Led by CU-Boulder doctoral candidate Teresa Segura and her adviser, Professor Owen B. Toon, the team used Mars photos and computer models to show that large asteroids or comets hit the planet some 3.5 billion years ago. These impacts apparently occurred about the time major river channels were formed on the Red Planet, said Segura.

According to the available evidence, roughly 25 huge impactors, each about 60 miles to 150 miles in diameter, slammed into Mars roughly every 10 million to 20 million years during the period, blowing a volume of debris equivalent to a global blanket hundreds of yards thick into the atmosphere. The material is believed to have melted portions of subsurface and polar ice, creating steam and scalding water that rained back on Mars at some six feet per year for decades or centuries, causing rivers to form and flow, according to the study.

But the study belies the warm, wet, Mars theory of rivers and oceans embraced by many planetary scientists, since such impactors were so infrequent. "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, currently a visiting researcher at NASA-Ames in California.

A paper by Segura, Toon, CU-Boulder graduate Anthony Colaprete -- now at NASA-Ames -- and Kevin Zahnle of NASA-Ames, will appear in the Dec. 6 issue of Science.

"When the river valleys on Mars were confirmed in the 1970s, many scientists believed there once was an Earth-like period with warmth, rivers and oceans," said Toon, director of CU-Boulder's Program in Oceanic and Atmospheric Sciences and a professor at the University's Laboratory for Astrophysics and Space Physics. "What sparked our interest was that the large craters and river valleys appeared to be about the same age."

In between such catastrophic events, the planet was likely very cold, dry and inhospitable to any life forms, said Toon. "We definitely see river valleys but not tributaries, indicating the rivers were not as mature as those on Earth."

The rare, hot rains pelting Mars that likely came from water in asteroids and comets hitting the planets and the evaporation of some ice from polar caps and ice beneath the impacts would have been spectacular, said Segura. "We believe these events caused short periods of a warm and wet climate, but overall, we think Mars has been cold and dry for the majority of its history."

According to Toon, previous theories that carbon dioxide gas and clouds warmed Mars during its early history "just have not worked out quantitatively." There is no evidence on Mars of large limestone deposits from the first billion years, which would be directly linked to large amounts of C02, a greenhouse gas, he said.

There also is no evidence that another greenhouse gas, methane -- which can be created naturally by volcanic eruptions or produced by primitive life -- was present in the Martian atmosphere. But even CO2 and methane combined would not be enough to warm the planet as greenhouse gases did on Earth and Venus in their early histories, Toon 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," wrote the authors in Science. "In contrast, we envision a cold and dry planet, an almost endless winter broken by episodes of scalding rains followed by flash floods.

"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. Although temperatures in the subsurface of Martian soil may have exceeded the boiling point during the impact period and provided a possible refuge for life underground, the short duration of warm periods predicted by the researchers would have made it difficult for life to ever establish itself on Mars, the team concluded.

Note: If no author is given, the source is cited instead.

• Mars
• Solar System
• Space Exploration
• Space Missions
• NASA
• Astronomy

• Exploration of Mars
• Phoenix (spacecraft)
• Near-Earth asteroid
• Asteroid belt
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• Equatorial bulge