One ancient volcanic vent inside the vast Caloris Basin of Mercury is twice the size of Washington state, they said. It has piled lava as deep as the Washington Monument in a flow that would extend as far as Baltimore.
The volcanic activity is "testimony to a long history of volcanic flooding of the interior of the Caloris Basin," said James W. Head III, a Brown University geologist on the team. He said the discovery "adds new life to what many had thought might be a dead planet."
The $446 million Messenger mission was designed and built at the Johns Hopkins University's Applied Physics Laboratory near Laurel and is being operated from a control center on the APL campus.
Launched by NASA in 2004, the spacecraft is the first to visit Mercury since Mariner 10 made a series of flybys in 1974 and 1975.
Messenger will zip past the planet again Oct. 6, and a third time in September next year. If all goes well, it will slip into orbit in March 2011 for at least a year of close-up observations.
Discoveries from Mercury are contributing to scientists' understanding of the early history of all the "terrestrial" planets, including Venus, Earth and Mars.
The disclosure of water molecules in Mercury's tenuous "exosphere" was made by the spacecraft's Fast Imaging Plasma Spectrometer, part of a seven-pound instrument developed by the APL and the University of Michigan.
It's a kind of nose, designed to identify low-energy ions encountered as they're lifted off the planet's surface and swept up in the solar wind.
"The magnetic protection layer of Mercury is not as impenetrable as the one on Earth," Zurbuchen said. "The solar wind is filling the entire volume around Mercury ... and even touches the planet at multiple locations," tossing surface material into space where Messenger scooped them up.
"Every single element seen here is really unprecedented," he said. But the "real surprise" was the water ions. Much of the planet may be sizzling so close to the sun, but in shaded craters, surface temperatures can dip to minus-256 degrees Fahrenheit, and water might be preserved.
Messenger's finding fits with observations from Earth that have found highly reflective regions near the planet's poles that theorists suspected might be water ice. Zurbuchen said small comets or meteors may have carried water to Mercury during the early history of the solar system.
There has been similar evidence of water ice in deep, shaded craters at the poles of the Earth's moon, too. Such water resources could help sustain manned lunar bases, supplying drinking water, breathing oxygen and hydrogen fuel for rocket engines.
frank.roylance@baltsun.com
