Douglas Isbell
Headquarters, Washington, DC                   October 14, 1998
(Phone:  202/358-1753)

Jane Platt
Jet Propulsion Laboratory, Pasadena, CA
(Phone:  818/354-5011)

RELEASE:  98-188

JUPITER'S "WHITE OVALS" TAKE SCIENTISTS BY STORM  

     As powerful hurricanes pummel coastal areas on Earth, NASA 
space scientists are studying similar giant, swirling storms on 
distant Jupiter that have combined to spawn a storm as large as 
Earth itself.

     Three separate cold storms, called "white ovals" because of 
their color and egg shapes, have been observed in one band around 
Jupiter's mid-section for half a century.  Two of the storms 
recently merged to form a larger white oval, according to 
scientists studying data from NASA's Galileo spacecraft, the 
Hubble Space Telescope, and the Agency's Infrared Telescope 
Facility atop Mauna Kea, HI. 

     "The newly merged white oval is the strongest storm in our 
Solar System, with the exception of Jupiter's 200-year-old 'Great 
Red Spot' storm," according to Dr. Glenn Orton, senior research 
scientist at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA.  
"This may be the first time humans have ever observed such a large 
interaction between two storm systems."  

     Each of the white ovals that merged were about two-thirds the 
diameter of the Earth before the merger, when they combined to 
form a feature as large as the Earth's disc.  Although scientists 
have observed the end result of the merger of the two white ovals, 
the actual "collision" took place under cover of darkness while 
Jupiter was turned away from view.

     This new, powerful white oval has a mysterious trait, 
according to Orton.  "We can see it, along with the other white 
ovals, at visible light and some infrared wavelengths, but we 
cannot see the new white oval at certain infrared wavelengths that 
peer underneath the storm's upper cloud layers,"  Orton said.  
This might mean the storm is in a transition stage, undergoing a 
rebirth after the merging of the two storms.

     "With mature white ovals, we can see the upwelling of winds 
in the center, which in turn leads to downwelling around it," 
Orton said.  The new white oval has a very cold center (about -251 
Fahrenheit or -157 Celsius) that is about one degree colder than 
its surroundings.  "Because of this, the oval may have generated a 
thick cloud system which obscures the downwelling," Orton said, 
which could explain the new oval's "disappearing act" at some 
wavelengths.

     Adding to the mystery is the fact that a nearby storm 
rotating in the opposite direction to the new white oval used to 
be warmer than its surrounding.  "This probably means that the 
feature contained mostly downwelling winds," said Orton.  However, 
Galileo's photopolarimeter radiometer instrument showed this 
feature had cooled down to temperatures that were about the same 
as its surroundings.  

     Orton suspects that this storm somehow lost power and is no 
longer spinning as fast or downwelling as strongly as a year ago.  
This storm was once positioned between the two smaller white ovals 
that merged, and Orton theorized that when this storm system lost 
power, it removed the buffering mechanism that kept the two 
original white ovals apart.

     Orton and his colleague, Dr. Brendan Fisher, a Caltech 
postdoctoral fellow at JPL, based their conclusions about the 
temperatures using data gathered by Galileo on July 20, 1998, 
during the spacecraft's 17th orbit of Jupiter and its moons.  
Although much data from the flyby of Europa in that time period 
was lost because of a problem with the spacecraft's gyroscope, 
Galileo's photopolarimeter radiometer gathered the new data on the 
white ovals before the anomaly occurred.

     The photopolarimeter radiometer measures temperature profiles 
and energy balance of Jupiter's atmosphere, helping scientists 
study the huge planet's cloud characteristics and composition.  
Scientists believe that the bright, visible clouds of the white 
ovals are composed of ammonia.

     Galileo has been in orbit around Jupiter and its moons for 2 
1/2 years, and is currently in the midst of a two-year mission 
extension, known as the Galileo Europa Mission.  JPL manages the 
Galileo mission for NASA's Office of Space Science, Washington, 
DC.  JPL is a division of Caltech, Pasadena, CA.  

     Related images and information on the Galileo mission are 
available on the Internet at the Galileo website: 

             http://www.jpl.nasa.gov/galileo

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