Instrument: Mercury Atmospheric and Surface Composition Spectrometer (MASCS)Of Interest: The graphic shown here is more than just an artistic representation of Mercury’s calcium exosphere – it is the result of a detailed model based on over four years of MASCS observations! This model image of Mercury’s calcium exosphere shows the source region is on the dawn hemisphere of the planet. The MESSENGER spacecraft is the first ever to orbit the planet Mercury, and the spacecraft’s seven scientific instruments and radio science investigation are unraveling the history and evolution of the Solar System’s innermost planet. In the mission’s more than four years of orbital operations, MESSENGER has acquired over 250,000 images and extensive other data sets. MESSENGER’s highly successful orbital mission is about to come to an end, as the spacecraft runs out of propellant and the force of solar gravity causes it to impact the surface of Mercury in April 2015.Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington