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Science / Medicine : Magellan Craft Refutes Theories on Venus Mountains : Space: The probe’s radar mapper finds that a vast highland is neither an ancient continent nor an extremely recent creation.

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TIMES SCIENCE WRITER

Scientists had expected the Magellan spacecraft to determine which of two leading theories about the formation of a huge mountain range on Venus is correct. Now they may have to come up with another idea.

Data from the spacecraft, now being analyzed at the Jet Propulsion Laboratory in Pasadena, suggests that both of the theories probably are wrong, according to mission scientist Steve Saunders.

“We are just starting to see the images now,” Saunders said. “Everybody is very excited about it.”

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The images from Magellan’s radar mapper show the western region of the great equatorial highland, called Aphrodite, that extends two-thirds of the way around the planet. The highland, about 2,000 miles wide and 15,000 miles long, has long fascinated planetary geologists who wonder how such a huge geological structure could have formed on a planet that does not have the same kind of plate tectonics that created giant mountain ranges on Earth.

“Everybody has had different theories about what formed it,” Saunders said. Some think it was formed by something like the Mid-Ocean Ridge (which created a long mountain range beneath the Atlantic Ocean on Earth) and others think it was an ancient continent. Those views represent two ends of a vast spectrum of possibilities that make the feature extremely recent or the oldest on Venus.

“But as one might expect, nature was smarter than all of us,” Saunders said. “It’s none of the above. I think we are seeing a fairly young surface that is rather complex and doesn’t fit simply” with current theories about the forces that create great mountain ranges on Venus. He said that scientists will continue to analyze the data, and they could be ready by March to say what they think formed the Aphrodite region.

Magellan, which is run by JPL for the National Aeronautics and Space Administration, has mapped more than half the surface of Venus with its unique system. The spacecraft uses a “side-looking” radar that images 10-mile-wide swaths of the surface as it orbits over Venus. The long images are blended to form mosaics of the Venusian surface, yielding a comprehensive picture of a planet that has never been seen clearly before.

Since the imaging system uses radar beams instead of visible light, it penetrates through the thick, poisonous atmosphere that hides the surface of Venus from other cameras. Magellan also has a precise altimeter that measures the topography of the surface as the craft zips over, giving scientists the information they need to create three-dimensional images of the planet.

The latest images released by JPL show a tortured landscape that is molded largely by extreme volcanism. Some of the photos show surface bulges 30 to 140 miles wide. The bulges, called “arachnoids” because of their resemblance to spiders, apparently were formed when molten rock pushed up beneath the surface. The ground cracked from the strain, creating long features radiating out from each bulge that look like spider legs.

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Scientists theorize that the arachnoids were formed by a process similar to the one that created the Hawaiian Islands on Earth. A “hot spot” in the Earth’s mantle sends molten rock up toward the Earth’s crust. It eventually melts through the surface and oozes out, forming the volcanoes that created the islands.

Venus is believed to be volcanically active today, although Magellan has not confirmed that. It may be that after the craft completes its first survey of the planet this May and begins to cover the same territory again, some images will show features that were not there when the craft passed over the first time. That would confirm active geological processes.

It is expected to take about two years for the craft to complete its $744-million mission. By the time Magellan is finished, scientists expect to have detailed maps of more than 80% of the surface. That resource should, in turn, help them understand more about Earth, a planet that started out similar to Venus but turned out so differently.

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