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 Venus still a hot mysteryBy Bjorn Carey  Surface temperatures on Venus average more than 860 degrees Fahrenheit. RELATED
YOUR E-MAIL ALERTS(SPACE.com) -- After the sun and the moon, Venus is the third brightest body in our sky, and because of this it received lots of attention from all the great ancient civilizations, including the Mayans, the Egyptians, and the Greeks. As early as 1600 B.C. the Babylonians kept detailed records of its movement across the sky. But in modern times our nearest neighbor has been somewhat neglected and only three missions have been dedicated to studying the heavily clouded planet. As a result, there is still a lot astronomers don't know about Venus. Even the things they do know fairly well, such as the contents of its atmosphere, speed of rotation, and texture of its surface, continue to raise questions. The European Space Agency plans soon to launch the instrument-loaded Venus Express satellite, which will orbit the "evening star" for two years. Over the course of the mission the satellite will bounce radio waves off the planet's young surface, snap pictures of its swirling atmosphere, take the temperature of the clouds, and search for hints of a magnetic field. (Full story) All this data will help scientists better understand a planet that from the outside resembles an early Earth and perhaps reveal clues of the existence of life. Trouble in the atmosphereClear skies, temperatures around 80 degrees Fahrenheit, and a light breeze are the recipe for a perfect day for a walk on Earth, but you'd be hard pressed to find a day like that on Venus. First of all, there's no such thing as clear skies on Venus. The entire planet is covered by a dense layer of clouds about 40 miles thick. The atmosphere is so dense that walking on Venus' surface would be like walking under 3,000 feet of water here on Earth. You'd be crushed. And although Venus reflects about 80 percent of the sun's radiation -- Earth soaks up more -- you can forget having an 80-degree day. Venus' atmosphere is about 95 percent carbon dioxide, which traps heat from the Sun so efficiently that surface temperatures average more than 860 degrees Fahrenheit. That makes Venus hotter than Mercury, even though it's twice as far from the Sun and receives only 25 percent of the solar radiance. You might get that light breeze, however. Winds in the upper atmosphere blow at more than 200 mph. Wind on the surface creeps just a few miles an hour. But here's the kicker: Since the air it's pushing around is so dense, a soft breeze in the face on Venus would feel like getting hit by a truck, scientists say. Express rideVenus was formed about the same time as Earth, around 4.6 billion years ago, but you wouldn't know from looking at it. "The surface of Venus looks geologically young, about 500 million years old," said Steven Hauck a planetary geologist at Case Western Reserve University. "And we know this from the apparent lack of impact craters." The planet is essentially devoid of impact craters, suggesting that at some point there was a significant geologic event that resurfaced the entire planet. This is another debated idea, though, since the surface shows no obvious plate tectonics or seismic activity, and we also can't tell whether it is still volcanically active. "We don't see the results of horizontal motions that are part of plate tectonics that we know on Earth," Hauck said. "And we can't distinguish if this was a long period that came to an end or a global, catastrophic resurfacing of the planet." The planet rotates counterclockwise, opposite Earth and most other planets, and does so exceedingly slow -- one day on Venus equals 243 Earth days. Scientists have speculated that a massive collision with an asteroid long ago may have reversed and slowed down Venus's spin. This event may have also set off a global volcanic reaction that led to the planet's resurfacing. It will take Venus Express less than half a year to travel to Venus, and considering our lack of knowledge of our sister planet and the possibility that her clouds might harbor life, it seems somewhat surprising that Venus Express is only the fourth mission dedicated to studying Venus. "We need more Venus missions to really answer the biggest mysteries about the planet," Grinspoon said. "Venus Express will be a great mission and will tell us a lot about the planet, but I think to really make the next leaps in understanding Venus we're going to have to do something more than just orbit the planet. We need to take the plunge and explore the clouds and the surface directly." Copyright © 1999-2006 SPACE.com, Inc.
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