A NASA spacecraft gliding over the surface of Mercury has revealed that the planet's atmosphere, magnetosphere, and its geological past display greater levels of activity than scientists first suspected. The probe also discovered a large impact basin named "Rembrandt" measuring about 430 miles in diameter - equal to the distance between Washington and Boston.

These new findings and more are reported in four papers published in the May 1 issue of Science magazine. The data come from the Mercury Surface, Space Environment, Geochemistry, and Ranging spacecraft--MESSENGER for short. On Oct. 6, 2008, MESSENGER flew by Mercury for the second time, capturing more than 1,200 high-resolution and color images of the planet.

"This second Mercury flyby provided a number of new findings," said Sean Solomon, the probe's principal investigator from the Carnegie Institution of Washington. "One of the biggest surprises was how strongly [Mercury's magnetosphere] had changed from what we saw during the first flyby in January 2008."

The magnetosphere is a region of space around Mercury enveloped by the planet's magnetic field. Gusty solar wind pressing against the bubble of magnetism can potentially trigger magnetic storms and other space weather-related phenomena.

"During the first flyby, MESSENGER measured relatively calm dipole-like magnetic fields close to the planet. Scientists didn't detect any dynamic features other than some Kelvin-Helmholtz waves," said James Slavin of NASA's Goddard Space Flight Center. Slavin is a mission co-investigator and lead author of one of the papers.

But the second flyby was a totally different situation," he says. MESSENGER observed a highly dynamic magnetosphere with "magnetic reconnection" events taking place at a rate 10 times greater than what is observed the Earth during its most active intervals.

"The high rate of solar wind energy input was evident in the great amplitude of the plasma waves and the large magnetic structures measured by the spacecraft's magnetometer throughout the encounter."

Another exciting result is the discovery of a previously unknown large impact basin. The Rembrandt basin is more than 700 kilometers (430 miles) in diameter and if formed on the east coast of the United States would span the distance between Washington, D.C., and Boston.

Rembrandt formed about 3.9 billion years ago, near the end of the period of heavy bombardment of the inner Solar System, suggests MESSENGER Participating Scientist Thomas Watters, lead author of another of the papers. Rembrandt is significant, not only because it is big, but also because it is giving researchers a peek beneath the surface of Mercury that other basins have not.

"This is the first time we've seen terrain exposed on the floor of an impact basin on Mercury that is preserved from when it formed," explains Watters. "Landforms such as those revealed on the floor of Rembrandt are usually completely buried by volcanic flows."

Half of Mercury was unknown until a little more than a year ago. Globes of the planet were blank on one side. Spacecraft images have since revealed 90 percent of the planet's surface at high resolution. This near-global coverage is showing, for the first time, how Mercury's crust was formed.

"After mapping the surface, we see that approximately 40 percent is covered by smooth plains," said Brett Denevi of Arizona State University in Tempe, a team member and lead author of a paper.

"Many of these smooth plains are interpreted to be of volcanic origin, and they are globally distributed. Much of Mercury's crust may have formed through repeated volcanic eruptions in a manner more similar to the crust of Mars than to that of the moon."

Another finding of the flyby is the first detection of magnesium in Mercury's exosphere. The exosphere is an ultrathin atmosphere where the molecules are so far apart they are more likely to collide with the surface than with each other. Material in the exosphere comes mainly from the surface of Mercury itself, knocked aloft by solar radiation, solar wind bombardment and meteoroid vaporization.

The probe's Mercury Atmospheric and Surface Composition Spectrometer instrument detected the magnesium. Finding magnesium was not surprising to scientists, but the abundance was unexpected. The instrument also measured other exospheric constituents including calcium and sodium. Researchers believe that big day-to-day changes in Mercury's atmosphere may be caused by the variable shielding of Mercury's active magnetosphere.

"This is an example of the kind of individual discoveries that the science team will piece together to give us a new picture of how the planet formed and evolved," said William McClintock of the Laboratory for Atmospheric and Space Physics at the University of Colorado at Boulder. McClintock co-investigator and lead author of one of the four papers.

"The third Mercury flyby is our final dress rehearsal for the main performance of our mission, the insertion of the probe into orbit around Mercury in March 2011," said Solomon.

"The orbital phase will be like staging two flybys per day and will provide the continuous collection of information about the planet and its environment for one year."

"Mercury has been coy in revealing its secrets slowly so far, but in less than two years the innermost planet will become a close friend."