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Cassini peeks up Jupiter's skirt

Back in 2000, Cassini took a rare shot of Jupiter from directly underneath the Giant Planet's south pole -- red spot and all.

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NASA/JPL/Space Science Institute

Most of our snaps of Jupiter look pretty similar: an angled view from the side, showing the planet's distinctive banded cloud. An image snapped by Saturn probe Cassini, though, on its flyby on December 11 and 12, 2000 en route to Saturn, shows the Giant Planet's posterior in elaborate detail -- directly from below.

Composed of 18 images -- one photo snapped in near-infrared and another in blue, every hour for nine hours, then composited into one image very close in hue to what the human eye would see when gazing at Jupiter -- the photo shows the planet's South pole and chaotic stormy atmosphere.

The alternating white-and-brown bands of cloud, segregated latitudinally, are made up of dense layers of ammonium crystals, ammonium hydrosulfide and water. The lighter zones and the darker belts -- coloured orange and brown by, scientists hypothesise, compounds such as sulfur, phosphorus and hydrocarbons that change colour when exposed to the sun's ultraviolet light -- have conflicting circulation patterns. The zones are colder, consisting of ascending air; the belts are warmer, consisting of descending air. This results in turbulent storms; the highest wind speed ever recorded on Jupiter is upwards of 600 kph.

This stormy atmosphere accounts for the Great Red Spot, an anticyclonic storm that ranges between 24,000 and 40,000 km across. It has been raging, records show, for at least since 1831 -- its first officially recorded appearance -- although it's possible that the "Permanent Spot" described by Gian Domenico Cassini in 1665 is the selfsame storm.

Data captured by Hubble shows that the Great Red Spot is shrinking -- although what this means is unclear. Is it merely a fluctuation, or will the storm diminish to the point of vanishing? Does the storm come and go, like a heart that takes centuries for a single beat?

In the accompanying map of the north pole, you can see other features: bright spots in the orange belt represent thunderstorms bearing lightning.

Although this image is pretty detailed -- the smallest object is 120km across -- new information about Jupiter is soon to start filtering back to Earth. NASA's Juno, launched in 2011, is due to reach Jovian orbit in August 2016, to start performing the same service for Jupiter that Cassini does for Saturn.

The ESA, too, also plans a Jupiter probe. The JUICE explorer has just been greenlit for implementation, with a planned launch date of 2022.