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Mars once could have been a planet shaped by networks of rivers and home to primitive life forms like those that arose on the Earth. Or it could be a dry place that has been lifeless forever, scientists say.

Whether or not life evolved on Mars, and is perhaps still there in some elemental form, depends largely on whether or not there once was liquid water on the Red Planet, the specialists said last week at the World Space Congress."It all comes down to water and its essential role to life," said Rocco L. Mancinelli of NASA's Ames Research Center. If the planet is dry, and always has been, "it makes the prospects for life bleak for all of Mars."

Bruce M. Jakosky of the laboratory for astronautics and space physics at the University of Colorado said there is a rich body of evidence that Mars once had a lush supply of liquid water, and there are suggestions from spacecraft studies that the planet still has huge volumes of water locked in the soil as ice.

Mancinelli said if there was liquid water at some time on the primitive Mars, then life could have evolved from basic chemistry, just as many scientists believe occurred on Earth.

Amos Benin of Hebrew University in Israel said studies by the Viking Mars lander and analysis of meteorites knocked from Mars and landed on Earth show that the Red Planet has all of the chemical elements for life.

Benin said science knows that 17 elements make up what he called the "chemical code of life" and that studies have confirmed that each one of these is present, in one form or another, on Mars.

Some elements, such as carbon, are in short supply on Mars, Benin said, but "there appears to be enough for the beginning of evolution."

Another Ames scientist, Wanda L. Davis, said that if Mars had liquid water early in its history, life could have arisen. With water, there is no chemical or physical reason that would prevent the evolution of life there, she said.

Diverse life is thought to have evolved on Earth by 3.5 billion years after the planet formed, she said, and there is no reason that early Mars could not have followed the same pattern. Both planets are thought to be about 4.75 billion years old.

If Mars lost most of its water billions of years ago, then the evolution of life could have been halted at that primitive point. This prompts suggestions that future missions to Mars should search for fossils of early life forms, the scientists said.

Jakosky said there could be ice accumulated in the soil just three feet or so beneath the surface. Views of Mars throughout its 687-day year show variations in color that suggest a seasonal collection of ice on the surface at the poles.

High-resolution photographs show serpentine features that resemble dry river and stream beds, he said. There are even alluvial fans, or depositions of soil such as those formed by flooding rivers on Earth. Also, there are features that appear to be water erosion on the walls and floors of Martian craters.

Much of the water that once was on Mars could have been lost to the planet forever because the planet has only about half of Earth's gravity.

Jakosky estimated that water sufficient to cover the planet to a depth of 300 feet has been lost over geologic time. But by some estimates, he said, there remains enough water trapped as ice to flood to a depth of 90 feet to 150 feet.

The international gathering of the World Space Congress attracted about 5,000 scientists, engineers and astronauts.