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Exploring trihydrogen auroras, by Jove!

Using an infrared telescope, two research teams have independently produced the most detailed images ever of auroras in Jupiter's atmosphere. These radiant regions -- shaped like a pair of croissants over vast sections of Jupiter's north and south polar areas -- vary in brightness on time scales as short as an hour.

The infrared glow stems primarily from ions of trihydrogen -- a simple molecule that many astronomers believe may trigger complex chemical reactions in interstellar gas clouds. For more than a decade, scientists searched in vain for trihydrogen ions far beyond our solar system, in remote, gaseous regions of the Milky Way. Then, in 1989, the elusive molecule turned up close to home, in infrared emissions from the hydrogen-rich atmosphere of Jupiter.

That discovery prompted researchers to obtain the new infrared images, which show the spatial distribution of the trihydrogen emissions, says Sang J. Kim of the University of Maryland in College Park. He and his colleagues conducted one of the two imaging studies undertaken early this year with NASA's Infrared Telescope Facility in Hawaii.

Jovian auroral storms, like those on Earth, develop when electrically charged particles crash into the magnetic field surrounding the planet and then spiral inward toward the north and south magnetic poles. When these particles hit the upper atmosphere, they excite atoms and molecules there, causing them to glow.

The ions that strike Earth's atmosphere come from the sun. But Jupiter lies five times father from the sun, and astronomers have speculated that most of the particles bombarding its atmosphere come instead from ions spewed out by volcanoes on Io, one of Jupiter's moons.

The infrared studies call that scenario into question, says Kim. "Everything [in the new findings] is against common sense, everything is against our predictions," he asserts. For example, he says, researchers have calculated that ions from the doughnut-shaped plasma cloud surrounding Io should intercept particular regions above Jupiter's north and south magnetic poles, triggering auroras within those regions. The infrared images of trihydrogen, however, only partly coincide with the apparent locations of the ion phenomenon.

Trihydrogen hotspots -- regions of high-intensity emissions in the northern and southern auroras -- pose another puzzles, he notes. Previous observations of ultraviolet and far-infrared Jovian auroras indicate they typically have a single, nearly stationery hotspot. But Kim's group found that the southern trihydrogen aurora contains two hotspots that vary rapidly in brightness and appear to rotate at a speed of several kilometers per second relative to the face of Jupiter. These hotspots may thus bask in sunlight at all times, which suggests the sun may modulate their activity, says Kim. The single northern hotspots moves more slowly and occassionally migrates out of Earth's view, he says.

In March, while Kim and his colleagues obtained images with NASA's infrared telescope, they simultaneously recorded the intensity of trihydrogen spectra at several wavelengths using the nearby Canada-France-Hawaii Telescope. A second team, led by Richard Baron of the University of Hawaii in Honolulu, had used the infrared telescope to image auroras in January and February. Both groups report their work in the Oct. 10 NATURE.

In a commentary accompanying those reports, Alexander Dalgarno of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., notes that the Jupiter findings provide new evidence that trihydrogen ions exist outside Earth-based laboratories. He and others have devised a theory to explain how the ions might form beyond the solar system: Cosmic rays ionize molecular hydrogen on interstellar dust grains; the ions then combine with neutral hydrogen molecules to create trihydrogen ions, which in turn trigger the formation of nearly 100 types of complex interstellar molecules.

Dalgarno says he and his colleagues have now tentatively identified trihydrogen ions in infrared emissions from supernova 1987A -- a finding that would represent the first such discovery beyond our solar system.
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Title Annotation:auroras in Jupiter's atmosphere
Author:Cowen, Ron
Publication:Science News
Date:Oct 12, 1991
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