Our Cratered Sister Planet

Abstract

A new radar map of Venus-the most detailed such map ever obtained -shows that surface of cloudcovered planet pocked with craters. Or at least one swatch is-an area 910 miles across, about size of Alaska. region contains a dozen large craters ranging from 21 miles across to one 100 miles across and possibly many smaller craters invisible to radar. craters are all relatively shallow. 100-mile-wide crater only a quarter of a mile deep. This area of Venus appears to be as crater-infested as moon, says Richard M. Goldstein, head of radar mapping team that produced map at Jet Propulsion Laboratory (JPL) in Pasadena. whole area itself basically flat, says Howard C. Rumsey, team member who devised computer techniques for making map. area may vary by no more than about 3,300 feet in altitude. The surprising thing to discover any craters at all, he says. Why are large craters so shallow? That question, says Goldstein, is bound to be important relative to internal and external processes that have gone on [and may still be going on] on Venus. shallow craters could be result of preatmospheric bombardment of planet and then subsequent isostatic readajustment. Extensive internal degassing may have flooded craters with a fill such as lava. Or subsequent erosion on surface could have filled craters with dust. The shallower craters are, says Harold Masursky of U.S. Geological Survey, the more dynamic crust of a planet is. He, along with many others who have been studying craters on earth, moon and most recently Mars, ecstatic about craters on Goldstein's map. Several of us are convinced that surface of Venus will help us understand earth's crust and dynamics that lead to continental drift. He suggests that Venus' crust may be even more mobile than earth's crust, because Venus still hot. But to determine whether craters were made shallow by isostatic readjustment or by lava fill, much-higher-resolution radar maps of Venus' surface would be needed. Masursky suggests that these might be obtainable from a spacecraft placed into orbit around Venus equipped with a radar altimeter and side-looking radar. NASA has been studying such a mission. Certainly seeing through to surface of a cloud-shrouded planet from earth-such as Goldstein's group has done-was no easy task. It took radar. an array of sophisticated computer techniques and signal-processing equipment and a year of painstaking work. just now completed. results are images of surface with a resolution of about six miles-five times better than map of Venus team produced in 1970. JPL scientists used two antennas -the 210-foot dish and 85-foot dish at Goldstone site near Pasadena. 210-foot dish beamed a series of .I.: