http://www.eurekalert.org/pub_releases/2004-12/uoi-rpc121604.php<br /><br />Public release date: 16-Dec-2004<br /><br />Contact: Gary Galluzzo<br />gary-galluzzo@uiowa.edu<br />319-384-0009<br />University of Iowa<br /><br />Researchers present Cassini findings at Saturn<br /><br />As NASA's Cassini spacecraft approached Saturn last July, it found<br />evidence that lightning on Saturn is roughly one million times stronger<br />than lightning on Earth.<br /><br />That's just one of several Cassini findings that University of Iowa<br />Space Physicist Don Gurnett will present in a paper to be published<br />Thursday, Dec. 16 in Science Express, an online version of the journal<br />Science, and in a talk to be delivered Friday, Dec. 17 at a meeting of<br />the American Geophysical Union in San Francisco.<br /><br />Other findings include:<br /><br />o Cassini impacted dust particles as it traversed Saturn's rings.<br />o Saturn's radio rotation rate varies.<br /><br />The comparison between Saturn's enormously strong lightning and Earth's<br />lightning began several years ago as the Cassini spacecraft prepared<br />for<br />its journey to Saturn by swinging past the Earth to receive a<br />gravitational boost. At that time, Cassini started detecting radio<br />signals from Earth's lightning as far out as 89,200 kilometers from the<br />Earth's surface. In contrast, as Cassini approached Saturn, it started<br />detecting radio signals from lightning about 161 million kilometers<br />from<br />the planet. "This means that radio signals from Saturn's lightning are<br />on the order of one million times stronger than Earth's lightning.<br />That's just astonishing to me!" says Gurnett, who notes that some radio<br />signals have been linked to storm systems observed by the Cassini<br />imaging instrument.<br /><br />Earth's lightning is commonly detected on AM radios, a technique<br />similar<br />to that used by scientists monitoring signals from Cassini.<br /><br />Regarding Saturn's rings, Gurnett says that the Cassini Radio and<br />Plasma<br />Wave Science (RPWS) instrument detected large numbers of dust impacts<br />on<br />the spacecraft. Gurnett and his science team found that as Cassini<br />approached the inbound ring plane crossing, the impact rate began to<br />increase dramatically some two minutes before the ring plane crossing,<br />then reached a peak of more than 1,000 per second at almost exactly the<br />time of the ring plane crossing, and finally decreased to pre-existing<br />levels about two minutes later. Gurnett notes that the particles are<br />probably quite small, only a few microns in diameter, otherwise they<br />would have damaged the spacecraft<br /><br />Finally, variations in Saturn's radio rotation rate came as a surprise.<br />Based upon more than one year of Cassini measurements, the rate is 10<br />hours 45 minutes and 45 seconds, plus or minus 36 seconds. That's about<br />six minutes longer than the value recorded by the Voyager 1 and 2<br />flybys<br />of Saturn in 1980-81. Scientists use the rotation rate of radio<br />emissions from the giant gas planets such as Saturn and Jupiter to<br />determine the rotation rate of the planets themselves because the<br />planets have no solid surfaces and are covered by clouds that make<br />direct visual measurements impossible.<br /><br />Gurnett suggests that the change in the radio rotation rate is<br />difficult<br />to explain. "Saturn is unique in that its magnetic axis is almost<br />exactly aligned with its rotational axis. That means there is no<br />rotationally induced wobble in the magnetic field, so there must be<br />some<br />secondary effect controlling the radio emission. We hope to nail that<br />down during the next four to eight years of the Cassini mission."<br /><br />One possible scenario was suggested nearly 20 years ago. Writing in the<br />May 1985 issue of "Geophysical Research Letters," Alex J. Dessler, a<br />senior research scientist at the Lunar and Planetary Laboratory,<br />University of Arizona, argued that the magnetic fields of gaseous giant<br />planets, such as Saturn and Jupiter, are more like that of the sun than<br />of the Earth. The sun's magnetic field does not rotate as a solid body.<br />Instead, its rotation period varies with latitude. Commenting earlier<br />this year on the work of Gurnett and his team, Dessler said, "This<br />finding is very significant because it demonstrates that the idea of a<br />rigidly rotating magnetic field is wrong. Saturn's magnetic field has<br />more in common with the sun than the Earth. The measurement can be<br />interpreted as showing that the part of Saturn's magnetic field that<br />controls the radio emissions has moved to a higher latitude during the<br />last two decades."<br /><br />The radio sounds of Saturn's rotation -- resembling a heartbeat -- and<br />other sounds of space can be heard by visiting Gurnett's Web site at:<br />http://www-pw.physics.uiowa.edu/space-audio.<br /><br />Cassini, carrying 12 scientific instruments, on June 30, 2004 became<br />the<br />first spacecraft to orbit Saturn and begin a four-year study of the<br />planet, its rings and its 31 known moons. The $1.4 billion spacecraft<br />is<br />part of the $3.3 billion Cassini-Huygens Mission that includes the<br />Huygens probe, a six-instrument European Space Agency probe, scheduled<br />to land on Titan, Saturn's largest moon, in January 2005.<br /><br />###<br /><br />The Cassini-Huygens mission is a cooperative project of NASA, the<br />European Space Agency and the Italian Space Agency. JPL, a division of<br />the California Institute of Technology, Pasadena, Calif. manages the<br />Cassini-Huygens mission for NASA's Office of Space Science, Washington,<br />D.C. JPL designed, developed and assembled the Cassini orbiter. For the<br />latest images and information about the Cassini-Huygens mission, visit:<br />http://www.nasa.gov/cassini<br /><br />STORY SOURCE: University of Iowa News Services, 300 Plaza Centre One,<br />Suite 301, Iowa City, Iowa 52242-2500.<br /><br />RESEARCH CONTACT: Don Gurnett, Donald-gurnett@uiowa.edu<br /><br />MEDIA CONTACT: Gary Galluzzo, Writer, 319-384-0009,<br />gary-galluzzo@uiowa.edu