Some raised moon areas are dark in contrast to surrounding areas because of volcanic basalt flows, previous researchers found.
On other areas, however, plains and highlands were indistinguishable based on color.
When Apollo 16 landed near some of those highlands, astronauts saw "impact processes all over the place. The smooth plains actually came from ejecta from surrounding impact basins," Head said.
So when Mariner 10 passed Mercury in the 1970s and also saw little contrast between high- and low-elevation areas, planetary scientists theorized that the planet had likewise been blasted by myriad impacts.
MESSENGER has now put that theory to rest.
"We absolutely see evidence for the smoking gun we didn't see before—the volcanic vents," Head said.
MESSENGER also sent images of odd, kidney-shaped deposits that could be volcanic in origin, along with large impact craters, near the Caloris Basin, the youngest known large impact basin on Mercury.
Planetary Magnetism
But what's going on under the surface has planetary scientists truly puzzled.
Earth and Mercury are the only known rocky planets that still have a global magnetic field, though Mercury's magnetic field is weaker than Earth's by a factor of a hundred.
Earth's moon and Mars both possess some magnetized rocks and regions—perhaps the last vestiges of once-global magnetism.
Mercury's magnetic field could mean its core remains partially molten and is releasing energy as it cools and churns—a phenomenon called dynamo. Or it could be the last whispers of energy release from the already solidified crust.
"With the data obtained to date, I am agnostic about the origin of the magnetic field," said MESSENGER scientist Jean-Luc Margot of Cornell University, who led a 2007 study on the topic but is not a co-author on any of the new papers.
"We know that a partially molten core is required for a dynamo. We know that the core is partially molten. It does not follow that a currently active dynamo is generating the magnetic field."
Brian Anderson is a space physicist at Johns Hopkins University and lead author on another of the new studies.
"The fact that the magnetic field appears to be fairly simple and hasn't changed in strength since Mariner 10's visit would seem to limit the field of viable dynamo models," Anderson said.
Researchers also found more slip faults than Mariner 10, revising estimates of how much Mercury compressed during its early years upward by a third.
That means the planet once collapsed in on itself and shrunk dramatically as it cooled, MESSENGER scientists suggest. The resulting wrinkles on its surface are up to 370 miles (600 kilometers) long.
More to See
Researchers hope to reveal more about the lively planet later this year. During MESSENGER's next flyby this fall, the spacecraft is expected to image another 30 percent of the planet's surface.
Meanwhile, the European Space Agency is planning a 2013 launch for BepiColombo, a mission with two orbiters that will study the planet's magnetic field.
That's a good thing, Anderson said—Mercury remains a mysterious place.
There's the case of the missing iron, for example. Even though Mercury is more than 60 percent iron by weight, iron is relatively scarce on Mercury's surface. That's unusual compared to the other inner solar system planets.
And a report led by Thomas Zurbuchen of the University of Michigan in Ann Arbor reveals that Mercury's surface is generating a high amount of charged particles that surround the planet and form a long, cometlike tail.
"You see ions from the atmospheres of Venus, Earth, and Mars, some sputtering from the moon," Anderson said, but "this is the first time we've seen this sputtering from the actual surface of a planet."
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