Washington, July 4 : Scientists have confirmed that Mercury's surface had been formed by volcanic activity early in the planet's history.
The conclusion is based on an image taken by the Messenger spacecraft as it flew past Mercury in January 2008, which showed a kidney-shaped volcanic vent, surrounded by a halo-like ring and a fainter outer ring.
Earlier, 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.
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," said Head. "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," he added.
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.
Armed with the higher quality images and greater coverage, Head and his team said 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.