University of Colorado Experiment on Cassini Mission Produces First Spectral Images of Jupiter

October 17, 2000

A $12 million University of Colorado at Boulder-built instrument package on the Cassini mission en route to Saturn has produced the first spectral images of Jupiter, including the planet's aurora and a gigantic glowing ring of gas encircling the planet.

Launched in 1997, the joint NASA-European Space Agency Cassini spacecraft began making observations of Jupiter on Oct. 1. The CU-Boulder Ultraviolet Imaging Spectrograph, or UVIS, has been taking images in the UV portion of the light spectrum invisible to the human eye.

The doughnut-shaped gas ring encircling Jupiter, known as the Io torus, is made up primarily of charged particles streaming from active volcanoes on Io, one of Jupiter's moons, said CU-Boulder Professor Larry Esposito, principal investigator for UVIS. Glowing with a power greater than all the electricity generated on Earth, the massive torus is almost 10 times as large as Jupiter itself.

"Our pictures are the first-ever imaging spectroscopy of the torus, although it was first discovered by Earth telescopes and later studied by spacecraft, including the Voyager and Galileo missions to Jupiter," said Esposito, who is affiliated with CU-Boulder's Laboratory for Atmospheric and Space Physics. Imaging spectroscopy combines photography with UV light emission observations, providing additional scientific information about specific targets.

The UVIS images show multiple overlapping exposures of the torus, each in the characteristic light emitted by sulfur and oxygen atoms, he said. Since the emissions can only be seen in the ultraviolet light that the two CU telescopes detect, "We see the entire donut of glowing gas in all its invisible colors," Esposito said.

During the first observations, Cassini instruments focused on Jupiter for an entire rotation of its atmosphere, which is one Jupiter "day," or about 10 Earth hours. Because the light-emitting atoms are trapped by Jupiter's tilted magnetic field, the torus wobbles back and forth. The researchers produced a movie of the gyrating torus from Oct. 3, which was generated by the first set of Jupiter pictures transmitted to Earth from the spacecraft.

The enormous torus reaches temperatures of more than 100,000 degrees Fahrenheit as it rotates with the giant planet. But even though it is about 600,000 miles across, its density is so low that all of its particles, if compressed to solid form, would fit inside Fenway Park.

The UVIS instrument package was built at LASP from 1990 to 1997, said Ian Stewart, team leader for the UVIS planning and science analysis and a senior research associate at LASP. Other UVIS team members include LASP's George Lawrence, Bill McClintock, Charles Barth and Justin Maki, a 1996 CU graduate now working at the Jet Propulsion Laboratory in Pasadena, which is overseeing the Cassini mission for NASA.

Cassini was launched on Oct. 15, 1997, and has twice flown by Venus and once by Earth to gain speed and momentum. It is now using Jupiter's gravity to speed it toward Saturn. In the next 6 months, UVIS will continue to observe Jupiter, the Io torus, and Jupiter's moons and aurora, said Stewart.

Upon its arrival at Saturn in 2004, the spacecraft is slated to drop a European-built probe on Saturn's moon, Titan, then spend four years orbiting Saturn and its 18 known moons and providing a flood of new data on what many view as a miniature solar system. UVIS will be used to study the atmosphere of Saturn, the surfaces and atmospheres of its moons and the structure and dynamics of the fabulous ring system.

The CU spectrometers will take chemical fingerprints of Saturn's atmospheric gases and measure their temperatures and compositions, said Esposito. UVIS also will be used to study the atmosphere of Titan, which is thought to be 10 times more dense than Earth's and contain a wealth of hydrocarbons, the building blocks of life.

A LASP-built photometer on UVIS is expected to resolve the size and shape of Saturn's ring particles -- thought to consist of ice and rock pieces ranging in size from sugar granules to houses -- five times better than the CU polarimeter on Voyager, Esposito said.
Contact: Larry Esposito, (303) 492-5990
Ian Stewart, (303) 492-4630
Jim Scott, (303) 492-3114
Oct. 18, 2000

University of Colorado at Boulder

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