Hidden Territory on Mercury Revealed
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Nov. 3, 2009: The MESSENGER spacecraft's third flyby of the planet Mercury has given scientists, for the first time, an almost complete view of the planet's surface and revealed some dramatic changes in Mercury's comet-like tail.
"The new images remind us that Mercury continues to hold surprises," says Sean Solomon, principal investigator for the mission and director of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington.
The probe flew by Mercury on Sept. 29th, executing a critical gravity assist maneuver designed to help MESSENGER enter Mercury-orbit in 2011. Despite shutting down temporarily because of a power system switchover during a solar eclipse, the spacecraft's cameras and instruments revealed 6 percent of the planet's surface never before seen at close range, including this picturesque region pocked by impact craters and molded by volcanic activity:
Above: This enhanced-color view was created with a statistical technique that highlights subtle color variations seen in the 11 filters of MESSENGER's wide-angle camera. The colors are often related to the composition of underlying material material. [more] 
The bright region in the upper-right corner of the image surrounds a suspected explosive volcanic vent. The 290-km-diameter double-ring basin near the bottom of the image has a smooth interior that may be the result of effusive volcanism.
One of the spacecraft's instruments conducted its most extensive observations to date of Mercury's ultrathin atmosphere or "exosphere." Material in the exosphere comes mainly from the surface of Mercury, knocked aloft by solar radiation, solar wind bombardment and meteoroid vaporization: diagram. This wispy gaseous envelope is stretched by solar radiation pressure into a long, comet-like tail, which seems to be changing as Mercury moves around the sun.
"A striking illustration of what we call 'seasonal' effects in Mercury's exosphere is that the neutral sodium tail, so prominent in the first two flybys, is now significantly reduced in extent," says participating scientist Ron Vervack of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.
Above: These illustrations show how Mercury's comet-like tail has decreased in size since MESSENGER's 2nd flyby in Oct. 2008.to view the original data. [more] [ ]
"This difference is related to expected variations in solar radiation pressure as Mercury moves in its [elliptical orbit around the sun]," adds Vervack. "Mercury's exosphere is one of the most dynamic in the solar system."
The observations also show that calcium and magnesium in the exosphere exhibit different seasonal changes than sodium--a difference that researchers do not yet fully understand. After MESSENGER enters Mercury orbit in 2011, it can make a continuous study of seasonal changes in all exospheric constituents. That will provide key information on the relative importance of the processes that generate, sustain, and modify Mercury's atmosphere.
Approximately 98 percent of Mercury's surface now has been imaged by NASA spacecraft. After MESSENGER goes into orbit, it will see the polar regions, which are the only remaining unobserved areas of the planet.
Click here for more images and data from the 3rd flyby.
MESSENGER -- home page
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, Md., 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.