A Change in Jovian Weather

Jupiter’s famous Great Red Spot is showing signs of age. The spot, known to be caused by a great, uninterrupted storm in the Jovian atmosphere, has been shrinking for many decades. But the visible evidence of it has never been as clear as now.

In this comparison image the top photo was taken by Hubble’s Wide Field Planetary Camera 2 in 1995. The spot is shown at a diameter of just under 21,000 km. The second image shows a 2009 photo of the Hubble Wide Field Camera 3 (“WFC3”) showing a diameter of just under 18,000 km. Finally, the lower photo shows the newest image from WFC3 taken in 2014 with the spot at its smallest yet, with a diameter of just 16,000 km.

Historic observations of the Great Red Spot date as far back as the late 1800s, when it may have been as much as 41,000 km at its widest point–wide enough to fit three Earths comfortably side by side. In 1979 and 1980 the NASA Voyager fly-bys measured the spot at a shrunken 23,335 km across.

“Recent Hubble Space Telescope observations confirm that the spot is now just under 16 500 kilometres across, the smallest diameter we’ve ever measured,” said Amy Simon of NASA’s Goddard Space Flight Center in Maryland, USA.

Amateur observations starting in 2012 revealed a noticeable increase in the spot’s shrink rate. Its “waistline” is getting smaller by just under 1,000 km per year.

“In our new observations it is apparent that very small eddies are feeding into the storm. We hypothesize that these may be responsible for the sudden change by altering the internal dynamics of the Great Red Spot.”

Simon’s team plans to study the motions of these eddies, and also the internal dynamics of the spot, to determine how the stormy vortex is fed with or sapped of momentum.