December 10, 1999

Tantalizing Signs of Ancient Martian Ocean

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Tantalizing Signs of Ancient Martian Ocean

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By WILLIAM J. BROAD

cientists at Brown University said Thursday that they had found tantalizing evidence that the northern lowlands of Mars once bore a wide ocean that until it dried up had waves, hidden depths and long beaches.

The findings, reported in Friday's issue of the journal Science, were hailed as making the best case yet for the existence of a sea on primordial Mars.

"I consider it proved in the way we understand that dinosaurs once walked on the Earth," said Victor R. Baker, a planetary scientist at the University of Arizona who has long looked for signs of a Martian sea that may have existed more than a billion years ago.

Although the findings bore no direct evidence on the issue of alien life, large amounts of water would be an indication that life might once have thrived on the red planet.

The report was made by a six-member team at Brown led by James W. Head III, a planetary geologist. The team examined six categories of evidence, all of which turned out to support the ocean hypothesis. The data are from the Mars Global Surveyor, which has been orbiting the planet for two years.

The findings include hints of ancient shorelines and, parallel to them, curved terraces suggesting the leveling action of waves and other ocean processes as the Martian sea retreated. Another clue is that the area's central basin is unusually flat and smooth, implying thick accumulations of bottom muds and sediments.

"It's compelling," Dr. Head said of the evidence, adding that he was initially skeptical of the sea hypothesis. "It makes me want to look at the issue a lot more."

The history of water on Mars, he added, "is linked to questions about the possibility of life there and changes in climate and things on Earth," like long-term shifts in the weather.

Tim Parker, a planetary geologist at the Jet Propulsion Laboratory in Pasadena, Calif., who was an early Mars ocean theorizer, said the new evidence, already widely discussed among scientists, was proving to be widely persuasive.

"There's always going to be skeptics," he said. "But most of the community is accepting this as almost smoking-gun proof that there were oceans on Mars. Ultimately, I think we're going to have to dig holes in the shoreline and look for sand."

The new findings underscored the magnitude of the recent loss of the $165 million Mars Polar Lander, which disappeared a week ago today while trying to land near the Martian south pole to search for water. Editors at Science said the loss of the spacecraft had no impact on the timing of the ancient-ocean paper. In September, yet another craft, the Mars Climate Orbiter, vanished as it approached the planet.

Mars today is an icy desert with dust storms and reddish dunes. The planet also has thin, tattered clouds, but the only evidence of water in any quantity is at the north and south poles, where there seems to be a mix of carbon dioxide and water, like frozen seltzer.

Scientists agree that Mars in its early days was much warmer and wetter, and they have even found channels in the planet's surface that once apparently carried rushing water into the northern lowlands.

Dr. Parker in 1989 and Dr. Baker in 1991 went further. They proposed that the flowing water collected in large standing bodies or seas. At the time, their ideas were considered wild. The Brown study has given that hypothesis its most rigorous test. The main tools of the investigation were thousands of photographs and altitude measurements made by the Mars Global Surveyor, a $150 million craft that went into orbit around Mars in 1997 and began its prime mission in 1998. In May, based on data from the Surveyor, scientists unveiled the first three-dimensional map of the planet.

The Brown team extended the Surveyor work. Among other things, it analyzed height measurements made with great precision as the Surveyor bounced laser beams off the planet's surface, allowing land elevations to be determined by measuring the time of the round trip.

Among the important findings, based on the altimetry and photographs were:

• The border between two geologically dissimilar areas in the northern lowlands is nearly level in elevation, suggesting an ancient coastline.

• The topography below this possible shoreline is much smoother than that of the region above at higher altitudes, which is consistent with smoothing by sedimentation.

• The volume of the putative sea is within the range of previous estimates of water on Mars.

• A series of terraces run parallel to the apparent shoreline, bolstering the idea of receding waters.

• Low areas bear what appear to be mud cracks, like those in dry terrestrial lake beds.

• Scars from impact craters suggest ground water or ice in the northern lowlands is near the surface.

Dr. Head said that the putative Martian sea would have covered about one-sixth of planet and ranged up to a mile deep. By terrestrial measure, he said, it would have been bigger than the Mediterranean Sea and Arctic Ocean combined.

In addition to Dr. Head, the authors of the Science paper are Harald Hiesinger, Mikhail A. Ivanov, Mikhail A. Kreslavsky, Stephen Pratt and Bradley J. Thomson. Dr. Ivanov and Dr. Kreslavsky also have posts at, respectively, the Vernadsky Institute of the Russian Academy of Sciences and the Kharkov Astronomical Observatory in the Ukraine.

Some scientists, while praising the new findings, said the thesis needed further evidence to be considered a sure thing. Michael H. Carr, a planetary geologist with the United States Geological Survey and author of "Water on Mars" (Oxford, 1996), said he particularly admired the evidence of wave-pounded terraces, which he called "very convincing." But he questioned some of the team's imagery interpretations, like their conclusions about the depth of ground ice or water, and the proposed edges of the putative sea. He said that further searches for ocean terraces, based on the Surveyor's altimetry data, could be conclusive.

Dr. Head suggested that further research might also include an analysis of meteorites from Mars and samples from landing sites on the planet, which could be checked for the presence of salts that might be related to an ancient ocean.

"My philosophy," he said, "is that something this important is worth a lot of my research time."