Saturday, May 15, 2010

Jupiter loses one of its belts


Jupiter and its system of belts and zones. The Red Spot is a hurricane-strength storm that's been present on the planet for at least the past few hundred years. Credit: NASA
Sunday morning when Jupiter and the moon came up together in the east I was tempted to remain outside with the birds and growing light and observe the planet, but it had been a long night and I desperately needed sleep. As the sun continues to move away from Jupiter, the planet will rise higher and in time become more conveniently placed for viewing. Still, intrepid sky watchers with small telescopes may want to consider making an early morning pilgrimage to the king of the planets to see for themselves how Jupiter's gotten something of a makeover.
Last winter changes were already underway as the South Equatorial Belt, one of the two most prominent dark "stripes" on the planet, began to fade. Most years you look at Jupiter and besides the four little moons lined up on one side or another of the planet, you'll see two prominent dark grey bands, the north and south equatorial belts. These and Jupiter's other belts are separated by lighter-colored zones giving it a striped appearance. Both belts and zones are composed of ammonia ice crystals which freeze out at 108 degrees below zero, a temperature easily attainable at Jupiter's half-billion mile distance from the sun. Materials like sulfur and phosphorus mixed in with the ammonia are believed responsible for creating the clouds' curious red, brown and yellow tints.


Watch Jupiter rotate and its cloud belts move with the winds. The time-lapse video was made using images shot during Voyager 1's flyby of the planet in 1979.
The origin of all these belts and zones comes from deep below the planet. Bubbles of warmer air rise to the upper atmosphere and condense into clouds where they're are blown into alternating bands by 350+ mile per hour winds. Jupiter's rapid rotation is at the source of its ferocious winds -- a day on the planet whisks by in just 9.9 hours. This speedy spin coupled with Jupiter's gaseous nature is also the reason the planet is flattened like a squashed meatball instead of being more nearly spherical.

Look at the dramatic change in the planet from less than a year ago. The South Equatorial Belt (SEB) has faded away leaving just the north belt (NEB) viewable in small telescopes. Credit: Anthony Wesley
So here's the surprise. That bad boy south equatorial belt (SEB) has completely faded away. Point your scope at the planet any morning soon and you'll see only one obvious dark stripe, the North Equatorial Belt. Jupiter with only one belt is almost like seeing Saturn when its rings are edge-on and invisible for a time -- it just doesn't look right.
The SEB is one of the most active areas on the planet for weather changes. Every 3-15 years, the belt, which is normally dark reddish-brown in color and typically divided in two by the south equatorial belt zone, fades from view. After some weeks or months a brilliant white spot forms within that zone and begins spouting dark blobs of material which get stretched into filaments and ovals by Jupiter's fierce winds into a new SEB. Within a few weeks (or longer) the belt is back and Jupiter presents its familiar dual "tire track" appearance through a telescope.


When the South Equatorial Belt fades, the Great Red Spot, here seen at the planet's left edge, usually becomes darker and more prominent. Credit: Anthony Wesley

The phenomenon goes by the name of the South Equatorial Belt Disturbance, and the changes are dramatic enough to see their evolution night by night. That's why I'd like to urge you to set the alarm and take a look at Jupiter in the next few weeks. Later this season, when the planet grows a new face, you'll be able to appreciate how quickly a celestial object like a planet can change. It's part of the reason amateur astronomers observe them as often as they can much to their spouses' amusement.
To find Jupiter, go out about an hour before sunrise and look a fist or two above the horizon in the east-southeastern sky. It's the only bright "star" you'll see there.
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