by Fraser Cain on March 12, 2012

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Venus Cloud Tops Viewed by Hubble

Venus Cloud Tops Viewed by Hubble

Venus is the second planet from the Sun and is located between Mercury and the Earth. The planet has been known to exist since ancient times. Aside from the Sun and the Moon, it is the brightest object in the sky. Although the ancients knew about Venus, some of the cultures thought it was two separate celestial objects – the evening star and the morning star. A Greek astronomer was the first one to realize that the evening star and the morning star were actually one object. Many cultures have attributed the planet with their respective goddess of love and beauty. Venus is the Roman name for that goddess. The Babylonians named the planet Ishtar, and the Greeks called it Aphrodite.

The average distance from Venus to the Sun is 108.21 million km. This is an average distance, though, and Venus follows an elliptical orbit around the Sun. At the closest point of its orbit, called perihelion, Venus is only 107.48 million km from the Sun. And then at the most distant point of its orbit, Venus is 108.94 million km from the Sun.

Venus takes 224.7 days to complete one orbit around the Sun. But it takes 243.02 days to spin once on its axis. In other words, a day on Venus is actually longer than a year on Venus. Even stranger, Venus is rotating backwards compared to the rest of the planets in the Solar System. Seen from above the north pole, Venus is seen to be turning clockwise. If you could stand on the surface of Venus, you would see the Sun rise in the West, slowly move across the sky, and set in the East. The opposite of Earth.

Scientists consider Venus to be a twin of Earth because of its many similarities. For example, the radius of Venus is 6,052 km; 95% the radius of Earth. The mass of Venus is 81.5% the mass of Earth, and the density of Venus is 5.24 g/cm3 while the density of Earth is 5.51 g/cm3. If you could stand on the surface of Venus, you would experience about 90% of the gravity you feel on Earth.

A day on Venus lasts 243 days; which is unusual considering the fact that a year on Venus is only 224.7 days. In other words, Venus’ day is longer than its year. Furthermore, Venus is the only planet in the Solar System that rotates clockwise on its axis. All of the other planets rotate counter-clockwise.

Radar imaging of Venus’ surface has shown that it does have impact craters around the planet, and evidence of wide scale volcanism. The volcanoes on Venus are smaller though, and almost always low shield volcanoes. It’s believed that some large event reshaped the surface of Venus 300-500 million years ago, wiping out older impact craters and volcanoes. This resurfacing event also shut down the planet’s plate tectonics, trapping heat inside the planet. Without the heat escaping, convection at the core of Venus shut down as well, and the planet lost its worldwide magnetic field. The interior of Venus is similar to the Earth. The planet has a core of metal, surrounded by a mantle of rock, and a thin crust. But unlike Earth, Venus has no plate tectonics, and no carbon cycle which removes carbon from the atmosphere and stores it inside the planet. This is one of the problems that might have led to Venus’ runaway greenhouse effect.

Although Venus has many similarities to Earth, it has some enormous differences. Perhaps the biggest difference is in its atmosphere. The pressure of the atmosphere on the surface of Venus is 92 times thicker than at sea level. In fact, you would have to swim down to a depth of 1 km beneath the surface of the ocean to experience the same kind of pressure. This atmosphere is made up almost entirely of carbon dioxide, with thick clouds of sulphur dioxide. Because of the carbon dioxide atmosphere, Venus experiences the strongest greenhouse effect in the Solar System. The temperature on the surface of Venus is 460°C, no matter where on the planet you are. This is hot enough to melt lead, and has destroyed spacecraft within hours.

At one time scientists and science fiction writers alike thought that the surface of Venus was tropical in nature. Some believe that belief was the only reason that the Soviets launched a series of surface probes to the planet. After seven missions were crushed by the pressure before they could reach the surface, Venera 8 landed and disillusioned millions. Several other probes were able to land in the ensuing years. The last one launched was the Venus Express. It arrived in April of 2006 and studied the planet’s surface and atmosphere until it could no longer function.

Because of its thick clouds, Venus has been impossible to observe from Earth. Early observations showed that the planet went through phases, like the Moon, demonstrating that it was orbiting the Sun with in the orbit of Earth. But it wasn’t until the first spacecraft observations that astronomers finally got a sense of what was beneath the thick clouds. Radar images from NASA’s Magellan spacecraft imaged the entire planet, revealing a hellish world covered by rocks and ancient lava flows. Several Russian spacecraft actually landed on the surface of Venus, surviving a few hours and returning images from the surface of the planet.

Venus is often referred to as the Morning Star because it orbits closer to the Sun than our planet. This causes Venus to appear in the western sky after the sun sets and rise in the East before the sun breaks the horizon. In addition, only the sun and the Moon are brighter in our night sky. All of this makes Venus hard to ignore even with the unaided eye.

Venus has no moons or rings.

Astronomy Cast – Episode 50: Venus.
NASA Facts: Magellan Mission to Venus
NASA Star Child: Facts about Venus
NASA: Venus Planet Profile
NASA: Planet Venus
NASA Solar System Exploration: Venus



Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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