Red river valleys – planet

Red river valleys – planet – geologic evidence of atypical water channels on Mars

MARS TODAY IS A FROZEN DESert, but the photos sent back by the Mariner and Viking probes in the 1970s indicate its past was less bleak and more Earth-like. The images showed sinuous channels and valleys that were almost certainly carved by flowing water. More than 3 billion years ago, researchers believe, Mars went through a warm epoch, as vigorously active volcanoes spewed greenhouse gases into its atmosphere. Water that had been locked up as permafrost earlier in the planet’s history flooded over its surface, cutting the channels and perhaps even forming large lakes in some areas.

But one network of valleys, the ones etched on the slopes of an extinct volcano called Alba Patera, doesn’t fit this model. These valleys are much younger than the others. Judging from the small number of meteorite craters overprinted on the Alba Patera valleys, says planetary scientist Virginia Gulick of NASA’s Ames Research Center, they were carved only about a billion years ago. By then, most of Mars’ volcanoes had sputtered out, the planet had cooled, and all its water had once again become trapped in ice and permafrost. “The Alba Patera valleys formed at a time when we think Mars didn’t have a warm, wet climate,” says Gulick. “They formed well after that. The problem is how to explain them.”

Gulick and Christopher McKay, a NASA colleague, have come up with a unique explanation. A billion or so years ago, they speculate, a frozen lake covered the plains around Alba Patera, which was then still active. Since most of the other volcanoes had stopped spewing gases into the atmosphere, the temperature and atmospheric pressure on Mars were so low that ice from the lake surface sublimated–it transformed into vapor without first melting as water. The water vapor formed clouds that eventually dropped snow onto the volcano’s slopes. The volcano’s magma chamber then warmed the slopes enough to melt the bottom layers of the deep snowpack.

So while the rest of the planet was already dry, the Alba Patera region was running its own little water cycle, according to Gulick and McKay, and small torrents of snowmelt were cutting valleys into the volcano’s flanks. The pattern of the valleys supports this scenario, Gulick says. Whereas the far more massive melting of permafrost earlier in Martian history produced broad and comparatively straight channels, the valleys on Alba Patera form a complex network of tributaries much like the ones carved by spring snowmelt on our own mountainsides. Says Gulick: “They’re the most Earth-like valleys we see on the planet.”

COPYRIGHT 1994 Discover

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