Element found in Mercury's atmosphere in equatorial regions, and in higher concentrations near sunrise than sunset, which oddly persisted during all three Messenger flybys while most of the rest of the environment was ever-changing. It is a mystery why Sodium and Magnesium are distributed differently. Also detected in Mercury's tail. Emissions from ionized Calcium were observed near the equatorial plane in the exosphere by Messenger during its third flyby.
Mercury/Calcium Web Pages
Mercury/Calcium In the News
Magnesium Detected in Mercury's Atmosphere (May 2009)