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January 31, 2008—A color image taken by the MESENGER spacecraft shows the side of Mercury previously unseen by human eyes.

The shot, released yesterday, is a mosaic of images from the craft's 11 narrow-band color filters.

Subtle patterns revealed by the filters—which can capture light in wavelengths invisible to the naked eye—will help astronomers determine the mineral composition of the planet's surface.

The bright spots with a bluish tinge are relatively recent impact craters. Some of these have bright streaks, called “rays.” The streaks are made from crushed rock that was blasted outward during an impact.


The large, light-colored circle in the upper right of the image is the inside of the Caloris Basin. The only previous mission to Mercury, Mariner 10, viewed only the eastern (right) portion of this enormous impact crater.

MESSENGER has now shown that Caloris is filled with smooth plains that are brighter than the surrounding terrain—the opposite of the shading differences on Earth's moon.


Louise Prockter of Johns Hopkins University, a member of the MESSENGER science team, says she and her colleagues are mystified by a unique feature they're calling ''the Spider,'' seen above.

The formation lies in the middle of the large, 3.8-billion-year-old impact crater called Caloris Basin, and it consists of more than a hundred narrow troughs radiating from a complex central region.

The whole feature is raised off the floor of the basin. The scientists aren't sure whether the small crater near the center of the Spider has anything to do with its odd, raised shape.

One interpretation suggests that the troughs formed when materials in the Caloris Basin floor broke apart.


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).


The Mariner 10 mission to Mercury in the 1970s hinted at long, high cliffs traversing the planet's surface. But that mission imaged just under half of the planet—and astronomy's cameras have improved since then.

This image from the MESSENGER flyby shows a region, previously unseen by any spacecraft, that includes a large cliff crossing vertically through the far right.

The presence of many long, high scarps suggests a history for Mercury unlike that of any other planet in the solar system. The giant cliffs probably formed when Mercury's interior cooled and the entire planet shrank as a result.