NASA's latest flyby of Mercury upset theories about the planet's composition and volcanic history, even though the probe shut down about halfway through because of an unexpected power problem.

Closeup imagery from the Sept. 29 pass of the Mercury Surface, Space Environment, Geochemistry and Ranging spacecraft (Messenger) revealed signs of volcanic activity much more recently than planetary geologists had thought possible, while neutron spectroscopy showed much higher concentrations of iron and titanium at the planet's surface than can be explained by today's theories of Mercury's formation.

During its third and final flyby the probe also gleaned new information about Mercury's tenuous atmosphere that it won't be able to detect once it becomes the first spacecraft to enter orbit around the sun's closest planet in 2011. "Even though we obtained about half of the data that we planned to obtain, we saw surprises," said Sean Solomon, the mission's principal investigator.

Composite imagery that combined data from Messenger's wide- and narrow-angle cameras and statistical weighting of data from its 11 filters to produce false color turned up evidence of an explosive volcanic eruption near a volcanic crater scientists estimate to be about 1 billion years old -- 2 billion years more recent than earlier calculations of the end of volcanic activity on Mercury.

Neutron spectrometry from Messenger turned up data showing much higher concentrations of iron and titanium than had been seen in visible and near-infrared observations, which had puzzled scientists because the planet's large core is believed to be mostly iron. And the flyby took Messenger through the "tail" of the planet, where molecules and atoms liberated from the surface are pushed away from the sun.

Measurements of sodium, calcium and magnesium during the second and third flybys were sensitive enough to reveal "seasonal" variations in the tail, as Mercury follows its orbit around the sun. Those measurements won't be possible once Messenger goes into orbit, but other flyby data "give us a little taste of what we'll get from orbit," said Brett Denevi, a member of the imaging team.

Artist's concept: NASA