Venus
DISTANCE FROM SUN: 67.2 million miles (108.2 million kilometers).
REVOLUTION AROUND SUN: 225 days.
ROTATION: 243 days.
DIAMETER: 7,520 miles (12,100 kilometers).
DENSITY: 5.2 x that of water.
MASS: 0.815 x that of Earth.
SURFACE TEMPERATURE: 850°F (454°C).

Veiled in clouds, Venus is the second brightest object in the night sky. Traditionally it is thought of as Earth’s sister planet, but “evil twin” would more closely describe it. While Venus is similar to Earth in size, mass, density and volume, it is also surrounded by thick clouds of sulfuric acid. The atmosphere is primarily carbon dioxide. And surface temperatures are similar to those in a self-cleaning oven.

It is that cloud cover—the feature that makes Venus sparkle like a jewel in our sky—that is responsible for the planet’s inhospitable nature. It is a greenhouse nightmare. Heat from the sun is trapped in the atmosphere beneath the canopy to create a surface temperature of about 850°F (454°C), hotter even than on Mercury, which is much closer to the sun.

More than 80 percent of Venus is covered with lava flows and plains, plus other volcanic features. That’s because the planet has 430 volcanoes 12 miles (19 kilometers) or more in diameter, and tens of thousands of smaller ones. Lava flows have totally resurfaced the planet over the past 300 to 500 million years. There are still some craters from relatively recent meteorite impacts, but under the planet’s incredibly high atmospheric pressure—some 92 times that of Earth—ejecta does not range very far from the crater.

For centuries, the cloud cover prevented astronomers from mapping the surface of Venus. Some Earth-based radar studies revealed large formations. A handful of landers gave limited local information, but they were soon shut down by the harsh Venusian conditions. Finally, the Magellan orbiter of 1991-92 created a high-resolution radar map of the surface that showed raised continent-like land masses. Magellan mapped 84 percent of the planet in just one day—that is, one Venusian day, equal to 243 days on Earth.


A BLANKET OF CLOUDS From a distance, the Venusian atmosphere resembles a stationary shroud of clouds. Viewed up close in ultraviolet light, however, the sulfuric acid clouds are a swirling maelstrom that circles the globe every four days. Horizontal cloud formations, often appearing in a Y-shape, indicate Venus may experience atmospheric waves, like the high and low pressure cells that create weather patterns on earth.
Because Venus is between the Earth and the sun, it appears to go through phases, like the moon. When the planet appears brightest, dominating Earth’s evening or early morning sky, it is actually near its most distant position from Earth, the sun illuminating its entire face from our perspective.
Lower level clouds, discovered with instruments on NASA’s Pioneer probes, show a turbulent, cloudy middle atmosphere some 33 miles (53 kilometers) above the surface, and about 10 miles (16 kilometers) below the visible cloud tops.

MOUNTAINS AND PLAINS Rising seven miles (11 kilometers) above the Venusian surface is Maxwell Montes, the highest mountain on Venus. Scientists believe the mountain, which is really a series of parallel ridges, was created by uplift that may have resulted when one crustal plate pushed up against another. All across the face of Venus is evidence of plate tectonics. On Earth, such forces push up vast mountain ranges that cover huge regions of the surface, while on Venus, the effect is more localized, astrogeologists call the process blob tectonics.

Impact craters also pock the Venusian surface. Sometimes surface rocks melt from the heat of impact and the resulting flow spreads out over the surrounding terrain. This may result in vast, flat lava plains like Lakshmi Planum, a dark, smooth lava flow at the foot of Maxwell Montes.

VOLCANOES Venus is a smorgasbord of volcanic formations. Heat generated by radioactive decay in the planet’s interior creates pockets of molten rock. The temperature, composition, and gas content of the magma determine whether the resulting lava has the viscosity of motor oil or toothpaste. And that helps determine the size and shape of the formations.

Steep-sided volcanic domes, called pancakes by planetary geologists, are caused by lava oozing through the crust and cooling as it spreads in a circular pattern. They are not as alien as they may seem though: California is dotted with similar volcanic structures that become overgrown with plants and eroded by wind and water.