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
'"/>

Contact: Teresa Segura
segurat@colorado.edu
650-604-0321
University of Colorado at Boulder
5-Dec-2002