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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.

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