A Powerful Attraction
Twenty times larger than the sun, tens of thousands of times larger than Earth, the largest object in the solar system is a violent, radiation-filled magnetic bubble that surrounds Jupiter, according to new data resulting from the timely rendezvous of two spacecraft around the giant planet.
“This inner region is probably the most intense and hazardous environment in the solar system except for the surface of the sun. It’s the extreme of the extreme,” said Scott Bolton, a space physicist at the Jet Propulsion Laboratory who worked on several of the new studies.
Jupiter’s magnetic field arises from electric currents generated by the planet’s metallic interior. Earth has a magnetic field too, but it is much smaller and simpler than Jupiter’s. Both magnetospheres contract and expand dramatically as they are buffeted by the solar wind, the streams of charged particles that are hurled into space by the ferocious activity of the sun.
But the vagaries of Jupiter’s magnetosphere have remained a mystery. Many of the signals it generates are trapped; those that do escape can’t penetrate Earth’s protective atmosphere.
“The only way to measure these [charged particles and radio signals] is to take your radio receiver to the planet,” said William Kurth, a planetary scientist at the University of Iowa who worked on a number of the new studies.
In January 2001, scientists were able to tag-team Jupiter with two spacecraft: the soon to be retired veteran Galileo and a newcomer called Cassini-Huygens. Cassini, speeding toward a 2004 date with Saturn, passed within 6 million miles of Jupiter in order to use the planet’s gravitational field as a slingshot.
In addition to the two spacecraft, scientists were also able to employ the Hubble and Chandra space telescopes and an array of radio telescopes on Earth. The synchronized view gave scientists so much information they published a flood of results. In the current issue of the journal Nature, 82 planetary scientists have published seven papers, something physicist Michele Dougherty of Imperial College in London called “a very rich scientific harvest.”
Cassini reached Jupiter while the battered, radiation-soaked Galileo was still circling the planet. Launched in 1989, Galileo had been expected to give out by 1997, the year Cassini was launched. With some software reboots and a little baby-sitting by JPL engineers, Galileo kept plugging away and sending home streams of data on Jupiter and its moons. “It just kept going, and going and going,” said Linda Spilker, the deputy project scientist at JPL for Cassini.
Galileo’s extended lifespan gave scientists the unprecedented chance to examine another planet using two spacecraft at once, something Kurth called “an amazing stroke of luck.”
This gave scientists the opportunity to see just how much Jupiter’s magnetosphere is affected by the solar wind and how much of it is altered by internal factors coming from Jupiter or its many moons.
While one spacecraft measured the magnetosphere, the other measured the solar wind. “For the first time, we could see the connection,” said Spilker.
The magnetosphere cooperated as well.
“Nature chose to be kind on this occasion,” said Thomas W. Hill, an expert on planetary magnetospheres at Rice University in Houston. “The magnetosphere was unusually large--just large enough to reach out and touch the passing Cassini-Huygens spacecraft on two brief occasions.”
The papers show that Jupiter’s magnetosphere is indeed buffeted, squashed and stretched by the solar wind, and that the bubble is also influenced by strange dynamics that come from Jupiter and its moons.
Much understanding came from studying the auroras at Jupiter’s poles. Like the northern and southern lights on Earth, the glowing displays are visible manifestations of the magnetic energy surrounding a planet. The new results show that Jupiter’s auroras, unlike Earth’s, move around as Jupiter spins. They are even influenced by Jupiter’s moons. Electrical signals from the moons travel across currents to Jupiter, leaving glowing traces or “magnetic footprints” within the aurora.
Despite the unprecedented examination of Jupiter, the planet’s magnetosphere continues to hold its mysteries.
One surfaced when Bolton was able to use Cassini’s communication antenna to “listen in” on radio signals coming from the violent, high energy environment near Jupiter’s surface. He found that some electrons there were moving at nearly the speed of light--more than twice as fast as expected. The findings suggest that activity around Jupiter approaches the intensity of that found in some of the universe’s most violent regions: black holes and areas where stars are forming. Another team was stumped by a “pulsating hot spot of X-rays” coming from Jupiter’s polar regions. They say they have no idea of its source.
Although scientists will be studying the new data for years, the two spacecraft have moved on. Galileo is continuing to study Jupiter and its moons and will do so until its planned descent into Jupiter’s crushing atmosphere next year. Cassini is speeding toward Saturn and its rings. The gravity assist from Jupiter boosted Cassini’s speed by 4,900 mph; the success at the giant planet boosted the sprits of its scientific team.
“The Jupiter fly-by,” said Spilker, “gave us a taste of what we can expect at Saturn.”
