Mercury Craft Shows ''Spider,'' Asteroid Assaults

Mercury Image Show Volcanism, Asteroid Assaults
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Since Pluto's demotion to ''dwarf planet'' last year, Mercury is the smallest planet in the solar system. At just 3,030 miles (4,880 kilometers) across, it's barely bigger than Earth's moon. Still, it's been a frequent target for meteoroid and asteroid impacts.

NASA's MESSENGER mission photographed impact craters all over Mercury's surface during a January 14 flyby.

Some of the craters are 100 feet (30 meters) or bigger. Smaller craters were probably made by castoffs from larger impacts. In some of the craters, volcanic material has apparently oozed out to form a smooth floor.

The density of craters on a planet's surface can indicate the relative age of different areas. That's because craters made long ago would weather with time, creating a smoother surface, while younger regions sporting recent impacts would show sharper features.

In this image alone, 763 craters have been identified and measured (shown in green) along with 189 hills (shown in yellow).

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—Image by NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
 
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