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May 20, 2008

Atmospheric Chemistry

Hydroxyl Radical Found On Venus

Venus Express spacecraft detects elusive atmospheric species

Elizabeth K. Wilson

EXPLORING VENUS The European Space Agency’s Venus Express spacecraft captured this image, in false color, of the southern hemisphere of Venus last year. The spacecraft recently detected hydroxyl radicals in the venutian atmosphere.

The reactive hydroxyl radical, which helps sop up pollutant molecules in Earth’s atmosphere, has now also been found in the hellish atmosphere of Venus. The discovery promises to help scientists understand the roles of this reactive species, as well as other molecules such as ozone and oxygen, on our neighboring planet.

Sensitive spectrometers aboard the European Space Agency’s Venus Express spacecraft, which is orbiting Venus, detected infrared emissions from HO, reports planetary scientist Giuseppe Piccioni of the Instituto di Astrofisica Spaziale e Fisica Cosmica, in Rome, and colleagues (Astron. Astrophys., DOI: 10.1051/0004-6361:200809761).

Although HO had not been seen before around a planet besides Earth, it’s been thought to play a role in atmospheric chemistry and dynamics on both Venus and Mars. But it had been difficult to detect before because it’s rare and its spectroscopic bands overlap with those of other species.

Scientists are particularly interested in better understanding Venus’ atmosphere because the planet is approximately the same size and density as Earth. Indeed, the two planets may have started out very similarly, but at some point billions of years ago, their paths diverged. And now, Earth’s CO2 is largely sequestered in its rocks, but Venus’ CO2 covers the planet in a thick atmospheric blanket, including clouds of sulfuric acid, that keeps the planet at a sweltering 450 °C.

For the past year, Venus Express has been sending back data about the planet’s atmosphere, including evidence—in the form of hydrogen and oxygen ions streaming into space—that Venus may have lost what once was a body of water (C&EN, Dec. 3, 2007, page 14).

The new results add to understanding of Venus’ atmosphere, scientists say. On Earth, the reaction of atomic hydrogen with ozone generates atmospheric HO. The Venus Express group posits that the same may be true on Venus, although the radical is produced in much smaller quantities.

Previously, “we didn’t suspect that ozone could be the main mechanism of production of HO, due to ozone’s relatively low abundance” on Venus, Piccioni says. But recent models suggest that large enough quantities of ozone may exist on the night side of Venus—where the unstable molecules can enjoy longer lifetimes—to account for the levels of HO that the Venus Express team measured.

The discovery also indicates that HO may play a significant role in the oxidation of CO and SO2 at high altitudes and so might help answer the long-standing question of how Venus' CO2 atmosphere remains chemically stable, notes Yuk L. Yung, a geological and planetary sciences professor at California Institute of Technology.

The HO levels the team measured can be used to enrich atmospheric models of Venus, Piccioni says. “We can now better estimate the amount of HO, ozone, and hydrogen.” The group is now analyzing more of the same type of observations from Venus Express to verify their HO findings.

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ISSN 0009-2347
Copyright © 2009 American Chemical Society


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