December 30, 2000
Cassini Visualizes the Invisible, Tracks Giant Storms
NASA's Cassini spacecraft, which made its closest approach to Jupiter early today, is providing ways to make invisible features visible, to track daily changes in some of the planet's most visible storms, and to hear the patterns in natural radio emissions near the edge of Jupiter's magnetic environment.
In collaboration with NASA's Galileo spacecraft, which has been orbiting Jupiter since 1995, Cassini is also beginning to provide new insight in how the solar wind of particles speeding away from the Sun affects a huge magnetic region surrounding Jupiter.
Scientists using instruments on both Cassini and Galileo gave a preview today at NASA's Jet Propulsion Laboratory, Pasadena, Calif., of what they are beginning to learn in the joint studies, which will continue for another three months.
Large storms on Jupiter, which can be larger than Earth and last for centuries, gain energy from swallowing smaller storms, preliminary analysis of Jupiter movies from Cassini spacecraft suggest. The smaller storms pull their energy from lower depths, according to information collected by Galileo.
The Cassini spacecraft, which made its closest approach to Jupiter today at 2:12 a.m. Pacific Standard Time, has taken pictures of thunderstorms on Jupiter. As small storms pass each other, they can be ripped apart, or merged together. This shows that the small features in Jupiter's atmosphere harvest the energy from below the cloud surface, and the larger storms encompass the small ones, just as a big fish eats smaller ones for energy, said Dr. Andrew Ingersoll of the California Institute of Technology.
He said a better understanding of storms on Jupiter helps in understanding Earth's atmosphere, too. "The weather is different on Jupiter. You have a 300-year-old storm. We'd like to know why Jupiter's weather is so stable, and Earth's is so transient," he said.
Dr. Carolyn Porco of the University of Arizona presented planetwide movies of cloud movements on Jupiter, a sampling of the Cassini camera results that scientists will be examining in coming months.
"The camera has performed beyond our wildest imaginings - - and that's saying something, because we've been imagining this for a decade now," she said.
Both Cassini and Galileo have recently returned evidence of the variability in size of Jupiter's magnetosphere, a bubble of charged particles trapped within Jupiter's magnetic field. The bubble is so big that if it were visible to the eye, it would appear bigger to viewers on Earth than our own Moon does, despite its much greater distance. While Galileo was moving toward Jupiter this fall, it passed the magnetosphere boundary, but then the boundary moved inward toward Jupiter even faster than the spacecraft was moving, temporarily putting Galileo back outside the magnetosphere, said Dr. William Kurth of the University of Iowa.
Kurth played a sound recording based on natural radio emissions created by the energy of the area where the solar wind hits Jupiter's magnetosphere. The emissions were detected on an instrument onboard Cassini, which encountered the boundary this week much farther out from Jupiter than expected.
Another instument on Cassini is creating images never before possible of the entire magnetosphere. Dr. Stamatios (Tom) Krimigis of Johns Hopkins University's Applied Physics Laboratory presented one at the briefing that Cassini took this week, showing some features of the structure within the magnetosphere. Other Cassini measurements show that some sulfur and oxygen spewed from volcanoes on Jupiter's moon Io are distributed much farther from the planet than the extent of the magnetosphere, Krimigis said.
The evidence shows there is big nebula of material surrounding Jupiter, originating from the volcanoes on Io, he said.
Cassini passed about 9.7 million kilometers (6 million miles) from Jupiter today in order to use Jupiter's gravity for a boost to take Cassini to its main destination, Saturn. It will reach Saturn in 2004.
The images and sounds released at the briefing are available online from JPL at
Cassini is a cooperative effort of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages Galileo and Cassini for the NASA Office of Space Science, Washington, D.C.