ANCIENT RIVER

Surveyor's snapshots may also provide more clues about an age when Vulcan ruled. Where bedrock is exposed in features such as cliffs, gullies and steep spurs, the new images show distinct layers in the ancient rock, which is estimated to be 3.5 and 4.3 million years old. Most researchers think the layers are result of successive lava flows, possibly separated by soil and other debris that accumulated between volcanic eruptions. Others, however, suggest that the layers could also be sedimentary rock formed by either wind blown sand or deposited in bodies of water.

Another instrument on Surveyor, the Mars Orbiter Laser Altimeter (MOLA), made accurate measurements of the Martian topography that confirm the dramatic differences between the planets hemispheres. While the southern hemisphere is mountainous and heavily cratered, the northern is as flat as the Great Plains and slopes up gradually toward the equator, where the terrain becomes rougher.

To many observers, that suggests one thing: water. The northern hemisphere is as flat as the abyssal plains in Earth's oceans--so flat that it could have been shaped by a great sea. The exposed land to the south may well have been exposed to the battering of meteorites over the eons. In their paper in Science, the MOLA researchers estimate that the amount of water necessary to fill the great basin "is less than the upper limit of the volume of water estimated for early Mars."

POLAR ICE

The investigators further estimate that, were all the water transported to the polar regions, it would be sufficient to form two polar caps, each more than five miles thick and 10 degrees of Martian latitude in radius. Not stopping there, the MOLA data indicate that the layered terrain of the northern ice cap is in fact composed mainly of ice. (The southern ice cap consists of frozen carbon dioxide.) And the MOLA data have also provided the most accurate profiles of the great outflow channels and valley networks that were probably formed when liquid water flowed on Mars.

Meanwhile, the south polar region has yet another allure: it is to be the landing site of the next spacecraft to explore the Martian surface--the Mars Polar Lander, which will touch down in late 1999. Final selection of a landing site will depend heavily on Surveyor's maps, but the obliging spacecraft swooped in for a sneak preview.

The images confirm that the terrain at the frozen poles makes the now-familiar Martian landscapes observed by the Viking landers and Mars Pathfinder look benign. The lander, which is scheduled for launch on January 3, 1999, will search for subsurface water--possibly bringing the day of a manned mission to Mars a bit closer.

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