By Richard Lovett

A newly discovered crater on Mercury may have been geologically active as recently as a billion years ago. The discovery was made by NASA's Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft on its latest fly-by of the innermost planet of the Solar System on 29 September.

"It's the youngest terrain we've yet seen on Mercury," said Clark Chapman a planetary scientist at the Southwest Research Institute in Boulder, Colorado, and part of the MESSENGER mission team.

The crater, about 260 kilometres in diameter, does not have a formal name, but because it looks similar to a basin named Raditladi, discovered in early 2008, it has been informally dubbed Twin.

Both craters have bull's-eye structures, with a pair of concentric walls separating inner and outer sections of the crater floor. The main difference is that the inner ring of Twin is pocked by very few small craters, indicating that it has not been exposed to meteor bombardment for as long as Raditladi.

There's only one thing that could resurface part of the floor of an impact crater that recently. "That's just absolute proof of volcanism," Chapman said on 20 October at the annual meeting of the Geological Society of America, in Portland, Oregon. "It's hard to say, but it could be less than a billion years old." Previously, scientists had thought that Mercury's period of volcanic activity ended more than three billion years ago.

And although the areas around Raditladi's inner and outer rings have roughly the same number of craters, Twin's outer ring has three times more craters than its inner one. That suggests that the crater might have seen two successive stages of volcanic activity, says Chapman.

Complex crust

Twin is not the only MESSENGER discovery to reveal that Mercury's geology is more complex than previously thought.

It has been known for decades that Mercury is covered with long, steep escarpments, probably created as the planet cooled and shrank just after it first formed. But new fly-by data indicates that these scarps might be as much as a kilometre high. "Wherever we look, when there is a low enough Sun angle [to highlight them], we see these thrust faults all over the place," says Robert Strom of the University of Arizona's Lunar and Planetary Laboratory in Tucson.

The MESSENGER scientists are also seeing features that seem to be volcanoes, and include smooth areas that could be the residues from pyroclastic flows -- outbursts of hot rocks and gas. Such flows are a surprise because they come from magma that is rich in volatile compounds.

"We're getting an intriguing look that even as close to the Sun as Mercury there may be some process for delivering and retaining volatiles to the interior that we did not appreciate," says Sean Solomon, of the Carnegie Institution of Washington.

Elsewhere there are indications that Mercury has had a complex tectonic history. Four sets of overlapping cracks in the 715-kilometre-wide Rembrandt Basin may indicate that they were created in four stages of crust movement, says Louise Prockter of the Planetary Exploration Group in Laurel, Maryland. "[They] point to Mercury being an even more tectonically interesting place than we suspected either from Mariner 10 [in 1974] or the first two [MESSENGER] fly-bys," Solomon says.

Meanwhile, MESSENGER is cruising toward its final rendezvous with Mercury, on 18 March 2011, when it will brake into orbit and begin detailed mapping. "Stay tuned," says Solomon.