Jupiter’s most iconic feature is its Great Red Spot. An enormous storm system, the Spot is approximately 12,400 miles long and 7,500 miles wide —large enough to engulf the Earth and Mars side by side.

Unlike hurricanes and cyclones on Earth, which come and go in a matter of days, this iconic oval has endured for centuries. The spot was first recorded in a drawing made in 1831 by German amateur astronomer Samuel Heinrich Schwabe – and may have been churning long before that.

Generally reddish in color and slightly oval in shape, the source of the red coloration is unknown; suggestions range from compounds of sulfur and phosphorus to organic material, any of which could be produced by lightning discharges or by high-altitude photochemical reactions.

Like a hurricane on Earth, the center is relatively calm, but farther out, the winds scream at 270 to 425 miles per hour. Infrared data indicates that the Great Red Spot is colder and at a higher altitude than most of the other clouds on Jupiter.

The Great Red Spot may be powered by the condensation of water, ammonia, or both at lower levels in Jupiter’s atmosphere. Alternatively, it may draw its energy from the smaller eddies that merge with it or from the high-speed currents on either side of it. Its remarkable longevity is undoubtedly a result of its size, but an exact theory that explains both its source of energy and its stability remains to be developed.

About 100 years ago, the storm covered over 40,000 km of the surface. It is currently about one half of that size and seems to be shrinking. At the present rate that it is shrinking it could become circular by 2040. It is not known how long the spot will last, or whether the changes in size are a result of normal fluctuations.