Jupiter Loses Big Belt

Two wide stripes—known as the equatorial belts—normally circle the huge planet, products of the fast-moving jet streams that roar through Jupiter's atmosphere.

But pictures of Jupiter taken by an amateur astronomer show that sometime during the past couple weeks, the south equatorial belt completely faded from view.

"The basic view of Jupiter is of two dark belts. Now there is only one," said Alan MacRobert, senior editor of Sky & Telescope magazine.

"This is the most obvious change on Jupiter that I can recall," he said, adding that anyone with a backyard telescope should be able to see the difference. "Any instrument powerful enough to show any of Jupiter's surface features will easily reveal the change."

However, the planet's "belt buckle" remains: The lost stripe "means that Jupiter's Great Red Spot is now floating all alone in whiteness, whereas usually it is in an indentation in the south equatorial belt," MacRobert said.

The spot—actually a raging storm three times bigger than Earth—is rarely brick red, he added, and often becomes quite pale. MacRobert characterizes its current color as "somewhat orangy." (Find out why the Great Red Spot has been shrinking.)

Jupiter's Dark Belt Lost to Light Cloud Cover?

Despite the dramatic change on Jupiter, the lost belt doesn't concern experts. The planet has lost this stripe before, most recently in the 1970s and the early '90s. So far, the stripe has always reappeared.

Astronomers are, however, somewhat stumped for a complete explanation.

A dozen or so jet streams move alternately east-west and west-east on Jupiter, said planetary scientist Andrew Ingersoll of the California Institute of Technology. The clouds in between these jet streams create the planet's multicolored stripes and swirls.

Jupiter Auroras Fed by Largest Moon's Magnetic "Bubble"

That's one finding in new research that offers unprecedented details on interactions between Jupiter and two of its moons, the giant Ganymede and the volcanically active Io.

On Earth, auroras are created by the interaction of charged particles from the sun with Earth's atmosphere. But at a distance of 483,780,000 miles (778,570,000 kilometers), Jupiter is too far away for the sun to be feeding the planet's polar light shows.

Instead, the moon Io's volcanoes eject a constant flow of particles, which create a donut-shaped ring around Jupiter. As Io ploughs through the ring, it generates magnetic waves that then connect to Jupiter and send particles streaming toward the planet's poles, as seen in the above animation.

In addition, the moon Ganymede is big enough to maintain its own protective magnetic "bubble" even as it orbits inside Jupiter's powerful magnetic field. Scientists have suspected that this mini-magnetosphere might be involved in generating the auroras, but the connection has been unclear.

Energy in Motion

Bright dots known as auroral footprints are thought to show where streams of charged particles from Io and Ganymede are entering Jupiter's atmosphere. These dots move as the moons orbit the planet, creating tails that swirl around the auroras.

Above, sets of pictures show auroral footprints moving across Jupiter's north pole as seen in ultraviolet Hubble Space Telescope pictures taken in March 2007 (blue) and April 2005 (red).

Using thousands of such Hubble pictures, researchers with the University of Liège in Belgium studied the size of both moons' footprints and how changes over time relate to the moons' orbits.

The team found that the size of Ganymede's auroral footprint matches the size of the moon's magnetosphere, confirming its role in Jupiter's auroras.

In addition, the Hubble pictures show that the particles streaming from Io have different energy levels, which suggests that some particles don't lose all their energy when they hit Jupiter and so are able to penetrate deep into the planet's atmosphere, the authors say.

"Each of these auroral structures is telling an ongoing story about vast transfers of energy taking place far away from the planet," team member Denis Grodent said in a statement.