VENUS

Earth’s Evil Twin

The planet Venus graces our skies so brightly that it has often been called the morning or evening star depending on when it appears. That brightness is due partly to the fact that Venus is the planet “next door” and because it is shrouded in clouds, which reflect sunlight. The planet’s atmosphere is made mostly of carbon dioxide, a potent greenhouse gas that traps heat from the Sun. Hidden under Venus’s clouds is a rugged desert.

The combination of a 462°C (863°F) surface temperature, an incredibly high atmospheric pressure some ninety-two times that of Earth’s, and a history of volcanic eruptions has turned Venus into a hellish, alien landscape where no life could possibly exist. People often refer to Venus as Earth’s twin because it’s close to our planet’s size and density. However, the nasty climate there makes it clear that Venus is really Earth’s not-so-pleasant twin.

Venus Facts

1. Closest point to Sun: 107.4 million kilometers (66.7 million miles)
2. Most distant point from Sun: 108.9 million kilometers (67.6 million miles)
3. Length of year: 224.7 Earth days
4. Length of day: 117 Earth days
5. Tilt of axis: 177.3 degrees
6. Gravity: 0.9 Earth’s gravity

The History of Venus

Venus is the second planet out from the Sun and is classified as a “terrestrial” planet. This means that it, like Mercury, Earth, and Mars, is made primarily of silicate rocks and metallic elements. It has a solid, rocky surface shaped by volcanic activity that may still be occurring, tectonism (earthquakes), erosion, and weathering. These processes also work to modify the surfaces of other planets and some of the moons of the solar system.

Early in its history Venus may have had water on its surface, but for some reason the planet lost it very quickly and developed the thick carbon dioxide atmosphere we see today. Volcanism continued to resurface early Venus, flooding the impact craters created during a period called the “Late Heavy Bombardment” (which ended about 3.8 billion years ago). This was a period when the worlds of the inner solar system were particularly targeted by debris left over from the formation of the solar system.

Something also stopped Venus from developing plate tectonics—the large-scale motions of the outer crust of a world—and planetary scientists are still working to figure out more of Venus’s evolutionary history. There’s not a lot known about the interior of Venus, but the best models suggest that it probably has a partially molten core, surrounded by a mantle layer and a crust. The mantle absorbs heat from the core, and when this middle layer gets too hot, it weakens. This causes the top layer—the crust—to melt in on itself, spurring volcanic events that keep paving the surface over and over again.

The planet also has no internally generated magnetic field. However, there is a weak field that seems to be created as the solar wind interacts with Venus’s upper ionosphere. The European Space Agency’s Venus Express mission has observed interesting flux ropes high in the atmosphere above the poles. Other planets with stronger magnetic fields have these ropes, but they’re rare for Venus. Flux ropes are magnetic structures that arise in a magnetic field when a high-speed solar wind stream is flowing past a planet.

Craters on Venus

Venus has a surprising number of craters despite the volcanic activity that works to erase them over time. There are nearly a thousand craters ranging in size from about 4 to 280 kilometers across. The size of the craters imply that only objects larger than 50 meters across make it through the atmosphere to smash into the surface. Smaller impactors are simply vaporized in the heavy atmosphere.

The Runaway Greenhouse

Early in its history, the wet, temperate Venus that some scientists think existed began to change. The most likely explanation is that as the newborn Sun brightened up, it heated the early Venusian atmosphere. This created water vapor. Eventually the oceans boiled away, and all the water vapor escaped out to space. The carbon dioxide atmosphere remained, along with the sulfuric acid clouds that shroud the surface from our view.

Based on radar studies from Earth and multiple spacecraft missions to Venus, today we know that the planet’s upper atmosphere circulates around the planet in only four Earth days while the planet itself turns much more slowly. There are also intriguing double-eyed hurricane-like storms called polar vortices, which whip along at a speed sixty times faster than the planet below it.

Venus is sometimes cited as an extreme example of what could happen to Earth as our planet’s atmospheric carbon dioxide buildup continues. Nothing so drastic will happen to Earth, although the rise in greenhouse gases here is troubling. Understanding the Venusian greenhouse helps climate scientists see what can happen to an atmosphere that is made largely of carbon dioxide and smothered in heavy clouds.

Exploring Venus

Humans have been observing Venus for centuries. First we used telescopes to explore this cloudy world. Then, the space age offered us a way to go to Venus and study it up close. There have been thirty-eight missions sent from the United States, the former Soviet Union and Russia, the European Space Agency, and Japan to study Venus. Not every one has been successful—some were plagued with instrument failures and others were lost.

However, there have been a solid string of successful Venus explorations, including:

• The Venera 4 and 5, the first to enter the atmosphere and send information
• Venera 9, which sent back the first images from Venus
• The Magellan Mission, a thirteen-year-long Venus radar mapping project
• The Pioneer Venus orbiters
• The current Venus Express

All of these missions provided long-term observations of the atmosphere.