An unusually pure cloud of young, fresh ammonia ice has been found on Jupiter by NASA’s Galileo spacecraft, the first discrete cloud of ammonia ice ever seen, though the planet is known to contain ammonia gas.
      The spot, first discovered during Galileo’s first orbit of Jupiter, may be in the cross-current of opposing paths of uncommonly strong wind, which pulls up ammonia gas from below, forming a large cloud of ammonia ice. Scientists have named the spot the Turbulent Wake Anomaly because it lies downstream from Jupiter’s Great Red Spot, a 300-year old storm two times as Earth.
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      Seeing pure ammonia clouds is surprisingly rare, said Dr. Kevin Baines, research scientist at NASA’s Jet Propulsion Laboratory, Pasadena, Calif., who discovered the spot in infrared images and spectra taken by Galileo’s near infrared mapping spectrometer instrument during its first orbit of Jupiter. These clouds are normally covered up within a few hours of their formation by unknown substances that attach themselves to the ammonia ice particles, he said.
      "Detecting a localized ice cloud confirms the common belief that ammonia clouds do exist on Jupiter," said Baines, who presented his findings today at the American Astronomical Socieyt’s Division of Planetary Sciences meeting in Pasadena. "It is only because we got a clear shot of a fast-growing cloud in an unusually turbulent region that we were able to see the ammonia ice crystals before their surfaces were altered."
      Planetary scientists have known that Jupiter contains lots of ammonia gas, because the gas absorbs certain wavelengths of light. Scientists would then expect that the low temperatures in Jupiter’s outer atmosphere would cause the ammonia gas to condense into ice. Until now, localized ammonia ice clouds have never been seen, despite the cold temperatures and the abundance of ammonia.
      Closer examinations by Galileo’s spectrometer instrument in May 1999 and May 2000 revealed that the spot was a heavy concentration of ammonia ice particles. The cloud, which may be as much as 15 kilometers (over 9 miles) thick, is the first concentrated cloud of ammonia ice seen on Jupiter.
      Baines said that the ammonia cloud is located in an especially turbulent area, in the northwest corner of the Great Red Spot. A current flows around the Great Red Spot from east to west. Behind it, the current eddies and whirls, just as a rock in a stream creates whirlpools and whitewater behind it. Another powerful current moving in the opposite direction of the Great Red Spot’s current may intensify the Turbulent Wake Anomaly. This constant tumultuous motion pulls up ammonia from below the cloud level, creating fresh, dense clouds of ammonia ice.
      This spot will be further scrutinized by both Galileo and NASA’s Cassini spacecraft later this year. Baines and his colleagues are eager to look for other materials that may be dredged up to high altitudes by the strong Jovian currents. "This may be the best place to mine Jupiter," said Baines. "There is a whole host of materials seen in the upper atmosphere that cannot be explained. These rare materials must have been transported upward from far below the clouds. Perhaps the Turbulent Wake anomaly is the ‘gold mine’ that will reveal material being transported to the upper atmosphere."
      The ammonia cools rapidly to form ice in the 120 C (-184 F) atmosphere, and the flow keeps the newly formed ice moving downstream, weathering into the typical clouds seen on Jupiter as it flows. Pure ammonia clouds may alter in just a few hours into the common clouds that cover most of the planet. Most clouds on Jupiter would be relatively old compared to this newly observed cloud.
      Galileo has been orbiting Jupiter and its moons since December 1995. JPL is a division of the California Institute of Technology in Pasadena, and manages the Galileo mission for NASA’s Office of Space Science, Washington, DC.