Great Dark Spot
Encyclopedia
The Great Dark Spot is the name given to a series of dark spots on Neptune
similar in appearance to Jupiter's
Great Red Spot. The first one was observed in 1989 by NASA
's Voyager 2
probe. Like Jupiter's spot, they are anticyclonic storm
s. However, the interiors of Great Dark Spots are relatively cloud-free, and unlike Jupiter's spot, which has lasted for hundreds of years, their lifetimes appear to be much shorter, forming and dissipating once every few years or so. Based on observations taken by Voyager and since then with the Hubble Space Telescope
, Neptune appears to spend somewhat more than half its time with a Great Dark Spot. Almost all that is known about Neptune is based on the research carried out by the spacecraft Voyager 2.
-shaped spot (with initial dimensions of 13,000 × 6,600 km, or 8,100 × 4,100 mi) was about the same size as Earth
, and was similar in general appearance to Jupiter
's Great Red Spot. Around the Great Dark Spot, winds were measured blowing up to 2400 kilometres (1,491.3 mi) an hour, the fastest in the Solar System. The Great Dark Spot is thought to represent a hole in the methane
cloud deck of Neptune
. The spot was observed at different times with different sizes and shapes.
The Great Dark Spot generated large white clouds at or just below the tropopause
layer similar to high-altitude cirrus clouds found on Earth
. Unlike the clouds on Earth, however, which are composed of crystals of ice, Neptune's cirrus clouds are made up of crystals of frozen methane
. And while cirrus clouds usually form and then disperse within a period of a few hours, the clouds in the Great Dark Spot were still present after 36 hours, or two rotations of the planet.
Neptune's dark spots are thought to occur in the troposphere
at lower altitudes than the brighter upper cloud deck features. As they are stable features that can persist for several months, they are thought to be vortex structures.
, it had disappeared completely, leaving astronomers to believe that it had either been covered up or vanished. The persistence of companion clouds shows that some former dark spots may continue to exist as cyclones even though they are no longer visible as a dark feature. Dark spots may dissipate when they migrate too close to the equator, or possibly through some other unknown mechanisms.
However, an almost identical spot emerged in Neptune's northern hemisphere
. This new spot, called the Northern Great Dark Spot (NGDS), has remained visible for several years.
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
similar in appearance to Jupiter's
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,...
Great Red Spot. The first one was observed in 1989 by NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
's Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
probe. Like Jupiter's spot, they are anticyclonic storm
Anticyclonic storm
An anticyclonic storm is a weather storm where winds around the storm flow contrary to the direction dictated by the Coriolis effect about a region of low pressure. In the northern hemisphere, anticyclonic storms involve clockwise wind flow; in the southern hemisphere, they involve counterclockwise...
s. However, the interiors of Great Dark Spots are relatively cloud-free, and unlike Jupiter's spot, which has lasted for hundreds of years, their lifetimes appear to be much shorter, forming and dissipating once every few years or so. Based on observations taken by Voyager and since then with the Hubble Space Telescope
Hubble Space Telescope
The 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...
, Neptune appears to spend somewhat more than half its time with a Great Dark Spot. Almost all that is known about Neptune is based on the research carried out by the spacecraft Voyager 2.
Characteristics
The dark, ellipticallyEllipse
In geometry, an ellipse is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve. Circles are special cases of ellipses, obtained when the cutting plane is orthogonal to the cone's axis...
-shaped spot (with initial dimensions of 13,000 × 6,600 km, or 8,100 × 4,100 mi) was about the same size as Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, and was similar in general appearance 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. Around the Great Dark Spot, winds were measured blowing up to 2400 kilometres (1,491.3 mi) an hour, the fastest in the Solar System. The Great Dark Spot is thought to represent a hole in the methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
cloud deck of Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
. The spot was observed at different times with different sizes and shapes.
The Great Dark Spot generated large white clouds at or just below the tropopause
Tropopause
The tropopause is the atmospheric boundary between the troposphere and the stratosphere.-Definition:Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry...
layer similar to high-altitude cirrus clouds found on Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
. Unlike the clouds on Earth, however, which are composed of crystals of ice, Neptune's cirrus clouds are made up of crystals of frozen methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
. And while cirrus clouds usually form and then disperse within a period of a few hours, the clouds in the Great Dark Spot were still present after 36 hours, or two rotations of the planet.
Neptune's dark spots are thought to occur in the troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....
at lower altitudes than the brighter upper cloud deck features. As they are stable features that can persist for several months, they are thought to be vortex structures.
Disappearance
When the spot was to be photographed again in November 1994 by the Hubble Space TelescopeHubble Space Telescope
The 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...
, it had disappeared completely, leaving astronomers to believe that it had either been covered up or vanished. The persistence of companion clouds shows that some former dark spots may continue to exist as cyclones even though they are no longer visible as a dark feature. Dark spots may dissipate when they migrate too close to the equator, or possibly through some other unknown mechanisms.
However, an almost identical spot emerged in Neptune's northern hemisphere
Northern 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...
. This new spot, called the Northern Great Dark Spot (NGDS), has remained visible for several years.
See also
- Extraterrestrial cycloneExtraterrestrial cycloneExtraterrestrial cyclones are cyclones found on other planets.-Venus:Venus has two large long-lived pairs of inverted anticyclones, one pair near each pole , discovered in 2007 by the Venus Express probe.-Mars:...
- Oval BA
- Small Dark Spot
- Dragon Storm (astronomy)Dragon Storm (astronomy)Dragon Storm is a large, bright and complex convective storm in Saturn's southern hemisphere. The Saturnian storm appears to be long-lived and periodically flares up to produce dramatic white plumes which then subside...
External links
- Neptune's Great Dark Spot of 1989, Windows to the Universe
- Neptune's Great Dark Spot: Gone But Not Forgotten, Astronomy Picture of the DayAstronomy Picture of the DayAstronomy Picture of the Day is a website provided by NASA and Michigan Technological University . According to the website, "Each day a different image or photograph of our universe is featured, along with a brief explanation written by a professional astronomer."The photograph is not necessarily...
, May 8, 1996