New data from the Messenger probe show mass volcanic activity was involved in the formation of Mercury's plains.

Credit: NASA/Johns Hopkins University

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WASHINGTON: Volcanic activity played a key role in shaping the planet Mercury's crater-riddled surface, and not asteroid impacts as previously thought.

Announcing the results in the U.S. journal, Science, the scientists based their conclusion on numerous images transmitted by the U.S. spacecraft Messenger, which on January 14 flew near Mercury and took pictures of the mysterious planet, the smallest in the Solar System and the closest to the Sun.

James Head, an American planetary geologist heading up the research team at Brown University said it appears the planet "underwent an intense bout of changes to its landscape about three to four 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," added Head. "Now, we want to know when it had that pulse and what caused it to slow down and eventually stop."

The researchers studied some of the depressions found on the planet's surface and determined some were volcanic vents with remnants of lava forming a bright ring around it.

There was evidence of dramatic and large-scale volcanism, likely made up of titanic lava flows and violent eruptions, based on surface features seen in the images, the researchers said.

Ancient lava flows

They estimated that the volcanic activity took place between 3 billion and 4 billion years ago, but there wasis no evidence of volcanism today on the planet's surface.

"Everything we've seen so far would suggest that the activity on the surface dates from the first half of Solar System history rather than the last half," Head said.

The Caloris basin, with a diameter of 1,500 km, is one of the Solar System's biggest impact craters, formed more than 3.8 billion years ago when a large asteroid hit. Other areas showed molten rock that had oozed out from the planet's core and made its way to the surface.

Messenger's swing past Mercury allowed scientists to see a fresh perspective; 55 per cent of its surface has never before been seen.

Just how the craters on Mercury's surface formed has been the subject of debate since the first spacecraft to approach Mercury, the Mariner 10, flew by three different times between 1974 and 1975.

Some have believed the craters were caused by asteroids striking the surface of the planet.

Messenger, whose name is derived from Mercury Surface, Space Environment, Geochemistry and Ranging, came within 200 km of Mercury on 15 January 2008.

Two more fly-overs are planned for October 2008 and September 2009 before Messenger goes into orbit of Mercury in March 2011. The spacecraft was launched on 3 August 2004.