White Spots?
Overall, Saturn presents a less dynamic range of details on its globe than does Jupiter, mainly displaying varying belts and zones without the finer details. This is why most observers tend to focus their attention on the rings. Episodically, however, Saturn’s globe erupts with stunning detail. The planet’s 2020 apparition may be one of those years that brings us some visual surprises.
Chief among them may be the return of the Great White Spot — a fantastic high altitude storm that surfaces roughly every 20 to 30 years. The greatest storms can be as large as Earth, with tails that wrap entirely around the planet over the course of months as regions of Saturn’s globe transform into a tempest of dynamic change. Six such storms have been observed on Saturn over the past 140 years, alternating between the equator and midlatitudes. Up until 1990, all the storms had occurred on a roughly 30-year cycle, but a surprise Great White Spot broke the cycle, occurring at an unexpected time and latitude.
The storms usually occur when Saturn’s northern hemisphere is most tilted toward the Sun near the moment of saturnian midsummer. The spots appear to be caused by erupting plumes of warm moist air that rapidly bring ammonia ice crystals into the planet’s upper atmosphere, triggering thunderstorms.
Three types of storms have been recognized to date: small white storms (like those that occurred in 1994 and 2006) that measure about 1,240 miles (2,000 kilometers) across and appear as bright clouds for a few days; Great White Spots (like those of 2010 and 1990) that are up to 10 times larger than the small storms and last for months every two to three decades; and midsized storms (like those detected near Saturn’s north pole in 2018) that can last anywhere from 10 to more than 200 days.
Will a Great White Spot appear on its normal 30-year cycle in 2020? The answer remains unknown. In fact, much remains uncertain about these storms, such as: How periodic are they? Can they occur at any latitude? How are they triggered?