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Planetary News: Venus Express (2007)Venus Express Pulls Back the Veil from Earth’s Mysterious “Twin”By Amir AlexanderNovember 30, 2007
Venus, our closest planetary neighbor, is also one of the most mysterious worlds in our solar system. Whereas our other neighbor, Mars, has been comparatively welcoming to human probes, which have studied everything from its thin atmosphere to its surface geology, Venus has held tight to its secrets. Coated in a thick and impenetrable atmosphere, nearly 100 times denser than Earth’s, it has proven highly resistant to the probing of the 30 odd spacecraft that had been sent its way in the past four decades. Over the years the cumulative data acquired in these missions has nonetheless begun to give scientists a clearer picture of the planet and its evolution. And now, with the publication this week of nine papers in the journal Nature based on results from the ongoing Venus Express mission, the veil shrouding this reclusive world has been pulled back even further. Venus Express is a spacecraft of the European Space Agency (ESA), its design based on ESA’s highly successful Mars Express, which has been orbiting the Red Planet for the past four years. Launched from Baikonur in Kazakhstan in November 2005, Venus Express arrived at its destination on April 11, 2006, and began systematic observations of Venus two months later. The spacecraft has now completed its primary mission and is starting on an extended mission that will last for at least the next two years. Beginning in 2010 it will be joined in Venusian orbit by the Japanese Venus Climate Orbiter. If Venus Express is fully operational at that time, the two spacecraft will be able to work together, and by comparing their measurements will greatly improve the accuracy of their data collection. According to most measures, Venus is Earth’s “near twin.” It is a rocky planet with a radius of 6,051 kilometers compared to Earth’s 6,371, and a mass more than 80% that of our home world. It completes each orbit around the Sun in 224 Earth days at an average distance of 0.72 Astronomical Units (AU), with AU signifying the average distance of the Earth from the Sun. And yet in other respects it could not be more different: its surface and atmosphere are parched dry, the atmospheric pressure is 92 times greater than on Earth, and surface temperatures hover around 460 degrees Celsius. No form of life recognizable to Earth-dwellers could possibly survive in these hell-like conditions.
How a planet that was apparently so similar to our own at its inception developed into such a startlingly different world is the great guiding question of Venus studies. The latest results from Venus Express now add to scientists’ growing understanding of the processes that brought about this state of affairs. A key factor in Venus’s evolution is the fact that it is somewhat closer to the Sun, and therefore hotter than Earth. If, as most scientists believe, early Venus, like the early Earth, possessed oceans, the excessive heat would cause more water to evaporate than on Earth, increasing the proportion of water-vapor in the atmosphere. Now water vapor is a prime “greenhouse gas,” whose presence in the atmosphere raises temperatures on the surface. This in turn increases the evaporation of the oceans even further, sending more water vapor into the atmosphere, and raising temperatures on the surface ever further. At some point this positive feedback loop will raise surface temperatures above 100 degrees Celsius, and practically boil the oceans away. The masses of water vapor now saturating the atmosphere will then be exposed to solar radiation, which will split the water molecules into separate hydrogen and oxygen atoms. The lighter hydrogen particles will soon begin a long process of escaping into space. This possibility of runaway global warming is fully consistent with current conditions on Venus, where water is all but absent from both the surface and the atmosphere. And while scientists have long believed this scenario to be the general evolutionary history of the planet, Venus Express has now provided an additional clue. Among the hydrogen atoms present on Venus, the rarer hydrogen isotope deuterium is 100 to 150 times more common than it is on Earth. Now deuterium, which has an extra neutron in its nucleus, is nearly twice the mass of a common hydrogen atom, and is therefore less likely to escape into space. Its presence on Venus in such a high proportion is evidence of the amount of hydrogen lost to space since the planet’s formation, and thus provides indirect support to the theoretical models explaining the planet’s loss of water. Venus Express also contributed to scientists’ understanding of other characteristics of the planet. Like Earth, Venus has large areas of circulating “air” around the poles, known as “polar vortexes.” Thanks to the presence of clouds, the spacecraft was able to take three-dimensional images of these atmospheric features, as well as measure their temperature. The surprising result was that unlike on Earth, Venus’s vortexes are two-layered, with an inner dipole-shaped feature substantially warmer than the surrounding vortex. Researchers do not yet understand the causes of this phenomenon, but it may provide a clue to the dynamics of the planet’s high-pressure atmosphere.
The Nature papers also show that Venus Express has gathered vast amounts of information about the layers of the Venusian atmosphere, its circulation dynamics, and its interaction with solar radiation. But perhaps most surprising of all was the spacecraft’s detection of lightning on the planet. According to Andrew P. Ingersoll of Caltech there “shouldn’t” be lightning on Venus, because the planet’s sulfuric acid clouds are more like Earth’s smog clouds, which do not produce lightning. The three other celestial bodies known to produce lightning – Earth, Jupiter, and Saturn – all do so in the presence of water-vapor clouds. In fact, Venus Express never did observe actual lightning flashes within the thick atmosphere of Venus. What it did detect was “whistlers” – low frequency electromagnetic bursts that last for a fraction of a second, and which are believed to originate in electrical discharges in the atmosphere, more commonly known as lightning. Scientists do not yet understand how lightning is generated under Venusian conditions, but perhaps, Ingersoll suggests, that is not surprising. We need to think beyond the bounds of our Earthly experience, he said, in order to understand the dynamics of another world. So what has Venus Express taught us about the different evolutionary paths taken by Earth and its closest neighbor? According to project scientist Hakan Svedhem and team members Dmitry V. Titov, Fredric W. Taylor, and Olivier Witasse, who joinly authored one of the Nature articles, our growing understanding of Venus shows that it may yet be considered Earth’s twin. “The overall sense of the results” they write “is that the differences . . . between Venus and Earth are much less mysterious than previously thought. They are consistent with theoretical ideas and interpretations suggesting that the two planets had similar surface environments in the past, and that they evolved differently.” Taylor likes to summarize it this way: Venus, he said, is indeed “Earth’s twin, but separated at birth.” |
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