Slow Ride to the Red Planet

When the Mars Global Surveyor soared into orbit around the Red Planet last September, the spacecraft--sent to complete one year of mapping--was right on schedule. But trouble soon struck: as the craft began spiraling down through the thin Martian atmosphere, one of its solar panels faltered. The panel was acting as a sail to slow Surveyor so that it could slip into ever lower orbits, but the stress of this "aerobraking" was simply too much. Mission controllers decided to take it slow and revised the schedule: The craft will now circle Mars 900 times before reaching low-orbit.
Sand dunes

Though initially disappointing, this delay has actually proven to be a boon. To take advantage of Surveyor's slow ride into the Red Planet, scientists turned on its instruments and they have sent back a torrent of new data about Martian geology, landscape, weather and atmosphere, much of which was published in the March 13 issue of Science.

The new data may be only a tantalizing taste of what will come when the mapping begins in earnest in March 1999. The highest resolution images sent back by Surveyor's Mars Orbiter Camera (MOC) were taken during aerobraking orbits when the spacecraft dipped to 1,700 miles above the planet's surface; they reveal objects about 48 feet across. Granted, when Surveyor is in its final mapping orbit of 234 miles above the surface, the camera will pick up features only 7 to 9 feet across. But the current images are far more detailed than any provided by the Viking missions in the 1970s.

Layered strata

Surveyor's early results have already confirmed previous data, given new insights into our nearest planetary neighbor--and thrown up a few Martian mysteries. For example, the data reveal a windshaped landscape of barren plains, complex dunes, sandsheets and drifts; stratified bedrock that was probably formed by lava flows before the planet's volcanic activity ceased; an extrordinarily flat plain that may be more evidence of a former ocean; and an abundance of water at the north pole.

During its initial sweeps around the planet, Surveyor captured a bird's-eye view of the south polar ice cap, which consists of frozen carbon dioxide and possibly some water, as it evaporated in the Martian spring. The MOC pictures show a region of frost-covered ridges that intersect in a curious rectangular pattern. Many features are puzzling to geologists, who argue there may be more than springtime evaporation at work in forming them.

Another harbinger of spring on Mars is huge dust storms that sweep across the planet. Surveyor tracked the progress of one such storm that took place from November 27 to December 2, 1997. This observation of a Martian dust storm was the first since Viking 2 recorded one in 1977. Researchers hope to gain insights into the frequency and intensity of the dust storms by comparing data from the two spacecraft.

The Surveyor images show the disturbance beginning as a series of small local dust storms along the edge of the shrinking ice cap. As the storm grew in intensity, local storms stopped forming and water clouds in the Martian atmosphere disappeared--even in places on the planet where dust was not visible. A month after the storm subsided, clouds once again appeared in the sky, and local dust storms resumed along the edge of the ice cap.

Dry 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."

South Pole

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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