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       The Magellan spacecraft mapping the surface of Venus with imaging radar has swept over nearly 55 percent of the planet, an area comparable on Earth to the distance from Los Angeles to New Delhi, India.

       Magellan scientists said the radar data provides significant new knowledge about the surface of Venus and its atmosphere.

       All of the areas mapped show widespread evidence of volcanism, said Project Scientist Dr. Steve Saunders, along with evidence of tectonics, the process that produces mountains.

       "Venus and Earth are the only planets in our solar system that have linear mountain belts," he said. But the mountains on Venus are not deeply eroded by rainfall and running water as are the mountains on Earth.

       Magellan also confirmed the number of impact craters that scientists had expected to find, judging from their earlier Earth-based radar data.

       The smallest impact craters that have been seen so far on the surface are about three miles in diameter, which indicates the dense atmosphere has effectively shielded the surface from bombardment of smaller asteroids and comets.

       There is also evidence that the poisonous, thick atmosphere of Venus was not formed recently, Saunders said. Surface images indicate it may be from 400 million to 800 million years old or even older. The Venus atmosphere is 90 times heavier than that of Earth and is composed primarily of carbon dioxide with significant amounts of sulfuric acid at upper levels.

       Saunders said scientists see in the images that linear mountain belts are being pulled apart by gravitational forces on the planet.

       New styles of volcanism have been found and lava channels hundreds of miles long occur at several places on the plains. Although lava channels have been found on Earth, none are as long or as regular as those seen on Venus.

       Another new type of volcanism is being referred to by scientists as "pancake" domes; these structures appear to be up to 20 miles across and nearly a mile high and form on the plains. Scientists believe the domes are formed by an outflow of a pasty, thick lava, similar to silicon-rich lavas on Earth.

       Volcanic domes also form on Earth, but they are much smaller and form in volcanic calderas, Saunders said.

       The images also reveal indications of turbulent surface winds on the planet. The evidence is in the form of wind streaks in the lee of topographical obstacles, such as the small, low shield volcanoes on the plains.

       "Careful mapping of those wind streaks over the entire planet may give us meteorological data about circulation of the atmosphere near the surface," Saunders said.

       To date, there have been 118 days of mapping and 973 orbits. A total of 819.6 mapping orbits have been received on Earth, 45 percent of the planet's surface, Project Manager Tony Spear said.      Fifteen days of mapping were lost, as expected, to superior conjunction, the period last November when Earth and Venus were on opposite sides of the sun and data transmission between the two planets was impossible.

       There have been other losses of data since mapping began last Sept. 15, due to spacecraft problems and difficulties at the various Deep Space Network stations, Spear said. But of the area covered by mapping orbits, only 2.4 percent has been lost.

Editors note: Three pictures will be released with the story. The pictures will be available Friday, Jan. 25, at the Jet Propulsion Laboratory and at NASA Public Affairs Office in Washington, D.C.


1/24/91 JJD