NASA Mercury Messenger Finds Surprises

On its third swing past Mercury, NASA’s Mercury Messenger spacecraft discovered an unexpectedly young lava plain, rapid rufflings of the planet’s weak magnetic field and an unanticipated dance of elements in the thin atmosphere.

“I think the biggest surprise for the community is that the planet is turning out to be much more dynamic than people appreciated,” said Sean C. Solomon of the Carnegie Institution of Washington who is the principal investigator for the Messenger mission.

The flyby occurred in September, when the spacecraft swooped within 142 miles of Mercury’s surface at 12,000 miles per hour, but the findings of that flyby just appeared in three papers the journal Science published last week on its Web site.

Within the 180-mile-wide double-ring Rachmaninoff crater, Messenger photographed flat, smooth plains that scientists interpreted as the hardened outflow of lava. Based on the number of smaller impact craters, the age of the volcanic deposits within Rachmaninoff is probably less than two billion years, said Louise K. Procktor of the Johns Hopkins University Applied Physics Laboratory and lead author of the paper that examined the crater.

While not recent, that would still suggest that Mercury was volcanically active well into its middle age, given that it formed 4.5 billion years ago with the rest of the solar system.

“It is quite a big surprise,” Dr. Procktor said.

Mercury, not much more than 3,000 miles in diameter, is by far the smallest of the planets, and thus many thought it would have cooled off in less than a billion years. But Mercury’s mantle may not rise and fall in convective patterns as in larger planets like Earth, and that may have kept the heat inside for more than two billion years. By imparting heat and relieving pressure, the impact of the meteoroid that formed the crater may have helped melt rocks that were already close to melting point.

Messenger’s measurements also detected rapid fluctuations in Mercury’s magnetic field as it shifted from one side of the planet to the other. Such fluctuations occur around Earth in a matter of hours. Around Mercury, the fields shifted in minutes.

“It looked as though we were seeing a complete collapse of the magnetic field,” Dr. Solomon said. “This is an incredibly dynamic magnetosphere.”

The third paper looked at Mercury’s tenuous atmosphere, which consists of atoms knocked off the surface by sunlight, charged particles and dust-size meteoroids. One surprise was that near the equator, the concentration of calcium was higher near sunrise than sunset, but that effect was not seen for sodium or magnesium.

Because only calcium and not the other elements exhibits this behavior, “We can’t explain it,” said Ronald J. Vervack Jr., another scientist at the Johns Hopkins laboratory and lead author of that Science paper. “They just don’t look the same, and they don’t look the same anywhere we look,” he said. “Which is a bit puzzling, because we expected there to be some similarities.”

A tail of sodium that had been seen behind Mercury during the first two flybys, in January 2008 and October 2008, was much diminished in the third flyby. The reason was that Mercury was in a different part of its elliptical orbit, and the resulting change in velocity diminished the power of sunlight that was sweeping the sodium atoms away from Mercury into the tail.

A clearer picture should start to emerge next year. The spacecraft, launched in 2004, has been doing a gravitational do-si-do with the inner planets to slow down as it spirals inward and gets in position to enter orbit around Mercury in March.