Scientists have long anguished over how little is known about Mercury, the innermost of the four terrestrial planetary bodies in our solar system. The gaps in knowledge covered such basic information as the planet's geology, how it was formed and evolved and whether its interior was still active.

In 1975, the Mariner 10 spacecraft returned intriguing images that showed smooth plains covering large swaths of Mercury's surface. But scientists could not determine whether the plains had been created by volcanic activity or by material ejected from below the surface when objects had collided into it. Thus, they could not reach a consensus over Mercury's geologic past.

Now, a research team led by Brown University planetary geologist James Head has determined that volcanism played a central role in forming Mercury's surface. In a paper that appears in the July 4 issue of Science, part of a special section describing the MESSENGER spacecraft's first flyby of Mercury, the researchers have found evidence of past volcanic activity, suggesting that the planet underwent an intense bout of changes to its landscape about 3 to 4 billion years ago – and that the source for much of that reshaping was within.

"What this shows is that Mercury was not dead on arrival," says Head, the paper's lead author. "It had a pulse for a while. Now, we want to know when it had that pulse and what caused it to slow down and eventually stop."

A major clue to Mercury's geologic past came from the scientists' finding of volcanic vents along the margins of the Caloris basin, one of the solar system's largest and youngest impact basins. The group zeroed in on a kidney-shaped depression that was surrounded by a bright ring, lending a halo-like impression to the landscape. The scientists determined that the depression was a volcanic vent, and the bright ring around it was pyroclastic, remnants of lava that had been spewed outward, much like a volcanic fountain on Earth. Another larger ring surrounding the vent and halo ring showed that another type of volcanism, called effusion, in which molten rock from within the planet oozes outward and covers the surface, had occurred. Together these deposits create a surface feature shaped like a volcanic shield – a clear sign to scientists that volcanic activity helped form the surrounding plains.

Mariner 10 snapped pictures of roughly 45 percent of Mercury's surface. MESSENGER (an acronym for MErcury Surface, Space ENvironment, GEochemistry, and Ranging), which flew past Mercury last January, mapped 20 percent more of the planet, in the process taking more detailed, higher-resolution images of its surface. (The spacecraft is scheduled for two more flybys of Mercury and a yearlong orbit of the planet in March 2011.) Armed with the higher quality images and greater coverage, Head and his team write that they found many impact craters and areas between craters that were flooded with lava, bolstering their belief that volcanism had been widespread on Mercury and had contributed significantly to the planet's formation.

In one such area, the team examined circular wrinkle ridges outlining an impact crater roughly 60 kilometers in diameter and filled to the rim with material. Similar patterns are found on the Moon and on Mars, and they indicate that the craters were flooded by volcanic activity. If that is the case with the wrinkle-ridge ring on Mercury, the scientists say, the finding suggests that the crater is filled with lava about 2.7 kilometers deep.

"That's a lot of lava," Head says. "It shows the planet was really active in its early history."

The new insights will help scientists link Mercury's surface evolution to its interior history and to compare the planet with the geologic histories of the Earth, Venus, the Moon and Mars.

The NASA Discovery Program funded the MESSENGER mission and the research.