Analysis of these new findings and more are reported in four papers published in the May 1st issue of Science magazine. The data come from the Mercury Surface, Space Environment, Geochemistry, and Ranging spacecraft, known as MESSENGER. On October 6, 2008, the probe flew by Mercury for the second time, capturing more than 1,200 high-resolution and color images of the planet. The probe unveiled another 30 percent of the planet's surface that had never been seen by previous spacecraft, gathering essential data for planning the remainder of the mission.
The spacecraft also made the first detection of magnesium in Mercury's thin atmosphere, known as an exosphere. This observation and other data confirm that magnesium is an important constituent of Mercury's surface materials. The probe's Mercury Atmospheric and Surface Composition Spectrometer instrument detected the magnesium. Finding magnesium was not surprising to scientists, but seeing it in the amounts and distribution observed was unexpected. The instrument also measured other exospheric constituents, including calcium and sodium. The variability that the spacecraft observed in Mercury's magnetosphere, the volume of space dominated by the planet's magnetic field, so far supports the hypothesis that the great day-to-day changes in Mercury's atmosphere may be a result of changes in the shielding provided by the magnetosphere.
The MESSENGER project is the seventh in NASA's Discovery Program of low-cost, scientifically focused missions. The Johns Hopkins University Applied Physics Laboratory of Laurel, Maryland, designed, built and operates the spacecraft and manages the mission for NASA's Science Mission Directorate in Washington. Science instruments were built by the Applied Physics Laboratory; Goddard; the University of Michigan in Ann Arbor; and the University of Colorado in Boulder. GenCorp Aerojet of Sacramento, Calif., and Composite Optics Inc. of San Diego provided the propulsion system and composite structure.