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Jupiter's Great Red Spot

Kulvinder Singh, News reporter
May 4, 2015, 1:42 UTC

Sen—The Great Red Spot (GRS) is planet Jupiter's most famous feature, as distinctive as it is mysterious. It's the most consistent, single weather system ever-observed by humanity, occurring in the largest planetary atmosphere in the Solar System. Notably observed with a telescope by Italian astronomer Giovanni Cassini in 1665, the GRS may even have been spotted earlier.

Jupiter is the largest planet in our Solar System with a mean diameter of 141,000 kilometers, nearly 11 times that of Earth. The only part visible to us is the churning, dynamic cloud layer, which is separated into different-coloured bands called 'zones' and 'belts'. These bands travel in different directions across the planet causing turbulence and wind shear. In amongst these bands, vortices often appear. The GRS is just such a vortex—the biggest one on the planet. 

The near constant observation of the GRS since the 19th century has allowed astronomers to see that the GRS has been shrinking, and the reason isn't really known. In May 2014 Hubble Space Telescope observations showed that the oval-shaped GRS, 16,496 kilometers across at its widest point, was half what it was at the end of the nineteenth century. By comparison Earth's mean diameter is 12,742 kilometers. By 2012 the rate of shrinking had accelarated. The GRS is currently reducing by 935 kilometers per year. Scientists at NASA's Goddard Space Flight Center think that eddies—small vortices caused by turbulence—are sapping the GRS of its energy. But more observations are needed to determine if this is the case.

The GRS is an anticyclone—a high-pressure region with outward-flowing winds, rotating in an opposite direction to the planet. The winds inside the GRS typically travel at 360 kilometers per hour. It's situated 22° from the equator, where wind jets from counter-travelling cloud bands confine it to that latitude. However, it can and does move freely longitudinally. Although often likened to hurricanes on Earth, this is perhaps misleading. Hurricanes are cyclones—low-pressure regions with inward-flowing winds.

Because the GRS is a high-pressure region, the distinctive red colour was long-thought to originate from chemical elements dredged up from deeper layers. But observations from NASA's Cassini spacecraft—which flew by the planet in 2000 on its way to Saturn—suggest that this might not be the case. Instead ammonia and acetylene are thought to produce red-coloured material when exposed to ultraviolet light from the Sun. Experiments conducted by scientists at NASA's Jet Propulsion Laboratory are consistent with this theory.

The reasons for the GRS's origin and longevity are still not fully understood, but scientists from Harvard University have developed 3D models of vertical flows of gas into and out of the system. Their simulations show that such vertical gas flows can feed energy into the GRS and can keep it going. It may also provide clues as to why this long-sustained feature is shrinking, and whether it will continue to do so.

Video based on NASA Cassini spacecraft observations of Jupiter, showing the Great Red Spot trapped withing counter-rotating cloud bands. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio,Cassini Imaging Team, CICLOPS, Cosmos Studios, Andrew Ingersoll (CalTech).