The atmosphere of Venus continues to recover from an intense shower of sulfuric "acid rain," according to January 24 images from the Hubble Space Telescope (HST) that have given scientists a first-time look at the planet in the ultraviolet spectrum. The images, taken when Venus was about 113.6 million km from Earth, reveal that the amount of sulfur dioxide in the atmosphere has declined by a factor of 10 since 1978 when the Pioneer Venus Orbiter flew.
"If Venus is a twin of the Earth, it's an evil twin," says Larry W. Esposito, a planetary astronomer at the University of Colorado at Boulder. While the Earth is surrounded by water-vapor clouds, Venus is enshrouded by clouds of sulfuric acid. In addition, the temperature on Venus is about 427°C.
Scientists believe that learning more about the atmosphere of Venus will shed light on processes at work in Earth's atmosphere. The Venus data provide a check on scientists' models for acid rain and global warming, Esposito says.
Scientists are still trying to figure out what kicked the sulfur dioxide above the clouds in the first place to trigger the acid rain. A leading - though controversial - theory holds that a gigantic volcanic eruption blasted the sulfur dioxide above the clouds, and in consequence, it has been raining on Venus ever since. But a "volcano is one way, there are other plausible ways," Esposito says.
New evidence from the NASA Magellan mission, which concluded last October after mapping more than 98% of Venus with imaging radar, is also consistent with the eruption thesis, but it has not yet been proven true. "We would have hoped Magellan would have found the smoking gun, but that was not the case," Esposito says.
The sulfuric rain on Venus is similar to what happens on Earth when sulfur dioxide emissions are broken apart in the atmosphere and are converted to sulfuric acid to make acid rain. Esposito explains. On Earth, however, it happens in limited areas, while on Venus the phenomenon is planetary.
The HST images are also giving scientists the first long-term coverage of Venus and Mars. Earlier unmanned craft offered only brief looks at the weather on these planets.
In this ultraviolet-light image, cloud patterns are distinctive. For example, a horizontal "Y"-shaped cloud feature is visible near the equator. Similar features were seen from the Mariner 10, Pioneer Venus, and Galileo craft. This feature might indicate atmospheric waves, analogous to high and low pressure cells on Earth. Bright clouds toward the Venus' poles appear to follow latitude lines.
The polar regions are bright, possibly revealing a haze of small particles overlying the main clouds. The dark regions show the location of enhanced sulfur dioxide near the cloud tops. From previous missions, astronomers know that such features travel east to west along with the Venusian prevailing winds, making a complete circuit around the planet in four days. —Image by NASA and Larry Esposito, University of Colorado, Boulder
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