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Friday, 10 March, 2000, 16:29 GMT
Underground 'rivers' revealed on Mars
MGS can look below the surface of Mars
By BBC News Online science editor Dr David Whitehouse

Some of Mars' best kept secrets, long buried beneath the surface of the Red Planet, have been revealed by instruments on Nasa's Mars Global Surveyor (MGS) spacecraft.

New observations of Mars show that the world's flat northern lowlands could have been a region of rapid water accumulation in the planet's youth.

MGS has found evidence of ancient underground channels
Elevation and gravity measurements, used to probe beneath the surface of Mars, suggest the planet went through a period of rapid cooling early in its history. The data also provide evidence for large, buried channels that could have carried enormous volumes of water.

"The crustal thickness map shows that, as for Earth, Mars has two distinct crustal provinces," explained Dr Maria Zuber of the Massachusetts Institute of Technology, lead author of a study published in the journal Science.

Beneath the rough southern highlands of Mars, the crust, estimated at 80 km (50 miles) thick, gets thinner progressively from the south pole toward the north. In contrast, the northern lowlands have a crust of uniform thickness, about 35 km (22 miles) deep.

Submarine channels

Researchers say that the crustal structure accounts for the elevation of the Martian northern lowlands, which in turn controlled the northward flow of water early in the planet's history. This would have produced a network of valleys and outflow channels.

The features are about 201 km (125 miles) wide and over 1,600 km (1,000 miles) long, with characteristics that can be explained by water flow on the surface or in a submarine environment later buried by sediments.

Standing four times higher than Everest, Olympus Mons is the highest peak in the Solar System
The large size of these channels implies that any bodies of water in the northern lowlands could have accumulated rapidly. The now buried channels may represent the means for filling an early ocean.

The gravity and topography also provide information about the cooling of Mars over time.

The period of rapid interior heat loss may correspond to the time when Mars had a warmer climate, liquid water flowed on the surface, and the planet's surface was shielded from the solar wind by a global magnetic field.

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