Because Venus, of all the other planets, most nearly matches Earth in size, mass, and density, and because its orbit is closest to us, it is often called Earth's sister planet. But unlike Earth, Venus has a dense atmosphere and thick clouds that are opaque to visible radiation, making its surface completely invisible from the outside at optical wavelengths. Figure 9.5 shows one of the best photographs of Venus taken with a large telescope on Earth. The planet resembles a white-yellow disk and shows occasional hints of cloud circulation.

Figure 9.5 This photograph, taken from Earth, shows Venus with its creamy yellow mask of clouds.

Atmospheric patterns are much more evident when examined with equipment capable of detecting ultraviolet radiation. Some of Venus's atmospheric constituents absorb this high-frequency radiation, greatly increasing the cloud contrast. Figure 9.6 is an ultraviolet image taken in 1979 by the U.S. Pioneer Venus spacecraft at a distance of 200,000 km from the planet. The large, fast-moving cloud patterns resemble Earth's high-altitude jet stream more than the great cyclonic whirls characteristic of Earth's low-altitude clouds. The upper deck of clouds on Venus rotates around the planet in just 4 days—much faster than the planet itself.

Figure 9.6 Venus as it was photographed by the Pioneer spacecraft's cameras some 200,000 km from the planet. This image was made by capturing solar ultraviolet radiation reflected from the planet's clouds, which are probably composed mostly of droplets of sulfuric acid (the highly corrosive acid used in car batteries).

Early spectroscopic studies of sunlight reflected from Venus's clouds revealed the presence of large amounts of carbon dioxide but provided little evidence for any other atmospheric gases. Until the 1950s, astronomers generally believed that observational difficulties alone prevented them from seeing other atmospheric components. The hope lingered that Venus's clouds were actually predominantly water vapor, like those on Earth, and that below the cloud cover Venus might be a habitable planet similar to our own. Indeed, in the 1930s scientists had measured the temperature of the atmosphere spectroscopically at about 240 K, not much different from our own upper atmosphere. (Sec. 4.4) Calculations of the surface temperature—taking into account the cloud cover and Venus's proximity to the Sun, and assuming an atmosphere much like our own—suggested that Venus should have a surface temperature only 10 or 20 degrees higher than Earth's.

These hopes for an Earth-like Venus were dashed in 1956, when radio observations of the planet were used to measure its thermal energy emission. Unlike visible light, radio waves easily penetrate the cloud layer, and they gave the first indication of conditions on or near the surface. The radiation emitted by the planet has a blackbody spectrum characteristic of a temperature near 730 K! (Sec. 3.4) Almost overnight, the popular conception of Venus changed from that of a lush tropical jungle to an arid, uninhabitable desert.

Radar observations of the surface of Venus are now routinely carried out from Earth using the Arecibo radio telescope. (Sec. 5.4) This instrument can achieve a resolution of a few kilometers, but it can adequately cover only a small fraction (roughly 25 percent) of the planet. The telescope's view of Venus is limited by the planet's peculiar near-resonance, as just described, and also because radar reflections from regions near the "edge" of the planet are hard to obtain. However, the Arecibo data have been usefully combined with information from probes orbiting Venus to build up a detailed picture of the planet's surface. Only quite recently, with the arrival of the Magellan probe, were more accurate data obtained.