FOR scientists working with the Magellan planet-mapping spacecraft, their survey of cloud-shrouded Venus has been a concept-shattering revelation.
Venus is nearly as massive as Earth and has a similar composition. Planetary scientists were calling it "our sister planet" when Magellan reached it on Aug. 10, 1990.
Now that the spacecraft's cloud-penetrating radar has nearly finished its mapping mission, these scientists "have found out it's certainly not Earth's twin," says planetary geologist James W. Head of Brown University in Providence, R.I.
Magellan's ability to produce images that are 10 times sharper than any seen before has revealed what Dr. Head calls "a distinctly different" world.
When Magellan project scientist R. Stephen Saunders, of the California Institute of Technology's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., makes the same point, he also notes that "for the first time in human history, we've produced a [nearly complete] map of a planet the size of Earth."
One of Magellan's primary objectives was to map more than 95 percent of the Venusian surface. Its mapping now covers more than 97 percent of that surface. Moreover, it has imaged 20 percent of the surface in stereo - taking matching images from different angles to aid three-dimensional analysis.
Magellan radar scientist Gordon H. Pettengill, of the Massachusetts Institute of Technology in Cambridge, Mass., expresses the radar team's satisfaction when he says, "We've exceeded [our objectives] and then some!"
Venus, with Magellan in orbit around it, currently is passing behind the sun. When it reemerges as a brilliant evening "star" this autumn, controllers plan to change Magellan's orbit to fulfill the last of the mission's primary objectives. This is the task of making a detailed map of Venus's gravitational field.
The overfulfillment of Magellan's mapping objectives has been a tough technical challenge for the control team at JPL, which manages the mission for the US National Aeronautics and Space Administration (NASA).
Magellan began this year with a failed primary-data transmitter and a backup unit whose transmissions were degraded by a spurious whistle. Engineers learned to minimize the whistle's interference, however. They restored data transmission to half its usual rate.
The mission plan is laid out as a series of cycles. Each of these lasts 243 days, the time it takes for the planet to rotate once beneath the spacecraft's orbit. Magellan now is well into its third mapping cycle. Its radar images show surface details ranging in size from about 120 meters (390 feet) near the equator to 300 meters (980 feet) near the north pole. Previous American and Russian radar scans imaged details no finer than 1,000 to 2,400 meters (0.7 to 1.5 miles) across.
JPL's Dr. Saunders notes that Magellan now has produced a planetary map of Venus with sharper average detail than shown on any comparable map of Earth. Planetary geologist Head explains that you would have to drain the ocean to chart Earth in that detail.
Head notes that Magellan's images show Venus to be as geologically active a planet as is Earth. Also, like Earth, it appears to have remained geologically active throughout its history. Its features are not frozen in time as is the case with Mars and the moon, where tectonic activity generally ceased long ago. Beyond this the similarity ceases. Venus hides a distinctly unearthly face beneath its veil of cloud.
EARTH'S crust is shaped by continuous recycling. Its crust is broken into tectonic plates that move relative to one another. New crust wells up along volcanically active mid-ocean ridges. It spreads out horizontally from these ridges until in disappears back into the interior. This generally happens in deep-sea troughs where an oceanic plate dives beneath a continental plate.
Head explains that on Venus, "There doesn't seem to be much crustal recycling as known on Earth." The Venus surface appears to be evolving by building crust vertically as lava flows out of volcanoes or meteorite craters. This is more akin to what has happened on the moon and Mars than to the way Earth's surface evolves. It challenges Magellan scientists to try to decipher how Venusian tectonics actually work.
Craters on Venus offer a different kind of challenge. Head calls crater research a "growth area for Venus science." Magellan images show about a thousand impact craters. They generally seem to be in pristine condition. This is another difference between Earth and its erstwhile "twin."
"Earth is a water-damaged planet," Head explains. Erosion is so powerful it's hard to find good craters - especially large meteorite craters - to study. Venus is a dry planet with apparently little erosion. The surface appears to be some 500 million years old.
"The good news is that the surface is preserved so we can study it," says Head. But, he adds, "The bad news is that everything that happened in any one place is still there so it's confusing." He explains that it's like looking at a multiple-exposure photograph. With features one on top of another, it's hard to sort out what happened when. Nevertheless, he says, "Venus has given scientists a laboratory in which they can study what happens when impact craters occur."
Meanwhile, planetary geologists look forward to charting Venus's gravity. After the current radar-mapping cycle ends in mid-September, controllers plan to lower Magellan's orbit so its closest approach to Venus's surface will drop from 186 miles to 111 miles. This will increase its sensitivity to slight differences in the gravitational field due to the planet's makeup. The spacecraft will speed up or slow down slightly as it passes over varying subsurface densities. These speed changes will show up as sl ight frequency changes in Magellan's radio "carrier" signal, which is not degraded by the spurious whistle.
The Magellan team would like to carry on this work through 1995. It should help in determining the underlying processes that shape Venus's puzzling surface. However, NASA currently plans to end the Magellan mission next April. Magellan scientists are appealing this decision. At this writing, NASA had not yet answered their appeal.