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Seeing Mercury in a new light

Times Staff Writer

The first NASA spacecraft to visit Mercury in 33 years is forcing scientists to reevaluate their conceptions of the solar system’s innermost planet, once dismissed as a dead world as barren as our own moon.

“This is a whole new planet we’re looking at,” said Robert Strom, a crater expert who was a member of the original Mariner 10 team that investigated Mercury in 1975.

Strom and other researchers on Wednesday presented the first detailed scientific results from the Messenger spacecraft, which made its first pass of Mercury two weeks ago.

Messenger, which stands for Mercury Surface, Space Environment, Geochemistry and Ranging, is scheduled to return in October for another pass before settling into orbit around Mercury in 2011, when the bulk of scientific studies will begin.

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As the craft whisked past the planet at an altitude of about 120 miles, it snapped 1,200 pictures that reveal a heavily cratered surface, with many craters dating to early in the planet’s history, nearly 4 billion years ago. There are huge cliffs, and some geological features stretch hundreds of miles across the planet’s surface.

The highest peak soars to about 15,000 feet, while the deepest basin plunges nearly two miles. Mercury’s craters also appear to be shallower than similar craters on the Moon.

Among other early surprises uncovered by Messenger’s seven scientific instruments is widespread evidence of volcanic activity. Images also reveal a strange feature that scientists dubbed “the spider.” Consisting of scores of narrow troughs radiating from a central plateau, it resembles the vein-like pattern left by a rock hitting a car windshield.

“The spider feature is unlike anything seen anywhere else in the solar system,” said Sean Solomon of the Carnegie Institution in Washington and Messenger’s principal investigator. Experts speculate that it could have been caused by something pushing up from below.

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The composition of the surface is thought to be similar to the Moon’s surface, which has been pounded into a fine dust by eons of meteor impacts. Mercury also possesses what the scientists call a “tenuous” atmosphere, consisting of sodium, potassium and oxygen, whose atoms are blasted off the surface by the solar wind.

A visitor would find Mercury to be a very unwelcoming place, with a daytime temperature of about 1,100 degrees Fahrenheit. At night the temperature plummets to minus 300 degrees. Because the planet rotates very slowly, a day on Mercury lasts longer than its year. It travels around the sun in only 88 Earth days. But 176 Earth days pass between one sunrise and the next.

Mariner 10 saw about 55% of the planet. Messenger, which launched in 2004, has added an additional 30% to that figure, the scientists said at a briefing in Washington.

“The best is yet to come,” Strom said. “Wait until we go into orbit.”

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john.johnson@latimes.com


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