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MAY 2014

Jupiter's Trademark Red Spot Is Shrinking

promo image showing three views of Jupiter's Red Spot from 1995, 2009, and 2014

Jupiter's shrinking
Great Red Spot

I magine a storm so large that Earth would fit inside it.

That Earth-sized storm is on Jupiter, the largest planet in the solar system. Called the Great Red Spot, the storm has been raging in Jupiter's cloudy atmosphere for at least a hundred years. Astronomers using the Hubble Space Telescope have discovered that the well-known storm has shrunk to 10,250 miles across, the smallest size ever measured. Observations during the late 1800s, however, showed that the storm was so enormous — 25,500 miles across — that three Earths could squeeze inside it.

Astronomers have been following its downsizing since the 1930s. Starting in 2012, however, amateur astronomers noticed an increase in the spot's shrinkage rate. The red spot is getting smaller by 550 miles a year. Even its shape has changed from an oval to a circle.

Astronomers do not know why the storm's size is decreasing. One possibility is that some unknown activity deep inside the planet's atmosphere may be draining energy and weakening the storm, causing it to shrink.

A swirling storm of rising air

The Great Red Spot shares some characteristics with hurricanes on Earth, including a circular motion and strong winds. Unlike a hurricane, the red spot rotates counterclockwise. The red spot also is a high-pressure system, where air rises. Hurricanes, on the other hand, are low-pressure systems, where air sinks. One reason the Jupiter storm may have lasted so long is that there is no solid surface to decrease the storm's energy and slow it down. Hurricanes, however, lose strength when they reach land.

The swirling storm towers six miles above Jupiter's multicolor cloud tops. Inside the red spot, some winds reach speeds of up to 400 miles an hour. The exact cause of the storm's signature red color is a mystery. One theory is that the material being pulled upward from deep inside Jupiter's atmosphere turns red when sunlight strikes it. The spot, however, is not always bright red. Sometimes its color is much lighter, and, rarely, can even be very pale.

promo image for the Voyager 1 image of Jupiter's Red Spot

Voyager's view of the red spot, 1979

promo image for the Voyager 1 movie of Jupiter's Red Spot

The Great Red Spot in motion, 1979

The first record of the Great Red Spot was a drawing made in 1831 by German amateur astronomer Samuel Heinrich Schwabe. The storm, however, may have been seen 200 years earlier, in 1665, by Italian astronomer Gian Domenico Cassini, who noted a monster storm he called the "Permanent Spot."

The red spot has been continuously observed since 1878. NASA's two Voyager spacecraft also made the first detailed observations of the feature in 1979, and Voyager 1 even took a movie of its six-day rotation.


The Star Witness

brings you "tele-scoops" from the Hubble Space Telescope