In its new, lower orbit, NASA's Magellan spacecraft has acquired high-resolution gravity data over about one-third of Venus by measuring the motions of the orbiting spacecraft through its radio signal.

The gravity data give scientists a glimpse inside Venus -revealing the internal structure that lies below its surface -by disclosing the relative density of different parts of the planet.

"These new data provide high resolution in the polar regions which previously was very poor due to the orbit's very high altitudes," William L. Sjogren, head of the Magellan Project's gravity team at NASA's Jet Propulsion Laboratory, told the 1993 fall meeting of the American Geophysical Union in San Francisco.

Sjogren's presentation was one of several given at the AGU meeting by gravity team members and other Magellan scientists.

On August 6, the spacecraft successfully completed a 70-day aerobraking phase which changed its orbit from highly elliptical to nearly circular and began high-resolution gravity mapping.

Two major areas of study on Venus, the Lakshmi and Maxwell Montes regions, were resolved for interior structure, Sjogren said.

Maxwell, a far northern mountain range, is the highest region on Venus. Sjogren said it is critically important to geophysicists who need to know the gravity field to realistically infer Venus' internal structure.

The mantle of Venus, as on Earth, is believed to consist of dense, rocky material that flows slowly in response to internal heating and brings heat energy from deep in the planet to the upper regions. Gravity observations can be used to identify regions of hot, upwelling mantle.

The Magellan spacecraft went into orbit around Venus in August 1990, and performed surface mapping of 98 percent of the planet with imaging radar for two years. In its fourth 243-day orbiting cycle, it began gravity mapping using only its radio signal.

A 243-day cycle represents one rotation of the planet beneath the spacecraft.

JPL manages the Magellan Project for NASA's Office of Space Science, Washington, D.C.

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