Plume expansion rates indicate that the gas is surprisingly hot with ~10,000K Na and ~15,000K Mg best reproducing the UVVS tail sweep measurements. Overall, it is impressive that modest impact events which occur about daily in frequency can produce such strong features in the exosphere, but discerning these signatures from the background relies on observing geometry that is serendipitous in the right place and time. The findings here hold promise that PHEBUS and MSASI will encounter similar impact ejecta plumes during the BepiColombo orbital mission. |
