Martian highlands: clues to a watery past?

For years, astronomers studying Mars have all but ignored the planet's southern highlands. This heavily cratered region, resembling the moon, lacks the allure of volcanoes and other geologic scars that mark the planet's northern face. But in reexamining images of a giant impact basin -- a gaping 2-kilometer- deep hole -- in the southern highlands, two researchers say they have found new evidence that liquid water once flowed on Mars.

Their study of images made by the Viking spacecraft more than a decade ago reveals that Argyle Planitia, the second-largest impact basin in the highlands, contains layers of material that could be sediment from a huge body of water held by the basin millions of years ago. In addition, three networks of channels appear to lead into the basin from the south. Other channels slope northward out of the basin, which has a diameter of some 1,200 kilometers.

Timothy J. Parker and Donn S. Gorsline of the University of Southern California in Los Angeles conjecture that Mars' atmospheric pressure was once high enough and the planet's southern polar ice cap once large enough to allow water to form and flow into Argyle Planitia. This would have created an icy lake in the basin, they suggest. Eventually, the water would have spilled over the side of Argyle Planitia, carving channels that would carry water northward.

"The [channels] are the smoking gun -- or squirting gun -- to support the contention that there had been standing water in the basin," says Parker, who reported the work last week at a meeting of the American Geophysical Union in Baltimore. He notes that the study supports the oft-debated idea that Mars once had lakes or planet-wide seas. But Parker adds that previous evidence for a watery Mars has come primarily from studies of the planet's northern hemisphere.

Even if current speculation about Argyle Planitia does hold water, Parker says, it's unclear whether the basin contained a windy lake or a still, ice-covered reservoir. He notes that the basin may be a prime site to look for organic material -- the possible precursor of primitive life --on the Red Planet.

The Mars Observer spacecraft, which will begin studying the planet in November, could shed further light on the history of the basin, Parker notes. A Russian craft, Mars 94, set for launch next year and scheduled to arrive at Mars in 1995, may also have a chance to photograph the basin.

COPYRIGHT Science Service Inc. 1993