cuts—planetary scientists seemed to have little to look forward to. The arrival of a robotic explorer called Magellan in orbit around Venus in August 1990 seemed almost like an afterthought.

Except, that is, to the scientists who had spent the better part of two decades working to make this happen. To them, Magellan offered a unique chance to study the one planet long considered by many scientists to be most like our own. Equipped with a radar-imaging system to pierce the cloudy veil, Magellan was designed to map the planet's surface in unprecedented detail. High on the list of the Magellan science team's questions was the most intriguing one of all: Is Venus still geologically active?

To be sure, much was known of Venus prior to Magellan's arrival. Soviet landers had proved that it is a place of broiling heat and crushing atmospheric pressure. The planet's surface features had been glimpsed in the preceding two decades, first using Earth-based radar and then from orbiting Soviet and American probes. But Magellan's pictures showed features as small as a few hundred meters across, far more detail than any previous views of Venus—in fact, better resolution than that available for vast areas of Earth, namely the ocean basins. From the first images the spacecraft radioed to Earth, the Magellan team of scientists—many of whom were veterans of several planetary missions—were stunned. The sheer variety of features was breathtaking. Everywhere, it seemed, there were volcanoes, ranging in size from tiny peaks at the limits of resolution to giant shield volcanoes hundreds of kilometers across. The smaller variety, several kilometers in diameter, numbered in the tens or even hundreds of thousands. Most of the planet's surface was covered with smooth, flat plains, also attributed to outpourings of molten lava. At first glance, it seemed that much of Venus' history was written in volcanism.

Elsewhere, huge belts of fractures sliced through the equatorial plains, extending nearly all the way around the planet. Looking closely, the scientists saw much finer fractures distributed over almost the entire planet. Elsewhere, they found complex jumbles of hills and valleys, now known as tesserae, that seemed to be in the process of destruction by tectonic forces. And there were some features found nowhere else in the solar system: giant welts called coronae, whose margins are jumbles of fractured rock but whose centers seem to have been formed by the buildup of lava. The smallest coronae are less than 100 kilometers

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