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Venus: surprising features sculpted in lava.

Venus: Surprising features sculpted in lava

Astronomers and geologists have long sought to penetrate the thick clouds shrouding Venus in the hope of discovering just how closely its geologic structures resemble those on Earth. Over the past two decades, several spacecraft have used radar and ultraviolet beams to view portions of the Venusian surface. But none offered the detailed resolution of the Magellan craft, which began mapping Venus last September and has so far surveyed 70 percent of the planet's surface.

Researchers have now unveiled the results of the first few months of Magellan's radar survey, which covered about 15 percent of the Venusian surface.

Analyses of these data reveal several novel geological features and confirm radar observations made by several U.S. and Soviet craft, says Magellan project scientist R. Stephen Saunders of the Jet Propulsion Laboratory in Pasadena, Calif. He and his colleagues describe Magellan's early results in a collection of reports in the April 12 SCIENCE.

Among the striking findings: thousands of small volcanoes dotting the mostly flat Venusian landscape, mountainous volcanic structures several hundred kilometers in diameter, and evidence of massive outpourings of lava.

Magellan also found several unusually high elevations never before seen in images of the planet, Saunders says. In a region near Alpha Regio, 30[degrees] south of Venus' equator, the orbiter detected seven flattened domes up to 30 kilometers in diameter and rising nearly a kilometer in height. The steep sides of these domed hills indicate that they formed from a viscous lava, he notes.

A more liquid lava -- common on Earth and believed to form the vast plains that make up 70 percent of the Venusian landscape -- would spread out more evenly and would not likely build the steep, high-altitude hills, Saunders explains. In addition, he says, Venus' scorching surface temperatures may cause lava to cool more slowly there than on Earth, perhaps accounting for Venus' relatively taller domes.

Saunders adds that a thick, viscous lava, typically rich in silica, usually indicates its underground predecessor -- molten rock, or magma -- has undergone considerable evolution, perhaps involving processes akin to those that created granite-like rock beneath Earth's continents. "The big question is, are we looking at [highly evolved] material like we find inside the Earth's continents, or are we maybe looking at some stage in the evolution of [more primitive] magma?"

Magellan also focused its radar-sharp eye on another giant structure, confirming that a 1983 Soviet craft had indeed observed the largest known caldera on Venus. Magellan images show that this mammoth volcanic depression, lying in a region known as Sacajawea Patera, covers 300 kilometers in diameter and 1 to 2 kilometers deep -- 10 times the area of any caldera on Earth. Saunders and his colleagues speculate that the caldera formed in a manner similar to those on Earth: When magma drained from an underground reservoir, the surface rock lost its support and collapsed to form a gaping depression.

Saunders emphasizes that the new findings represent just a taste of discoveries to come. "The purpose of this [mission] is to get a picture of the planet as a whole," he says, "and we haven't finished that job yet." During Magellan's second mapping cycle, the orbiter will complete a map of Venus' entire surface. That survey, scheduled to begin May 15, will include the south polar region -- an area never before studied.

Contributors to the first wave of Magellan reports include researchers from Washington University in St. Louis; Brown University in Providence, R.I.; the U.S. Geological Survey in Flagstaff, Ariz.; the Massachusetts Institute of Technology; Cornell University; University College London in England; Cambridge University in England; and the University of California, Los Angeles.
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Title Annotation:Magellan spacecraft observations
Author:Cowen, Ron
Publication:Science News
Date:Apr 13, 1991
Words:611
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