WASHINGTON – Volcanic activity has played a central role in forging the surface of Mercury, scientists said Thursday based on data collected by a NASA spacecraft that zoomed past the closest planet to the sun in January.

The seven scientific instruments on the car-sized MESSENGER spacecraft also sent back information about Mercury's magnetic field and about the fact that the extremely dense planet has been shrinking more than expected over the eons.

MESSENGER has begun to resolve some of the mysteries of Mercury, a sun-baked world about a third the diameter of Earth and a bit larger than the moon.

In 1975, Mariner 10, the last spacecraft to fly past heavily cratered Mercury, sent back images showing smooth plains covering large parts of its surface.

But scientists have debated whether these plains were created by volcanic activity or by debris settling back onto the surface after space rocks collided with the planet.

Images provided by MESSENGER from its Jan. 14 fly-by provide strong evidence that volcanoes played a critical role in forming Mercury's surface, according to geologist James Head of Brown University in Providence, Rhode Island.

The dramatic and large-scale volcanism probably took the form both of humongous lava flows and violent eruptions, based on surface features seen in the images, Head said.

He estimated that the volcanic activity occurred between 3 billion and 4 billion years ago, adding that there is no evidence of ongoing volcanism on the planet's surface.

'BRAND-NEW VIEW OF VOLCANISM ON MERCURY'

“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 960 miles, is one of the solar system's biggest impact craters, formed more than 3.8 billion years ago when a large space rock hit.

MESSENGER sent back images of a shield volcano with a distinct orange color about 60 miles wide on the southwestern edge of this basin that may be a source for the lava that formed smooth plains inside the basin.

These deposits look similar to basalt flows on the moon, but are very low in iron, representing an unusual rock type. The size of the plains imply the existence of large sources of magma in Mercury's upper mantle, the scientists said.

Inside the shield volcano they detected a kidney-shaped vent with a bright halo around it very similar to halos formed by explosive eruptions on Earth and the moon.

“MESSENGER has given us a brand-new view of volcanism on Mercury. It's provided insight into the formation of surface plains and given new life to what many thought might be a rather dead planet,” Head told reporters.

In other findings also published in the journal Science, scientists said Mercury's magnetic field originates in the planet's outer core and is powered by the core's cooling.

MESSENGER, which stands for Mercury Surface, Space Environment, Geochemistry and Ranging, was launched in 2004.

NASA's Mariner 10 flew past Mercury three times, mapping 45 percent of its surface. MESSENGER has imaged about 20 percent more and is due to fly by Mercury again this October and in September 2009 before starting a yearlong orbit in 2011.

With many scientists now considering Pluto a dwarf planet, Mercury holds the distinction of being the solar system's smallest planet, with a diameter of 3,032 miles.