Venus often is called Earth's planetary twin. But it is the differences, rather than the similarities, between it and Earth that now have caught geologists' eyes.
Besides being very similar to Earth in size and mass, Venus has a number of geological features similar to those of our planet. But it appears that Venus may be only at the beginning of the kind of crustal-shaping activity that now is the dominant geological action on Earth. That would make the planet "a powerful tool for understanding this powerful [crustal-shaping] process which has become the great unifying theme in Earth science," says Harold Masursky of the US Geological Survey.
This discovery has emerged from radar surveys of the Venusian surface carried out by the Pioneer Venus spacecraft since it began orbiting the planet in December 1978. Although some of the radar maps had been released earlier, the full range of data was presented here May 28 by the National Aeronautics and Space Administration (NASA).
The instrument responsible -- a radar altimeter -- was not originally a major part of the spacecraft's instrumentation. Pioneer Venus was designed to study the planet's atmosphere, not is surface. It carried four probes that sampled that atmosphere from top to bottom. But the altimeter was adapted for surface mapping and now has produced one the major scientific payoffs of the entire mission. For the fist time, scientists are being given a comprehensive view (some 93 percent) of the surface of this cloud-covered planet.
At a press conference, radar astronomer Gordon H. Penttengill of the Massachusetts Institute of Technology and Dr. Masursky, members of the Pioneer Venus radar team, outlined the main findings. Like Earth, Venus has uplands, mountains, and lowlands, although its sizzling surface -- hot enough to melt lead -- holds no oceans. However, these features are distributed quite differently from those of Earth.
There are mountains as high as any here -- one mountain massifcalled Maxwell is slightly higher than Mt. Everest. Plateaus are even higher than on Earth. But, unlike Earth's, much of the Venusian surface consists of relatively flat, rolling plains. In geologists' terms, about 60 percent of the surface is within 1,000 meters of the level that corresponds to sea level. Only 16 percent of the surface is significantly below this level, while Earth's ocean areas account for up to 70 percent of the surface.
Much of the Venusian surface appears to be ancient. Data from Russian landing craft suggest that material is granitic, like the continental material on Earth. Thus, Venus appears to have formed and preserved vast areas of granite-like crust.
Dr. Pettengill showed maps and artists' interpretations of different features. These included uplifted regions, mountains, rifts, and folds -- all of which indicate strong internal activity. Dr. Masursky indicated the importance of this finding by noting that Venus is the most geologically active body so far found in the solar system after Earth and Jupiter's moon, Io.
He said that if, five years ago, you had asked a planetary scientist what was the most important factor shaping a planet's geological development, the answer would have been: the planet's mass. Now the planet's place in the solar system seems more important.
Io is not large. But it is far more active than Mars or Mercury because of the stresses caused by Jupiter's gravitational pull and Io's interaction with the other Jovian moons. Earth's development may be partly linked to interaction with its large moon. Venus, which has no moons but is more massive than Mercury or Mars, now appears to be intermediate in geological activity.
Earth's surface has been shaped, and is constantly being reshaped, in large part by the action of what is called plate tectonics. Some 10 or so large plates make up the outer surface. These are constantly interacting. They are created at, and move out from, great rifts in the sea floor. Elsewhere, plate material is returned to the interior as one plate dives beneath another. Much of the earthquake and volcanic activity occurs along the margins where plates interact.
This is the "powerful process which has become the great unifying theme of Earth science" to which Dr. Masursky referred. It is this process which the radar views suggest may be just beginning on Venus.
The Pioneer Venus radar maps are produced by an instrument not designed for this purpose. Yet they have produced this important discovery. Now NASA is planning a new spacecraft dedicated to radar mapping -- the Venus orbiting imaging radar (VOIR). If funded by Congress, this would produce hihgly detailed images which Drs. Masursky and Pettengill say would enable scientists to study tectonic activity on a planet whose surface had, until now, been one of the most tantalizing mysteries of the inner planets.