Mars Soil 'Friendly' To Life, Tests Show

"This soil clearly has interacted with water in the past," said William V. Boynton of the University of Arizona in Tucson.

Phoenix was aimed at the north polar region because earlier surveys showed that is where water ice is most likely to be present. Scientists are convinced that surface water flowed on Mars billions of years ago, a conclusion reached by studying geologic features of the Martian landscape. Today, conditions on Mars do not allow for liquid water, in large part because the atmosphere is only 1 percent as dense as Earth's.

In 2002, the Mars Odyssey orbiter discovered that large amounts of water ice lay just beneath the surface in permafrost that covers much of the planet's far north. The region, which is notably flat and smooth, may have once been the bottom of a large ocean.

Scientists generally believe that water is necessary for life, but the recent discovery of microbes and bacteria living in extreme environments on Earth has led to a rethinking of where life can survive. Kounaves said, for instance, that some microbes have been discovered on Earth that live without water and, in effect, "eat rocks." He said similar organisms could be living below the Martian surface.

In describing the soil as compatible with life, Kounaves said he was not suggesting anything could grow on the Martian surface, which is bombarded by deadly ultraviolet rays. Any existing life forms would have to be in the soil and ice below.

The soil was deemed to be promising because, he said, "there's nothing about it that's toxic."

Hecht said the makeup of the soil -- or at least the sample tested so far -- could be important for future manned exploration because, if properly protected, the soil could be used to grow vegetables for astronauts.

"So far, we've found nothing extreme," Kounaves said. "It's all amazingly similar to what you might find on Earth."