Venus Express Orbiter Basks in Venusian Glory

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The European Space Agency’s Venus Express orbiter has spotted a very terrestrial phenomenon in the high-altitude cloud cover of Earth’s hellish ‘sister’ world.

While orbiting Venus, with the sun at its back, Venus Express used its cameras to observe a “glory” — the rainbow-like halo that is usually seen from an aircraft flying over clouds. This is the first time a glory has been observed on another planet.

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Glories appear as tight circular rainbow structures surrounding the shadow of an aircraft and are only formed when the cloud consists of tiny water droplets of approximately same size. On Earth, glories are a rare treat as the atmospheric conditions and location of the observer to the sun need to be just right. But if the conditions are met, sunlight can bounce off the water droplets, refracting back to the observer.

Venus, however, doesn’t have temperate, life-giving water droplets in its high-altitude clouds. But in an effort to determine the characteristics of the droplets contained within the Venusian sulfuric acid clouds, mission scientists waited until the sun was directly behind Venus Express and took this shot of the corrosive clouds far below.

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On July 24, 2011, the satellite spotted its first extraterrestrial glory in Venus’ cloud tops 70 kilometers (44 miles) above the planet’s surface. According to an ESA news release, the glory was 1,200 kilometers (746 miles) wide as seen from the spacecraft that was orbiting 6,000 kilometers (3,728 miles) above.

Simulated views of glory on Venus and Earth.
ESA/C. Wilson/P. Laven

From this observation, scientists have been able to determine the dimensions of Venus’ sulfuric acid cloud drops. They found the droplets to measure 1.2 micrometres across — approximately a fiftieth of the width of a human hair. The fact that the glory was 1,200 kilometers wide suggests that all the droplets within that area were around the same size.

But there were some surprises that may help us understand the complex chemistry at play in Venus’s atmosphere.

“The variations of brightness of the rings of the observed glory is different than that expected from clouds of only sulfuric acid mixed with water, suggesting that other chemistry may be at play,” writes ESA.

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The researchers believe that there is an unknown atmospheric component called a “UV-absorber” that creates mysterious dark patches in the Venusian atmosphere when viewed from space — the same chemical mechanism could be responsible for the unexpected variation of glory brightness.

Publication: “Glory on Venus Cloud Tops and the Unknown UV Absorber,” by W.J. Markiewicz et al, Icarus. doi:10.1016/j.icarus.2014.01.030

Source: ESA

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