Great White Spot
Encyclopedia
The Great White Spot, also known as Great White Oval, on Saturn
, named by analogy to 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.
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.
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.
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 GWS was extremely prominent, being visible in apertures as small as 60 mm. Was the earlier record simply poor, or was the 1876 GWS truly a first for the telescopic era? Some believe that neither scenario is likely.
Mark Kidger (1992) has described three significant GWS patterns:
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.
, 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.
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
, named by analogy to Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
'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.
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
Cassini-Huygens
Cassini–Huygens is a joint NASA/ESA/ASI spacecraft mission studying the planet Saturn and its many natural satellites since 2004. Launched in 1997 after nearly two decades of gestation, it includes a Saturn orbiter and an atmospheric probe/lander for the moon Titan, although it has also returned...
is tracking the storm.
Cassini data has revealed a loss of acetylene
Acetylene
Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...
in the white clouds, an increase of phosphine
Phosphine
Phosphine is the compound with the chemical formula PH3. It is a colorless, flammable, toxic gas. Pure phosphine is odourless, but technical grade samples have a highly unpleasant odor like garlic or rotting fish, due to the presence of substituted phosphine and diphosphine...
, 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 hemisphereNorthern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...
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.
- 1876 – Observed by Asaph HallAsaph HallAsaph Hall III was an American astronomer who is most famous for having discovered the moons of Mars in 1877...
. He used the white spots to determine the planet's period of rotation. - 1903 – Observed by Edward Barnard.
- 1933 – Observed by Will HayWill HayWilliam Thomson "Will" Hay was an English comedian, actor, film director and amateur astronomer.-Early life:He was born in Stockton-on-Tees, in north east England, to William R...
, comic actor and amateur astronomer. Until recent times the most celebrated. - 1960 – Observed by JH Botham (South AfricaSouth AfricaThe Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
). - 1990 – Observed by Stuart Wilber, from 24 September through November.
- 1994 – Studied by ground-based observers and the Hubble Space TelescopeHubble Space TelescopeThe Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
. - 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 GWS was extremely prominent, being visible in apertures as small as 60 mm. Was the earlier record simply poor, or was the 1876 GWS truly a first for the telescopic era? Some believe that neither scenario is likely.
Mark Kidger (1992) has described three significant GWS patterns:
- 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.
- The high-latitude GWSs recur at a slightly shorter interval than the equatorial GWSs (~27 versus ~30 years).
- 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 longitudeLongitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
, 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
Solstice
A solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky, as viewed from Earth, reaches its northernmost or southernmost extremes...
in Saturn's atmosphere has been seized on as proof that insolation
Insolation
Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter or kilowatt-hours per square meter per day...
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.
External links
- The Great White Spot at ESA/Hubble
- Christopher Go's Saturn Website Pictures of Saturn's Northern Electrostatic Disturbance (2011)