Map of Mars helps solve a dark mystery.

A new perspective on old images of the Red Planet may provide planetary scientists with a better understanding of several Martian features, including a vast dark region that has puzzled researchers for decades.

Astronomers have long speculated about the processes that formed and preserved Cerberus, a dark, gently sloping region just north of the Martian equator that has roughly half the width of the United States. Many areas of Mars are coated with a highly reflective layer of fine dust and appear bright red.

But Cerberus remains dark, its whaleshaped silhouette contrasting with its bright surroundings. And unlike other dim areas of Mars, Cerberus has no obvious origin. For example, it is not a depression, which could trap dark sand.

Researchers have suggested two theories for the creation of Cerberus' dim countenance. In one scenario, a wind of sand particles periodically scours this section of the Martian surface, removing dust and exposing the dark, underlying bedrock. In the second scenario, sand blankets the area instead of scouring it, forming a dimly reflective surface akin to the black sand beaches of Hawaii. But each proposal poses the same puzzle: Where does the sand come from?

A new global map of Mars, based on pictures taken by both Viking Orbiters between 1976 and 1980, should help solve the riddle. To better study the reflectivity and geology of Mars' surface, Alfred S. McEwen, Laurence A. Soderblom, and their colleagues at the U.S. Geological Survey in Flagstaff, Ariz., used computer technology to combine two types of images taken by the Orbiters.

One set of images, recorded with red and violet filters when the sun stood almost directly overhead, best reveals the color and reflectivity of the surface. However, the flat, overhead lighting makes it difficult to discern topography. In contrast, the other image set -- recorded when the sun was low in the sky -- has many more shadows, a feature that accentuates the shape and height of the Martian terrain. However, the sun's illumination angle obscures the color and reflectivity of the surface.

Merging the two image sets, a feat that required the processing of some 5,000 pictures, "provides the best of both worlds," McEwen says.

At the annual Lunar and Planetary Science Conference in Houston this week, his team displayed the fruits of their labors: a planetwide map of Mars that reveals both color and topography.

The map sheds new light on dim Cerberus. Researchers already knew from the orientation of bright and dark streaks that prevailing winds blow across the region from northeast to southwest. This suggests that if a source of dark sand exists, it lies in Cerberus' northeast region, McEwen says.

That's intriguing, he adds, because the northeast section has a terrain unlike any other part of Cerberus. The new map shows that this section consists of knobby remnants of ancient highland crust that stick out above a lava flow. The lava indicates past volcanic activity there.

A large body of evidence now suggests that Mars' highland crust contains the planet's major reservoir of water, stored as ground ice since an earlier, wetter epoch on the planet. When a volcano erupts near such a reservoir, the lava slams into the storehouse of ice, causing small explosions, McEwen notes.

The explosions create a glassy material known as palagonite, an excellent source of dark, sand-size material, he adds. Thus, suggest McEwen and his colleagues, a thick deposit of palagonite in the northeast corner of Cerberus appears to have created the entire 2,000-kilometer-long dark region.

With the mystery of Cerberus' formation most likely solved, McEwen says he and his team look forward to comparing their maps to those expected from future missions to the Red Planet, including NASA's Mars Surveyor.