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Mission: Saturn

Cassini Plasma Spectrometer (CAPS) reveals Saturn's tremendous magnetospheric structure, as well as new surprises

The narrow angle camera shot this view of Saturn's rings on June 21 while the spacecraft was at a distance of four million miles. Because the rings are primarily water ice, researchers believe different colors in the rings represent different amounts of rock or mineral contamination. The CAPS instrument has measured water group ions over parts of the rings. Cassini's science payload, combined with images such as this, will help determine the composition of the ring system. (NASA/JPL/Space Science Institute)

Moments after the Cassini spacecraft fired its engines to enter into orbit around Saturn on June 30, the Cassini Plasma Spectrometer (CAPS) successfully began its mission to measure the composition of plasma (ions and electrons) originating from the ringed planet and its satellites.

Led by Southwest Research Institute (SwRI), the CAPS team has seen a tremendous amount of spatial structure in plasmas surrounding Saturn, even with preliminary data. There are oxygen, water and hydrogen ions, and it appears there is a cloud of plasma floating over parts of the rings – illustrating that the magnetosphere at Saturn is very different from that of Earth.

As the plasma density increases towards the planet, the abundance of the water group ions increases. This reveals that plasma is being manufactured inside Saturn's magnetosphere through bombardment of the icy satellites by dust and radiation. In contrast, plasma in the Earth's magnetosphere is generated by the ionosphere and the solar wind.

This illustration shows ion and electron plasma fluxes measured by the Cassini Plasma Spectrometer on either side of closest approach to Saturn on June 30, 2004. Plasma energy increases going upwards: Ions from 1 to 50,000 electron volts and electrons from 1 to 30,000 electron volts. Flux intensities are color-coded, with red the most intense and blue the least. Time runs from left to right, beginning 27 hours before closest approach and going to 18 hours after. These data reveal that Saturn's giant rotating plasma disk consists mainly of water and hydrogen ions originating on the surfaces of Saturn's icy moons. Sharp vertical striations in the figure are caused by structures in the plasma disk. The disk, which extends roughly out to the distance of the moon Rhea, is approximately 1,100,000 kilometers (670,000 miles) in diameter - roughly three times the distance from Earth to the Moon.

Invisible to the human eye, plasmas are the most pervasive substance in the universe and can yield information that will help characterize the evolution of the Saturn system over its 4.5-billion-year existence. Flybys performed by Pioneer 11, Voyager 1 and Voyager 2 in the late 1970s and early 1980s resulted in plasma measurements from around the planet, but never before has a plasma instrument gathered such high-quality data while in orbit.

The largest, most complex space plasma instrument ever flown, CAPS consists of three sensors, one electron and two ion, developed for NASA by SwRI and an international team of scientists and engineers. The sensors are designed to determine the chemical composition, dynamics and temperature of plasma within and surrounding the saturnian system.

As Cassini moves inward through the magnetosphere, ion and electron distributions get denser and colder – forming a great, flat disk of plasma rotating with the planet's magnetic field. Voyager caught a glimpse of that process but did not have the resolution or range to determine its composition or energy distribution; Cassini will eventually reveal just that.

The team is now searching for evidence of a suspected, but never observed, ionosphere of charged particles over the rings. Over the optically thick part of the rings, there appears to be a cutout in the large plasma disk that is rotating with Saturn, but perhaps the most surprising preliminary result has been the discovery of a dense ionosphere of mostly oxygen ions over optically thin parts of the rings and inside the "Cassini division," the largest gap in Saturn's rings. The cause and composition of the unusual ionosphere is so far a mystery.

The atmosphere of Titan, one of Saturn's moons, consists of a photochemical smog that gives the moon a smooth, featureless glow. The spacecraft's powerful cameras and the Huygens probe, built by the European Space Agency, will eventually penetrate the atmospheric veil that clouds the moon. The narrow angle camera shot this composite image on June 10.

Cassini researchers also see very complex energy structures in the plasma of the magnetosphere. Voyager gathered energy measurements below 6 kilo electron Volts (keV) and above 30 keV. CAPS is taking measurements from 1 eV to 50 keV, covering the all-important 5-30 keV gap where much of Saturn's plasma is found.

More than a year before the spacecraft arrived at Saturn, CAPS made its first major discovery when it made the first in situ observations of interstellar pickup ions beyond the orbit of Jupiter. Data analysis revealed a strong depletion of hydrogen pickup ions compared to helium pickup ions in the region downstream of the Sun in the direction of flow of interstellar gas. SwRI researchers determined that this depletion, called the "interstellar hydrogen shadow," is produced by radiation pressure and ionization of the neutrals. Before Cassini, the particles were extremely difficult to measure. Models indicated that something like the interstellar hydrogen shadow existed but CAPS was able to take the first direct measurements of it.

Cassini carries 11 other instruments to image the saturnian system at infrared, ultraviolet and visible wavelengths and with radar, and to directly sample the dust, neutral gas, plasma wave and charged particle environments.

The spacecraft also carries the Huygens probe, which consists of six experiments to measure the composition, temperature and atmospheric dynamics of Saturn's largest moon, Titan. The probe will be released from the Cassini orbiter spacecraft on Christmas Eve of this year and be deployed into Titan's nitrogen-rich atmosphere on January 14, 2005.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory manages the mission for the NASA Science Mission Directorate.

Since its launch on October 15, 1997, the Cassini spacecraft has traveled 2.2 billion miles, using four gravity assists at Venus, Earth and Jupiter to gain enough speed to reach Saturn, before falling into Saturn orbit some 930 million miles from Earth. At this great distance, signals transmitted to Cassini from Earth (and vice versa) are delayed on average by a light travel time of 1 hour and 26 minutes.

An Exciting Early Look

Published in the Summer 2004 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.

Summer 2004 Technology Today
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