Weird "Spider," Volcanism Discovered on Mercury

Anne Minard
for National Geographic News
January 30, 2008
The solar system's smallest planet created a huge buzz today as NASA unveiled preliminary photos and findings from the first Mercury flyby in 30 years.

The photos reveal widespread volcanic activity, asteroid assaults, and a spider-shaped formation the likes of which have never before been seen in the solar system. (see some of the new images of Mercury.)

The MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft cruised past the innermost planet on January 14.

The approach was the first of three planned flybys that will eventually position the probe to settle into orbit around Mercury in March 2011.

Until MESSENGER, Mariner 10 had been the only spacecraft to scope out the planet. The craft had swung by in 1974 and 1975, but it left more than half the planet unseen by human eyes.

During the MESSENGER flyby, cameras captured 1,213 images, revealing another 30 percent of the tiny planet. (Related photo: "First View of Mercury's 'Other Face'" [January 16, 2008].)

"We were continually surprised," Sean Solomon, MESSENGER's principal investigator at the Carnegie Institution of Washington, D.C., said during a press briefing.

"It was not the planet we expected. It was not [like] the moon. It's a very dynamic planet with an awful lot going on."

Volcanic Oozing

With Pluto's demotion to "dwarf planet" last year, Mercury became the smallest planet in our solar system. The rocky world is 3,031 miles (4,879 kilometers) across, hardly bigger then Earth's moon.

But its petite stature has not spared it from massive bombardment, the MESSENGER photos revealed.

The surface is pockmarked by craters—some reaching 100 feet (30 meters) across or bigger—from asteroid impacts.

In some of the craters, volcanic material apparently oozed out to form a smooth floor.

"There's very little doubt that there have been widespread volcanoes on Mercury's surface," said Louise Prockter, a mission scientist from Johns Hopkins University in Maryland.

This volcanism, she said, was one of Mercury's biggest surprises.

Prockter and her colleagues are also intrigued by a unique feature they're calling "the Spider."

The odd formation consists of more than a hundred raised, narrow troughs radiating from a complex central region.

The Spider lies in the middle of a large, 3.8-billion-year-old impact crater called the Caloris Basin, about half of which sits on the side of Mercury that had gone unseen until the MESSENGER mission.

The scientists aren't sure whether a smaller crater near the center of the Spider has anything to do with its odd, raised shape.

In general, Mercury's extensive cratering record is "going to give us a lot of clues about the geologic history of Mercury and evolution of terrestrial planets," said Robert Strom, a science team member from the University of Arizona in Tucson.

"Mercury has been a big hole in that."

Future Focus

The astronomers stress that their announced findings from MESSENGER's first flyby are very preliminary.

Prockter, from Johns Hopkins, declined to elaborate on initial results showing possible sources for Mercury's volcanic history, noting only that she and her colleagues are studying several "intriguing features" on the surface.

Still to come are geologic maps of Mercury's surface and an analysis of its superthin atmosphere, where molecules of sodium and hydrogen barely hang together.

There's also the mystery of why a planet with temperatures reaching higher than 1,292 degrees Fahrenheit (700 degrees Celsius) seems to harbor ice.

Future data from MESSENGER could explain why Earth-based radar imaging of Mercury has revealed areas of high reflectivity—an indicator of ice—near the north and south poles.

And future data could add to our understanding of Mercury's puzzling magnetosphere, which appears to have poles like Earth's magnetic field.

(Read "Liquid Mercury: Tiny Planet Has Molten Core" [May 3, 2007].)

The Carnegie Institution's Solomon said he's most curious to know where Mercury's short-lived atmosphere comes from, how the atmosphere interacts with the magnetosphere, and how the magnetosphere interacts with solar wind.

"It's a very complex planet," he said.

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