Seasonal variations of Mercury's magnesium dayside exosphere from MESSENGER observations

•We describe the first orbital observations of Mercury's magnesium exosphere.•The observed brightness is consistent with very little year-to-year variability.•Mg is enhanced near the dawn terminator on both the day and nightside.•A warm source (average ∼6000K) dominates the dayside throughout the Mercury year.•Impact vaporization is the most probable main source of the dayside Mg exosphere. The Ultraviolet and Visible Spectrometer channel of the Mercury Atmospheric and Surface Composition Spectrometer instrument aboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft made near-daily observations of solar-scattered resonant emission from magnesium in Mercury's exosphere during the mission's orbital phase (March 2011–April 2015, ∼17 Mercury years). In this paper, a subset of these data (March 2013–April 2015) is described and analyzed to illustrate Mg's spatial and temporal variations. Dayside altitude profiles of emission are used to make estimates of the Mg density and temperature. The main characteristics of the Mg exosphere are (a) a predominant enhancement of emission in the morning (6am–10am) near perihelion, (b) a bulk temperature of ∼6000K, consistent with impact vaporization as the predominant ejection process, (c) a near-surface density that varies from 5cm−3 to 50cm−3 and (d) a production rate that is strongest in the morning on the inbound leg of Mercury's orbit with rates ranging from 1 × 105cm−2s−1 to 8 × 105cm−2s−1.