Study: Asteroids shaped Mars canyons

Martian wet seasons may not have supported microbes

By Richard Stenger
CNN

Valles Marineris, called the Grand Canyon of Mars, dominates the planet's southern highlands.

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RELATED Science magazine  Mars exploration (NASA)  BIG DITCH Valles Marineris is more than four times longer and deeper than the Grand Canyon. The red planet trench is 2,500 miles long and up to four miles deep.

(CNN) -- The authors of a new study say that while enormous surges of water carved out sprawling networks of valleys on Mars eons ago, the young red planet was hardly a steamy greenhouse with long-lasting oceans, as some scientists have theorized.

Researchers with NASA and the University of Colorado say the canyons most likely were formed during brief bouts of torrential rains, when asteroid and comet chunks collided with Mars and unleashed frozen water.

They propose that the flurry of impacts produced periodic layers of hot debris that blanketed the surface, warming it above the freezing point of water for decades or centuries at a stretch.

"We envision a cold and dry planet, an almost endless winter, broken by episodes of scalding rains followed by flash floods," the researchers wrote Thursday in the journal Science.

Deep gorges and gullies cut across the heavily cratered southern highlands of Mars, at about 4 billion years old considered the oldest terrain on the planet.

At the time, our solar system was quite young, and many asteroids and comets bombarded the inner planets. Mars still bears many of the crater scars. Earth has avoided the blemishes, with its oceans and still-evolving continents.

Space scientist Teresa Segura and colleagues used computers to calculate the effects of the impacts. They determined that the collisions unleashed enough water, whether from the space boulders, the ejected Martian material or exposed ice, to create the valleys.

Jumbo impacts could have injected steam into the atmosphere or evaporated subsurface or polar cap ice, fueling temporary rainstorms that lasted long enough to create rivers and carve the channels.

But could a wet season have lasted long enough for primitive microbes to take root on the red planet, as some have suggested? Segura seems to doubt it.

"The short duration of the warm episodes predicted here may have made it challenging for life to establish itself on Mars in the first place," she and her colleagues wrote.

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