Jupiter's "White Ovals" Take Scientists By Storm

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