From: Reuters
Published January 30, 2008 04:28 PM

Probe detects "spider" crater on Mercury

By Will Dunham

WASHINGTON (Reuters) - A NASA probe that shot past Mercury this month detected a puzzling geological feature that scientists on Wednesday labeled "The Spider" and found evidence of past volcanic activity on the closest planet to the sun.

The U.S. space agency's car-sized MESSENGER spacecraft on January 14 flew past Mercury, whose diameter is just a third the size of Earth's, and its seven scientific instruments gathered new information about the little understood planet.

Data collected by MESSENGER showed that a massive impact crater on its surface is larger than previously thought. The probe is due to fly by again this October and in September 2009 before beginning a yearlong orbit of the planet in 2011.

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While Mercury looks superficially like Earth's moon with a cratered, rocky surface, scientists said the new findings show they are quite different.

"We were continually surprised. It was not the planet we expected. It was not the moon," said Sean Solomon of Carnegie Institution of Washington, the mission's lead investigator. "It's a very dynamic planet with an awful lot going on."

Mercury is a mystery in many ways and its proximity to the sun has made it difficult to observe from Earth.

Mercury has been visited by a spacecraft only twice before, in 1974 and 1975 when NASA's Mariner 10 flew past it three times and mapped about 45 percent of its surface. The latest fly-by covered another 30 percent of the surface.

"The Spider" was the most striking feature described by the scientists. It is made up of more than 100 narrow, flat-floored troughs radiating from a central point, much as petals from a daisy or the legs of a spider.

"A REAL MYSTERY"

"The Spider" has a crater 25 miles wide near its center, but it is unclear whether this is related to the feature's original formation and scientists aren't sure what to make of it.

"It's a real mystery," said Louise Prockter of Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, who works on the mission.

"The Spider" is in the middle of Mercury's Caloris basin, one of the solar system's biggest impact craters, formed more than 3.8 billion years ago when a large space rock hit.

Based on the probe's new observations, the diameter of the Caloris basin is now thought to be 960 miles, larger than a previous estimate based on Mariner 10's data. The basin's interior looks like it was volcanically resurfaced by magma from deep within Mercury's crust or mantle.

Prockter said Mariner 10 data provided some evidence of volcanism, but it was not universally accepted. Based on the new observations, Prockter added, "there's very little doubt, I think, in the minds of most of us from the geology team that there has been widespread volcanism on Mercury's surface."

MESSENGER stands for Mercury Surface, Space Environment, Geochemistry and Ranging. It was launched in 2004 and flew past Venus twice and Earth once en route to Mercury.

MESSENGER also collected data on Mercury's magnetic field, its tenuous atmosphere and its topography.

Mercury's surface is a mix of craters caused by bygone impacts with space rocks, plains and long, winding cliffs. The spacecraft saw basins as deep as 1.7 miles and peaks jutting out as high as 3 miles above the surface.

With Pluto classified as a dwarf planet, Mercury is the solar system's smallest planet, with a diameter of 3,032 miles , only a bit larger than Earth's moon.

(Editing by Chris Wilson)

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