for National Geographic News
Our solar system's smallest planet has seen an enormous amount of volcanic activity, according to scientists studying information from the latest Mercury flyby.
Images returned earlier this month from NASA's MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft reveal about 3,600 cubic miles (15,000 cubic kilometers) of solidified lava inside a single crater on Mercury's western hemisphere.
That's enough lava to fill the entire Baltimore-Washington metropolitan area to a height 12 times that of the Washington Monument, according to Maria Zuber of the Massachusetts Institute of Technology, who is a co-investigator for the mission.
"That's an awful lot of volcanic material in one place for such a little planet," Zuber told reporters during a Wednesday briefing.
Scientists think this volcanism may have happened 3.8 billion to 4 billion years ago, Zuber said, adding that a more precise date would require studies of rock samples.
Mercury has a large iron core that makes up the majority of the planet's width, Zuber said, so "volcanic processes could have continued much further into time, but the preponderance of activity probably occurred early."
Planetary bodies tend to heat up slowly and cool off pretty quickly. Earth, which is the largest terrestrial planet in the solar system, is still active volcanically, Zuber noted, suggesting size may help slow the cooling process.
But smaller rocky bodies such as Mercury and our moon are believed to have seen most of their volcanism in their earliest history.
Still, the new data show that Mercury had even more eruptions than the moon, "which has always been the planetary body we've compared Mercury to," Zuber said.
The October 6 flyby was the second in a planned series of three close sweeps past Mercury that will ultimately put MESSENGER in orbit around the planet in 2011. (See an illustration of MESSENGER's flight path in our space blog.)
"The main purpose of this flyby was to have a trajectory correction to help us to get into orbit later," said Marilyn Lindstrom, a program scientist at NASA headquarters in Washington, D.C.
But astronomers have been using the close approaches as opportunities to collect more data about an otherwise largely neglected planet.
Until recently humans had seen just 45 percent of Mercury's surface, based on images taken during the Mariner mission in the 1970s.
MESSENGER's first flyby in January took hundreds of high-resolution pictures covering 30 percent of the planet's eastern hemisphere that had never been seen before.
(Related: "Weird 'Spider,' Volcanism Discovered on Mercury" [January 30, 2008].)
During its second flyby, MESSENGER went past the western side, revealing another 30 percent of Mercury's surface never seen before by spacecraft and allowing scientists to compare the two sides of the planet for the first time.
"This hemisphere of Mercury is smoother than the other hemisphere, and we need to think hard about why that's the case," Zuber said.
Scientists also noted a large "wrinkle ridge"—a feature that probably forms when a planet shrinks.
"We believe it is associated with the cooling of Mercury, probably very early in it's history," Zuber said.
"The planet probably has undergone a considerable amount of contraction, but we can't go so far as to say how much until we take many, many more measurements."
In addition, color imaging suggests that Mercury's crust is made of the same types of materials at depth as what's seen on the surface, unlike the defined layers of material seen in Earth's crust.
"Mercury's crust is more analogous to a marbled cake than a layered cake, said co-investigator Mark Robinson of Arizona State University.
MESSENGER's third and final flyby is scheduled for September 29, 2009.
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