WASHINGTON -- Volcanic activity played a key role in shaping the planet Mercury's crater-riddled surface, and not asteroid impacts as previously theorized, according to US research published Thursday.

Scientists based their conclusion on numerous images transmitted by the US 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.

Planetary geologist James Head's 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," according to the findings in the Journal Science.

"What this shows is that Mercury was not dead on arrival," said Head.

"Now, we want to know when it had that pulse and what caused it to slow down and eventually stop."

Scientists 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.

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 percent 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 kilometers (124 miles) of Mercury on January 15.

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