Seasonal variations in Mercury’s dayside calcium exosphere

•MESSENGER has observed Mercury’s exospheric Ca for eight Mercury years.•There is a persistent source of energetic Ca in Mercury’s dawn equatorial region.•There is a seasonal, but not year-to-year variation in the Ca source rate. The Mercury Atmospheric and Surface Composition Spectrometer on the MESSENGER spacecraft has observed calcium emission in Mercury’s exosphere on a near-daily basis since March 2011. During MESSENGER’s primary and first extended missions (March 2011 – March 2013) the dayside calcium exosphere was measured over eight Mercury years. We have simulated these data with a Monte Carlo model of exospheric source processes to show that (a) there is a persistent source of energetic calcium located in the dawn equatorial region, (b) there is a seasonal dependence in the calcium source rate, and (c) there are no obvious year-to-year variations in the near-surface dayside calcium exosphere.