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.