Great White Spot

This article is about a storm on Saturn. For the storms of white clouds on Jupiter, see white oval.

Saturn's great white spot.

The Great White Spot, also known as Great White Oval, on Saturn, named by analogy to Jupiter's Great Red Spot, are periodic storms that are large enough to be visible by telescope from Earth by their characteristic white appearance. The spots can be several thousands of kilometers wide.

Currently, a large band of white clouds called the Northern Electrostatic Disturbance (because of an increase in radio and plasma interference) is enveloping Saturn since 2010, and the Cassini orbiter is tracking the storm.

Cassini data has revealed a loss of acetylene in the white clouds, an increase of phosphine, and an unusual temperature drop in the center of the storm. As of April 2011, the storm underwent a second eruption.

Occurrence

The phenomenon is somewhat periodic at 28.5-year intervals, when Saturn's northern hemisphere tilts most toward the Sun. The following is a list of recorded sightings; years with spots generally considered to be part of the cycle are 1876, 1903, 1933, 1960, and 1990. The next appearance should be in 2016.

• 1876 – Observed by Asaph Hall. He used the white spots to determine the planet's period of rotation.
• 1903 – Observed by Edward Barnard.
• 1933 – Observed by Will Hay, comic actor and amateur astronomer. Until recent times the most celebrated observation.
• 1960 – Observed by JH Botham (South Africa).
• 1990 – Observed by Stuart Wilber, from 24 September through November.
• 1994 – Studied by ground-based observers and the Hubble Space Telescope.
• 2006 – Observed by Erick Bondoux and Jean-Luc Dauvergne.
• 2010 – First observed by Anthony Wesley.

That none were recorded before 1876 is a mystery, in some ways akin to the long observational gap of the Great Red Spot in the 18th and early 19th centuries; the 1876 Great White Spot (GWS) was extremely prominent, being visible in apertures as small as 60 mm. It is not known if the earlier record was simply poor, or if the 1876 GWS was truly a first for the telescopic era. Some believe that neither scenario is likely.

In 1992, Mark Kidger described three significant GWS patterns:

1. The GWSs alternate in latitude, with one apparition being limited to the North Temperate Zone (NTZ) or higher, and the following being limited to the Equatorial Zone (EZ). For instance, the 1960 GWS was high-latitude, and the 1990 GWS was equatorial.
2. The high-latitude GWSs recur at a slightly shorter interval than the equatorial GWSs (~27 versus ~30 years).
3. The high-latitude GWSs tend to be much less prominent than their equatorial counterparts.

Based on these apparent regularities, Kidger forecast that the next GWS will occur in the NTZ in 2016, and will probably be less spectacular than the 1990 GWS.

Characteristics and causes

A "classic" Great White Spot is a spectacular event, in which a brilliant white storm enlivens Saturn's usually bland atmosphere; all the major ones have occurred in the planet's northern hemisphere. They typically begin as discrete, literal "spots", but then rapidly expand in longitude, as the 1933 and 1990 GWSs did; in fact, the latter eventually lengthened enough to encircle the planet. Current theory (partly based on computer climate modelling) suggests that GWSs are massive atmospheric upwellings, perhaps due to thermal instability.

Seasonal variation : The rough coincidence of the GWSs with the summer solstice in Saturn's atmosphere has been seized on as proof that insolation is the main cause, though their occurrence has been more closely linked with time elapsed since the northern winter solstice. Whether similar phenomena occur during the winter solstice as well is unknown, as the rings effectively "block" a terrestrial view of the hemisphere.

See also

• Great Dark Spot

References

• Article on Saturn's Northern Electrostatic Disturbance on Sky and Telescope
• 1990/1 Hubble Space Telescope image
• 2006: observed with a 12" telescope by amateurs near Paris.
• Volunteers Help NASA Track Return of the Dragon

External links

• The Great White Spot at ESA/Hubble
• Christopher Go's Saturn Website Pictures of Saturn's Northern Electrostatic Disturbance (2011)