1990

Venus Mapped by Magellan

When Galileo peered through his early astronomical telescope in 1610, he was the first person to realize that Venus has phases. In contrast to his studies of the Moon and Jupiter, however, Galileo’s observation of Venus did not reveal any specific features or markings—and neither did those of any other astronomers. That’s not surprising: recent telescopic observations and space missions, such as the Soviet Venera orbiters and landers, have showed that the surface of Venus is shrouded from view by a thick atmosphere of carbon dioxide and featureless sulfuric acid clouds and hazes.

Fortunately, radio waves can see through clouds and hazes. Indeed, radar weather mapping instruments used by terrestrial meteorologists exploit the fact that radio waves pass through clouds but bounce off raindrops and snowflakes. In the 1960s, astronomers figured out that radio waves could penetrate the clouds of Venus by bouncing radar signals off the planet using the Arecibo radio telescope. Some surface markings were detected this way, allowing the very slow (243 Earth days), backward rotation of the planet to be discovered. The Arecibo findings helped justify the development of orbital radar mapping missions to Venus. The first successful missions were the Soviet Venera 15 and 16 orbiters in 1983–1984, which mapped about 25 percent of Venus’s northern hemisphere, revealing mountains, ridges, faults, volcanoes, and other landforms.

Venera’s results helped spur an even more ambitious Venus radar mapper: the NASA Magellan mission. Launched in 1989 by the space shuttle Atlantis, Magellan arrived in 1990 and systematically mapped 98 percent of the surface from pole to pole. Magellan data reveal the full scope of Venus topography—from high mountains to deep valleys—and provided geologists with images of a spectacular variety of volcanic, tectonic, impact, and erosional landforms. Magellan scientists discovered vast lava plains, pancake-shaped volcanic domes, and large Hawaiian-style shield volcanoes. They also discovered channels many thousands of miles long carved by very low-viscosity molten rock; vast networks of ridges and troughs, suggesting tectonic activity but not Earthlike plate tectonics; and very few impact craters, suggesting that most of the planet may have been resurfaced by massive outpourings of lava some 500 to 750 million years ago. While certainly not Earth’s twin, Venus turns out to have been (and still is?) just as active over geologic time.

A colorized elevation map (reds and whites are higher; greens and blues are lower) of Venus, using radar data from the NASA Magellan mission and the Arecibo radio telescope in Puerto Rico.