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Volcanoes behind craters on Mercury: researchers

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

Scientists based their conclusion on numerous images transmitted by the US spacecraft Messenger, which on Jan. 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,” the findings in the journal Science said.

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

“Now, we want to know when it had that pulse and what caused it to slow down and eventually stop,” he said.

“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 1,550 km, 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 95km 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.

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 200km of Mercury on Jan. 15.

Two more flyovers are planned for October and September next year before Messenger goes into orbit of Mercury in March 2011.

The spacecraft was launched on Aug. 3, 2004.

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 4,880km.
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