Scientists have argued about the origins of Mercury's smooth plains and the source of its magnetic field for more than 30 years. Now, analyses of data from the January flyby of the planet by the MESSENGER spacecraft have shown that volcanoes were involved in plains formation and suggest that its magnetic field is actively produced in the planet's core and is not a frozen relic. Scientists additionally took their first look at the chemical composition of the planet's surface material. The tiny craft probed the composition of Mercury's thin atmosphere, sampled charged particles (ions) near the planet and demonstrated new links between both sets of observations and materials on Mercury's surface. The results are reported in a series of 11 papers published in a special section of the July 4 issue of Science magazine.

"When you look at the planet in the sky, it looks like a simple point of light," remarked MESSENGER project scientist Ralph McNutt, of APL. "But when you experience Mercury close-up through all of MESSENGER's 'senses'Ñseeing it at different wavelengths, feeling its magnetic properties and touching its surface features and energetic particlesÑyou perceive a complex system and not just a ball of rock and metal. We are all surprised by how active that planet is and at the dynamic interrelationships among its core, surface, exosphere and magnetosphere."

"It's remarkable that this rich lode of data came from two days of imaging, just 30 minutes of sampling the planet's magnetosphere and exosphere, and less than 10 minutes carrying out altimetry and collecting other data near the time of its closest approach 125 miles to the surface," said principal investigator Sean Solomon, of the Carnegie Institution of Washington. "MESSENGER's first flyby was a huge success, both in keeping us on target for the rest of our journey and in advancing our progress toward answering the science questions that have motivated this mission."

MESSENGER is a NASA-sponsored scientific investigation of Mercury and the first space mission designed to orbit the planet closest to the sun. The Johns Hopkins Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages the mission for NASA. The spacecraft launched on Aug. 3, 2004, and in March 2011, after flybys of Earth, Venus and Mercury, will start a yearlong study of its target planet.

To read about the flyby's discoveries, go to: messenger.jhuapl.edu/mer_flyby1.html.

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