Scientists say they may have to re-think some of their best theories about the origins and evolution of the planet Mercury as new data from the Sept. 29 flyby of the planet by the Maryland-built Messenger spacecraft continue to surprise.
In their latest discussion of the mission's scientific findings, scientists said Tuesday they have found evidence that volcanic activity, including explosive eruptions, continued until unexpectedly recent times.
The evidence appears in photos of an unnamed volcanic crater, 180 miles wide with a double ring around it. Its interior is surprisingly smooth and free of subsequent meteor impact craters, suggesting there were lava flows into the center as recently as a billion years ago.
Scientists had thought Mercury's vulcanism, like that on Earth's moon, was among the first in the solar system to cease, at least 3 billion years ago. But "if the basin is young and the interior is even younger ... that may not be the case," said Brett Denevi, an imaging team member from Arizona State University in Tempe.
Also, neutron spectroscopy from the latest flyby has found signs of higher concentrations of iron and titanium on the surface than had been thought.
"Models of Mercury's formation and evolution will have to be reassessed to account for a large iron and titanium content. ... It will keep people busy for a while trying to understand this," said David J. Lawrence, a scientist with the mission from the Johns Hopkins Applied Physics Laboratory near Laurel. Messenger was built at APL, and the $446 million NASA-funded project is managed from a control center there.
Previous observations have shown that Mercury is the densest planet in the solar system, with a large iron core. Surface measurements, however, had found a surface rich in rocky silcates but poor in iron. So theoreticians constructed models - some including huge impacts that stripped away large volumes of iron - to explain how that contrast could have evolved from the planet's formation 4.5 billion years ago.
"It's going to be a volley back to our geochemists ... to come up with an explanation," said Sean Solomon of the Carnegie Institute of Washington, the mission's principal investigator.
Messenger was launched in August 2004. It is on course to enter orbit around Mercury in March 2011.