Mars May Not Have
Been Warm Or Wet
By Maggie Fox
Health and Science Correspondent

WASHINGTON (Reuters) - Mars may not have once been a warm, wet and hospitable planet that somehow lost its atmosphere, scientists said on Thursday, instead suggesting the dead planet was occasionally bombarded by melting meteorites that carved out its distinctive craters and valleys.
They paint a hellish picture of a planet slammed by huge balls of ice that spattered steam and scalding water onto the surface. The power of the impacts would not only have melted the asteroids, but may have melted ice just under the planet's surface, they said.
"Hypotheses of a warm, wet Mars, based on the presumption that the valley networks formed in a long-lasting greenhouse climate, imply that Mars may once have been teeming with life," Teresa Segura and colleagues at the University of Colorado in Boulder wrote in the report, published in the journal Science.
"In contrast, we envision a cold and dry planet, an almost endless winter broken by episodes of scalding rains followed by flash floods."
The idea that Mars may have been wet started when early planet-watchers thought they saw canals on its surface. Space missions found huge canyons, deeper than anything on Earth, and what looked like dried-up lake, sea and riverbeds.
Much evidence exists of water on Mars, including measurements taken from space that suggest ice lies under the surface, probably in a frozen mixture with sand and rock. Some studies also suggest water may seep to the surface in places.
All this led many experts to believe that Mars once had a wet climate but lost its atmosphere. This leaves open the possibility that water, and perhaps primitive life such as bacteria, exist below the surface.
"There apparently were some brief warm and wet periods on Mars, but we believe that through most of its history, Mars has been a cold, dry planet," said Segura, now working at the space agency NASA's Ames laboratory in California.
Segura and colleagues used photographs of the Red Planet's surface and computer models to show that large asteroids or comets hit the planet 3.5 billion years ago.
They find evidence of 25 huge asteroids or meteors, each about 60 to 150 miles in diameter, that punched craters in the surface of Mars every 10 to 20 million years.
The impacts would have injected steam into the atmosphere, both melted from the surface and from the asteroids and meteors themselves. This would have rained onto the surface "at a rate of about 2 meters (six feet) per year," they wrote.
This would explain the deep riverbeds that do not have tributaries, said Owen Toon, who directs the program in atmospheric and ocean sciences at the University. "We definitely see river valleys but not tributaries, indicating the rivers were not as mature as those on Earth," Toon said in a statement.
This scenario still leaves open the possibility of life on Mars, the researchers said.
"Only during the brief years or decades after the impact events would Mars have been temperate, and only then might it have bloomed with life as we know it," they wrote.
In a second study in Science, Timothy Titus of the U.S. Geological Survey in Flagstaff, Arizona and colleagues said they had found evidence of water frozen under Mars' south pole. Such ice has been seen at the north pole but not the south.
"Water ice that is in the top few centimeters (inches) of soil will probably be accessible to future robotic probes and, ultimately, human exploration," they wrote in their report.


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