Unveiling Mars' watery secrets.

Mars may once have been a far wetter place than researchers have realized.

Laying bare a network of long-buried secrets, new gravity maps suggest that the Red Planet has an extensive system of channels, now covered by sediment, that several billion years ago were on the surface and may have carried water. The numerous channels lie no more than a few kilometers beneath the vast expanse of the northern lowlands. Each is about 200 km wide and more than 1,600 km long--similar in size and shape to the hundreds of troughs still preserved on the Martian surface. More than 2 decades ago, spacecraft images of the bone-dry surface channels had tipped off scientists that Mars may have been wet and warm in the distant past.

Several of the buried features appear to be continuations of visible channels, says Sean C. Solomon of the Carnegie Institution of Washington (D.C.). The findings indicate that the flow of Martian water extended far into the northern plains and that the channels could have carried enough water to fill an early ocean. Moreover, such an ocean could have accumulated much more rapidly than scientists have appreciated, Solomon says.

Solomon and his colleagues, including Maria T. Zuber of the Massachusetts Institute of Technology and NASA's Goddard Space Flight Center in Greenbelt, Md., relied on data from the Mars Global Surveyor spacecraft, which has circled the planet since 1997. To map the features under the planet's surface, the researchers analyzed the radio waves emitted by Surveyor as it orbits. Gravity makes the craft speed up or slow down as it passes over regions of higher or lower density. The movements make the craft's radio broadcasts detected on Earth appear to vary in frequency. This so-called Doppler shift provides a view several kilometers deep into the Martian interior.

Surveyor found a series of elongated, low-density regions beneath the crust, indicating the presence of buried channels, as it passed over the northern lowlands. The researchers describe their work in the March 10 SCIENCE.