The Great Red Spot is arguably Jupiter’s most iconic feature. Mention the giant planet and most of us conjure up an image of striped gas ball with a big red beauty mark.
While the spot has been observed since the infancy of the telescope, we’ve come to accept it as a permanent part of the Jovian planet’s persona. Now it’s time to admit the truth. The Great Red Spot has been downsizing since the 1930s with particularly swift changes happening in just the last couple decades.
Ask any long-time amateur astronomer. Back in the 1960s the spot extended over a greater area and was more elongated or stretched out. In the past few years, its not only contracted thousands of miles but become more circular. Most of us have blamed the spot’s pale, watered-down color in recent years as the reason it’s become more difficult to see.
But that’s only part of the problem. Since 1995 it’s downsized by over 3,000 miles. That’s nearly half an Earth diameter in 20 years! Since 2012 it’s been losing girth at the rate of 580 miles a year. 130 years ago the spot spanned about 25,000 miles (40,000 km) and looked like a giant blimp riding Jupiter’s cloud belts. Even in the small 2-3 inch refracting telescopes popular at the time it would have hard to miss. Now you need at least a 6-inch telescope to see it clearly.
“In our new observations it is apparent very small eddies are feeding into the storm,” said Simon. “We hypothesized these may be responsible for the accelerated change by altering the internal dynamics and energy of the Great Red Spot,” said Amy Simon of NASA’s Goddard Space Flight Center.
The not-so Great Red Spot (GRS) is a hurricane-like storm that rotates anticlockwise in Jupiter’s southern hemisphere immediately south of the prominent South Equatorial Belt. It takes its swirly appearance from winds blowing at several hundred miles an hour with the spot’s cloudtops reaching 5 miles (8 km) above Jupiter’s cloud deck.
The spot used to rotate once every 6 days, but smaller eddies or vortices feeding into the spot that may be responsible for its changing appearance have shortened that to about 4 days.
What will become of the spot is anyone’s guess. It may continue to wither and disappear altogether. It is does go bye-bye, Red Spot Jr. waits in the wings. This new but considerably smaller red-tinted spot formed from the merger of three smaller oval vortices between 1998 and 2000. Or we could be completely surprised and see it revivified by the Jovian jet streams. Such is weather, whether here or on Jupiter, there will always be an element of unpredictability.
NASA’s Juno spacecraft is hurtling toward Jupiter now, due to reach the giant planet in July 2016. Up close examination by the probe will hopefully fill in holes in our knowledge of the planet’s turbulent and fascinating atmosphere.