Great White Spot

From Wikipedia, the free encyclopedia

The Great White Spot, also known as Great White Oval, on Saturn, named by analogy from Jupiter's Great Red Spot, is a name given to 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.

1 Occurrence
2 Characteristics and Causes
3 Notes
4 References
5 External Links

[edit] 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 in bold:

Wikisource has original text related to this article:

an account of Hall's use of the Great White Spot to calculate Saturn's rotation.

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.^[1]

[edit] 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.^[2] 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.^[3] Current theory suggests that GWSs are massive atmospheric upwellings, perhaps due to thermal instability.^[4]

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.^[5]

[edit] Notes

^ Ibid., 180.
^ Ibid., 178.
^ Ibid., 187-9.
^ Ibid., 211-12.
^ Ibid., 213-4.

[edit] References

1990/1 Hubble Space Telescope image
2006: observed with a 12" telescope by amateurs near Paris.
Volunteers Help NASA Track Return of the Dragon
Patrick Moore, ed., The 1993 Yearbook of Astronomy, Mark Kidger, "The 1990 Great White Spot of Saturn", 176-215, (New York: W.W. Norton & Company, 1992).
Moche, Dinah (1996). "Chapter 9: Planets", Astronomy: a self-teaching guide (paper), 4th edition (in English), John Wiley & Sons, 245. ISBN 0-471-53001-8. Retrieved on November 26, 2006.