PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011

FOR RELEASE FEBRUARY 22, OR THEREAFTER

       At least four competing theories about the nature of Aphrodite Terra, a continent-sized highland on Venus, are being tested by new Magellan data, a scientist at NASA's Jet Propulsion Laboratory said Thursday.

       The Magellan spacecraft, which began imaging the surface of the planet last Sept. 15, has mapped more than 58 percent of Venus.

       Aphrodite Terra, the largest of the highland regions on Venus, extends nearly two-thirds of the way around the planet. Magellan has completed mapping of the region, but the process of producing image mosaics lags behind actual data acquisition, said Project Scientist Dr. Steve Saunders.

       Still, scientists have been able to study radar images of the western portion of Aphrodite, called Ovda Regio.

       Earlier data produced by the Pioneer Venus Radar Mapper indicated that the regional topography may be similar to Earth's continents, Saunders said. The various theories are based on the Pioneer data and other earlier radar imagery, as well as topographic and gravity data.

       "All of the leading proponents of competing ideas for the nature of Venusian continents are part of the Magellan science team, and this leads to lively debates during the science analysis meetings," he said.

       The first hypothesis is the "ancient continent" model which holds that Aphrodite formed from lighter rock that crystallized early and literally floated on the dense mantle of Venus. There are similar formations on Earth and on the moon.

       A second theory is called the "spreading ridge" model. This compares the topography of Aphrodite to that of mid-ocean ridges on Earth where new crust is being formed as the continents drift apart.

       Another hypothesis advocates the "hot spot" model which says the equatorial highlands were pushed up by hot spots similar to the ones that lie beneath the island of Hawaii. Hot spots are regions of mantle that tend to rise, producing broad domes frequently resulting in huge volcanoes.

       The fourth hypothesis is that western Aphrodite is a region of "mantle downwelling". That means a downwelling plume of colder, more dense mantle material causes the surface crust to compress and thicken. The thicker region stands higher than the surroundings.

       "The study of Aphrodite will be extremely important for understanding the origin of continental regions of Venus, Saunders said, "and may help us better understand our own planet."

       Referring to the predicted models for the formation of Aphrodite, he added, "Someone should have predicted that none of the early hypotheses would be easily confirmed. Nature, once again, demonstrates that she is far more imaginative than we, and has provided a surface full of surprises."

       Magellan's primary mission cycle of 243 days, one Venus rotation, ends May 15, at which time more than 80 percent of the surface will have been mapped.

       JPL manages the Magellan mission for NASA's Office of Space Science and Applications.

Editors: Three new Magellan images to illustrate this story will be available at JPL and NASA Headquarters.


#####

2/21/91 JJD
#1349