Detection of Large Exospheric Enhancements at Mercury due to Meteoroid Impacts

The Ultraviolet and Visible Spectrometer (UVVS) on the MESSENGER spacecraft observed three large transient events in Mercury’s nightside “tail” in which the exospheric brightness increased by an order of magnitude. Meteoroid impacts are the best explanation given that the events are brief, can be simulated with instantaneous injections of vapor, and were not associated with unusual solar wind conditions. Data–model comparisons suggest that the impactors are 10–20 cm in diameter and produce vapor temperatures of ∼10 4 K, much warmer than usually assumed for impact vapor. We estimate the impact frequency to be on the order of once per Earth day for meteoroids 10 cm diameter and larger, consistent with a pre-MESSENGER prediction. UVVS observed three atomic species during one event: sodium, magnesium, and calcium. Na and Mg brightened simultaneously, and their modeled ejection ratio roughly matches Mercury’s surface abundance. Ca showed no sign of an enhancement, consistent with earlier predictions that Ca in impact vapor is bound in a molecule that is undetectable to UVVS. This event provides an unprecedented opportunity to see three species respond (or not) to a single source and has implications for our understanding of Mercury’s exosphere.