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Did volcanism scar Mars' canyons?

Did volcanism scar Mars' canyons?

There are many signs of volcanism visible on Mars, from peaks and ridges to huge "shields" formed by slowly oozing flows of magma. Less research, however, has explored for signs of volcanism on the floors of a huge canyon complex known as the Valles Marineris, according to Baerbel K. Lucchitta of the U.S. Geological Survey in Flagstaff, Ariz. Only volcanism, she says, cna so easily account for the variety of surface features within this canyon array, which stretches some 4,500 kilometers across the planet's northern hemisphere -- a span as great as the contiguous United States is wide.

Photos taken by two Viking spacecraft, some as early as 1976, show many dark patches on Valles Marineris' floors. Some of the patches follow fault-lines that could have been generated by volcanic stresses. Elsewhere, Lucchitta identified surface deposits with "lobate" (curved) boundaries that appear to have resulted from flowing, molten lava. Valley-floor features that appear to have emerged from nearby craters also evoke volcanic origins, she says.

These apparently volcanic, valley-floor deposits vary from "thin dust to several kilometers" in thickness, Lucchitta says. And in the August ICARUS, she suggest they seem to represent "the last major event in the history of the Valles Marineris" -- possibly occurring as recently as one billion years ago, barely a fifth of the planet's approximate age.

The dark material largely covering the valley floors reflects as little as 5 percent of the sunlight falling on it, Lucchitta says. Some dark patches show "feathery edges" that "may include explosive volcanic vents," she observes. The patches' crispness of detail, lack of any apparent dust cover, and visibility atop landslides all suggest "they are young."

Lighter-colored areas with about three times the reflectivity of the dark patches occur in smaller numbers within the canyons. Though their source is more difficult to identify, she notes they do occur near some craters that "may well be volcanic in origin."

Lucchitta also describes what she calls "mottled material" -- dark and light -- in a canyon known as Candor Chasma. Its origin is harder to identify, but an observed "dearth" of meteor impact craters within it indicates the material is young, she says, suggesting it was deposited at about the same time as the darker material.

Scientists with the International Astronomical Union named the canyons Valles Mariners, or Mariner Valleys, after the Mariner 9 spacecraft that began the process of photographing them in 1971.
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Publication:Science News
Date:Jul 28, 1990
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