The NASA Messenger mission continues to show that Mercury was mercurial well past the planet’s middle age — with vast volcanic flows carpeting the surface, according to a suite of papers published today in Science Express.

The papers are based on a third and final flyby in September 2009, after which just about all the gaps in mapping the innermost planet have been filled. After two flybys, the probe, combined with old Mariner images, had mapped 90% of the surface; Now, 98% is known. In March 2011, Messenger will fall into orbit and begin the prime phase of its mission. But already, the mission has turned up surprises — especially in terms of evidence for volcanism that persisted perhaps as recently as 1 billion years ago.

That was already suspected after the 2nd fly-by, but the latest views turned up a new 290-kilometre-wide impact basin, called Rachmaninoff (pictured), with especially smooth volcanic plains in its interior. That means that the surfacing was relatively fresh: recent enough so that meteorite impacts didn’t have enough time to pockmark the basin.

Image: NASA/Johns Hopkins University Applied Physics Laboratory/Arizona State University/Carnegie Institution of Washington