Cassini's recent pictures of Jupiter are providing scientists with never-before-seen images of the giant planet's magnetosphere and underlying dynamics.

NASA's $3.4 billion Cassini spacecraft is presently in a six-month flyby of Jupiter during a gravity-assisted swing toward Saturn and a four-year study of the ringed planet that will begin in July 2004. Researchers using the flyby as an opportunity to try out some of Cassini's advanced instrumentation are reaping scientific rewards.

"Every new spacecraft carries instruments that expand our ability to see things," says Stamatios Krimigis, Space Department head at APL and principal investigator for the Magnetospheric Imaging Instrument aboard Cassini. "With MIMI, we're able to visualize the invisible."

The MIMI instrument includes an Ion and Neutral Camera developed by APL, a spectrometer built by the University of Maryland under Douglas Hamilton and a high-energy particle detector developed by Stefano Livi of APL and a number of co-investigator institutions. "By detecting various energetic particles and discriminating among them according to energy and mass, the camera is able to obtain remote images of the global distribution of these particles," says APL's Donald Mitchell, who leads the camera science team.

From a distance of 6 million miles, MIMI's camera has recorded pictures of Jupiter's energetic particle-filled magnetosphere. Sequenced into a movie, these images will eventually provide a large-scale look at the compression and expansion of magnetospheres as they are buffeted by solar winds.

"These images, when combined with the other MIMI measurements, demonstrate the ability of the camera to capture not only the shape and dynamics of the magnetosphere but also elements of its chemical composition," Krimigis says. "They reveal that the particles we're detecting--primarily hydrogen, but also oxygen, sulfur and sulfur dioxide--are spewed from volcanoes on the Jovian moon Io and spun out into Jupiter's magnetosphere, where they are trapped, energized and accelerated to high velocities. Then, when collisions with other particles provide them with an electron, they become neutral and are able to escape the magnetosphere. And that's when we can detect them with our camera."

In addition to imaging the Jovian magnetosphere, MIMI's instruments have detected the presence of a huge nebula of particles enveloping Jupiter and extending out to at least 13 million miles from the planet, according to Hamilton, the developer of another MIMI sensor that detected oxygen, sodium, sulfur, potassium and sulfur dioxide. "All of these are constituents of the gas spewed out by Io's volcanoes, thrown out of Jupiter's magnetosphere and eventually picked up by the flowing solar wind," Hamilton says. Other observations include detection of flowing electrons along the planetary magnetic field inside Jupiter's magnetosphere, according to Livi, who developed the magnetic spectrometer sensor.

Related Web sites:
Cassini's pictures of Jupiter and its magnetosphere
APL's MIMI instrument