clouds Venus might be a steamy tropical world much like prehistoric Earth, but nothing could have been further from the truth. In 1963 Mariner 2, history's first planetary probe, showed that temperatures on Venus were too high to support liquid water. A decade later, Mariner 10 surveyed the Venusian upper atmosphere and clocked winds up to 300 kilometers per hour (Pasachoff, 1990, p. 179). Remarkably, while Venus rotates once every 243 days, its atmosphere whips around the planet in the other direction, once every 4 days.

To find out what lay beneath those clouds, astronomers had only one tool: radar. By the early 1970s, they were using radio telescopes like the 300-meter dish at Arecibo Observatory in Puerto Rico to bounce radar signals off the planet's surface. Good observations were possible only during inferior conjunction, the period when Venus is closest to Earth. Because Venus always shows the same face to Earth at this time—a result of a near commensurability of Venus' rotation rate and Earth's orbital period—Earth-based radar surveys were limited to about 25 percent of the planet. Images made from the radar data showed just enough to tantalize: circular features that seemed to be impact craters, bright spots thought to be volcanoes, and banded terrain that might be the result of crustal folding. Clearly, Venus' surface had been shaped by a variety of geologic processes, but no one would be able to say more without getting a better look.

Meanwhile, in 1975 the Soviet Union scored an astonishing success when two unmanned probes, Veneras 9 and 10, parachuted through a dense atmosphere laden with droplets of sulfuric acid and touched down on the torrid surface. The Veneras found surface temperatures of 450°C and pressures of 90 bars (i.e., 90 times the pressure at the surface of Earth). And in the brief minutes before they succumbed to these hellish conditions, the probes sent back black and white images of a rock-strewn surface. At Venera 9's landing site the rocks were surprisingly sharp edged, testifying to an almost total absence of erosion, while Venera 10's images showed rocks with a pancake-like appearance. In 1982 the Soviets bettered this spectacular achievement with Veneras 13 and 14, each of which survived on the surface for more than 2 hours. In addition to sending back higher-quality images, including some in color, each lander actually drilled a sample of the surface material and brought it inside a sealed chamber for chemical analysis. The results indicated a composition consistent with basalt, the iron- and magnesium-rich volcanic rock most common on the surfaces of the earth and moon.

As is so often the case in science, a single piece of information—that there is probably basalt on Venus—had great significance. Geologists



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