GREAT RED SPOT

One of the biggest features in Jupiter's atmosphere is the Great Red Spot. It is a huge storm that is bigger than the entire Earth. It is on record since at least 1831, and as early as 1665. Images by the Hubble Space Telescope have shown as many as two smaller "red spots" right next to the Great Red Spot. Storms can last for hours or as long as hundreds of years in the case of the Great Red Spot. The storm is visible through Earth-based telescopes with an aperture of 12 cm or larger. The oval object rotates counterclockwise, with a period of about six days. The maximum altitude of this storm is about 8 km above the surrounding cloud tops. The Spot's composition and the source of its red colour remain uncertain, although photodissociated ammonia reacting with acetylene is a robust candidate to explain the colouration.

The Great Red Spot is larger than the Earth. Mathematical models suggest that the storm is stable and will be a permanent feature of the planet. However, it has significantly decreased in size since its discovery. Initial observations in the late 1800s showed it to be approximately 41,000 km across. By the time of the Voyager flybys in 1979, the storm had a length of 23,300 km and a width of approximately 13,000 km. Hubble observations in 1995 showed it had decreased in size to 20,950 km, and observations in 2009 showed the size to be 17,910 km. As of 2015, the storm was measured at approximately 16,500 by 10,940 km and was decreasing in length by about 930 km per year.