Investigating the Mysteries of Venus

The first spacecraft to travel beyond Earth's moon, the U.S. probe Mariner 2, passed within 34,760 kilometers (21,600 miles) of Venus in December 1962. Mariner 2's microwave radiometer detected a surprising amount of heat, in the form of infrared radiation, seeping up from beneath the layer of clouds that surrounds the planet. This discovery shattered the theory that the climate of Venus was similar to the Earth's. Measurements by subsequent spacecraft showed that the atmosphere of Venus consists mainly of carbon dioxide and that the atmospheric pressure at the surface is 90 times higher than sea-level air pressure on Earth. This enormous blanket of carbon dioxide produces an intense greenhouse effect: The atmosphere lets sunlight reach the surface but prevents that energy—converted to infrared radiation by the heated planet—from escaping back into space. Due to the greenhouse effect, the average surface temperature of Venus is 462 °C (864 °F), hot enough to melt lead.

But had it always been like that? The answer to this question had to await the December 1978 arrival of two NASA spacecraft—Pioneer Venus 1, an orbiter, and Pioneer Venus 2, which carried four atmospheric probes. During its descent through Venus's atmosphere, one of the atmospheric probes measured a surprisingly large amount of deuterium—an isotope (variant form) of hydrogen—relative to normal hydrogen. This discovery led scientists to theorize that there had once been large amounts of water on Venus.

Several billion years ago, many scientists now believe, an ocean existed on Venus, and the climate was perhaps only somewhat hotter than Earth's is today. The heat, however, was sufficient to cause water from Venus's ocean to evaporate in large quantities. The vapor drifted into the upper atmosphere, where the water molecules were broken apart into hydrogen and oxygen by ultraviolet light from the sun. Only a tiny fraction of ocean water contains deuterium rather than normal hydrogen. The normal hydrogen, however, because it is lighter than deuterium, escaped Venus's atmosphere more easily. As a result, the atmosphere of Venus today is left with a large proportion of deuterium—the ghostly echo of an ocean long gone.

While the ocean evaporated, carbon dioxide from within the planet was building up in the atmosphere. On Earth, carbon dioxide is constantly being removed from the atmosphere by rainfall and plants and by the weathering of rocks to form compounds called carbonates. With none of these processes operating on Venus, the result was an overabundance of carbon dioxide and a runaway greenhouse effect.

Although the Pioneer Venus probes told scientists much about Venus's atmosphere, the surface of the cloud-shrouded planet remained a mystery. It was not until 1984 that the veil began to lift. In that year, twin orbiters launched by the Soviet Union—Venera 15 and Venera 16—successfully mapped about a fourth of Venus's surface using cloud-penetrating radar. The maps revealed a geology unlike anything seen on Earth. Volcanic activity was revealed in several huge regions of fractures on the surface, large volcanoes, and lava flows. But there was virtually no sign of the kinds of long, organized ridges and trenches that on Earth are characteristic of plate tectonics (the slow movement of rock plates making up our planet's crust).

More extensive mapping of Venus was carried out by NASA's Magellan spacecraft, which began orbiting Venus in 1990. Armed with synthetic aperture radar—an advanced kind of radar that can scan a planet's surface in much greater detail—Magellan mapped virtually the entire surface of Venus. The mountains of Venus shown in Magellan's images have an eerie incompleteness to them; they lack the canyons, gullies, and valleys that water carves into the mountainous regions of the Earth. The absence of such features confirmed that Venus has been without liquid water for most of its geologic history.

Magellan's radar maps also showed that the surface of Venus, compared to other rocky, waterless bodies in the solar system, is pocked by relatively few meteorite craters. Furthermore, the cratering is uniform across the entire surface—no one area has significantly more or fewer craters than any other area. To some geologists, these findings indicate that the surface of Venus is probably relatively young in geologic terms and that the entire present surface of Venus probably formed at about the same time. But what kind of cataclysmic event could have caused the resurfacing of an entire planet remains a mystery. One theory holds that heat builds up within the planet and that every few hundred million years, the entire crust breaks apart, melts, and then re-forms.