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The theory, identifying melting snow deposits as the likely cause of the many deep gullies on the planet, was announced Wednesday at a briefing at the National Aeronautics and Space Administration in Washington. Pictures from the Mars Odyssey spacecraft, which is orbiting the planet, inspired the suspected relation between snowmelt and gullies.
A week ago, other scientists reported studies showing that both Martian polar regions are capped almost entirely with ice, enlarging the planet's known reservoir of water. Until recently, the huge southern cap was thought to be predominantly, if not entirely, frozen carbon dioxide, or dry ice. In the report Wednesday, Philip Christensen, a planetary scientist at Arizona State University in Tempe, proposed that many of the eroded gullies were probably carved by water from melted snow flowing beneath a thick covering of packed snow. Like a protective blanket, the covering was said to have warmed the deeper snow, thawing some of it and sheltering the flowing water from refreezing or evaporating in the thin atmosphere. Christensen said spacecraft photographs showed gullies on the slopes of craters in the planet's middle latitudes. They appeared to be emerging from under remnants of extensive snowpacks.
"Melting begins first in the most exposed area, right at the crest of the ridge," Christensen said. "This explains why gullies start so high up." Over time, geologists said the flowing water left deep cuts in the surface in places thought improbable, like the crests of hills. Among previous explanations for the gullies were seeping groundwater, pressurised flows of groundwater or carbon dioxide and mudflows from collapsing permafrost. John Mustard, a planetary geologist at Brown University in Providence, Rhode Island, called the melting-snow hypothesis "very interesting and compelling" and said it presented a "fundamentally new perspective" on the origin of Martian gullies. The research report is to be published soon in the journal Nature, but it is already available on the journal''s website, www.nature.com/nature. A biologist, Lynn Rothschild of NASA's Ames Research Center in Mountain View, California, said the "possibility of liquid water on Mars right near the surface" was intriguing evidence that some forms of life could exist on the planet. Christensen said of the gully landscapes, "If life ever existed on Mars, I can think of no places where it would be more interesting to look for it."
When the first spacecraft flew close to Mars, in 1965, it determined that the planet's atmosphere was made up largely of carbon dioxide. Scientists concluded that the two huge polar ice caps were frozen carbon dioxide. Now Andrew Ingersoll of Caltech and a graduate student, Shane Byrne, reported in the February 14 issue of the journal Science that new spacecraft data had revealed that both polar caps were made mostly of frozen water. Examining pictures and infrared measurements, they found wide, deep pits in the upper layer of frozen carbon dioxide. The pits penetrated to a layer of ice that was too warm to be dry ice and must be frozen water, the scientists said. Source: The New York Times
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A new theory and a revised interpretation of observations have bolstered the idea that Mars has more water than was thought and have encouraged speculation about the possibility of life on the planet.
As he explained, the heaviest accumulation of snow on Mars is most likely to be on the coldest, pole-facing slopes, as seen in the pictures. The snow would presumably collect over the landscape through a period of deep cold, then would melt as the climate warmed.
In separate research, scientists at the California Institute of Technology in Pasadena said they had found evidence to correct a decades-old model of what the Martian polar caps were like. 
