Jupiter has lost a belt, giving sky-watchers an unusual view of the gas giant planet's pale bottom.

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."

(See a picture of a dark spot on Jupiter made by a recent impact.)

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.

Unlike atmospheric features on Earth, Jupiter's streams have remained at constant strengths and stayed in almost exactly the same locations since we first got a close look at them during the Voyager 1 probe flybys in 1979. (See a Voyager picture of Jupiter's Great Red Spot.)

The shifting upwellings and downwellings of north-south air currents may cause changes to the cloudy stripes by bringing up different substances, changing the belts' shapes and chemical compositions, Ingersoll said.

"If one band has an upwelling, at least the top of those clouds could spread out over [and obscure] neighboring bands," he said. "What's probably happened is that this very slow upwelling or downwelling circulation has changed, but we can't measure it."

But even if lighter clouds have covered darker ones, scientists don't yet know what causes the different colors of Jovian belts, Ingersoll added.

Jupiter's clouds may be made of ammonia, hydrogen sulfide, or other substances. Each chemical would react differently to environmental changes, such as exposure to sunlight.

"We still haven't figured the colors out," Ingersoll said. "It could be sulfur compounds, it could be organic compounds—or even more exotic things like phosphorous. We just don't know the chemistry."

Such mysteries about Jupiter may soon be solved by NASA's Juno probe, due to launch in 2011 to study the Jovian system.

Not Certain Jupiter's Lost Belt Will Return

In the meantime, people hoping to catch a glimpse of the "new" Jupiter might have to rise early.

Sky-watchers in mid-northern latitudes, including the U.S. and Europe, should look to the east in the brightening skies about an hour before sunrise, Sky & Telescope's MacRobert advised.

"It's by far the brightest thing low in the east," he said. "You can't miss it."

Many people might also be keeping their eyes peeled for the stripe's eventual return, which Caltech's Ingersoll said is likely—but not absolutely certain.