![Neptune's Great Dark Spot](https://upload.wikimedia.org/wikipedia/commons/d/df/Neptune%27s_Great_Dark_Spot.jpg)
The Great Dark Spot (also known as GDS-89, for Great Dark Spot, 1989) was one of a series of dark spots on
Neptune similar in appearance to
Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth t ...
's
Great Red Spot. In 1989, GDS-89 was the first Great Dark Spot on Neptune to be observed by
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
's ''
Voyager 2
''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, '' Voyager 1'', o ...
'' space probe. Like Jupiter's spot, Great Dark Spots are
anticyclonic storms. However, their interiors are relatively cloud-free, and unlike Jupiter's spot, which has lasted for hundreds of years, their lifetimes appear to be shorter, forming and dissipating once every few years or so. Based on observations taken with ''Voyager 2'' and since then with the
Hubble Space Telescope
The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...
, Neptune appears to spend somewhat more than half its time with a Great Dark Spot. Little is known about the origins, movement, and disappearance of the dark spots observed on the planet since 1989.
Characteristics
The Great Dark Spot was captured by NASA's Voyager 2 space probe in Neptune's southern hemisphere. The dark,
elliptically-shaped spot (with initial dimensions of 13,000 × 6,600 km, or 8,100 × 4,100 mi) of GDS-89 was about the same size as
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
, and was similar in general appearance to
Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth t ...
's
Great Red Spot. One major difference compared to Jupiter's Great Red Spot is that Neptune's Great Dark Spot has shown the ability to shift north-south over time, while the Great Red Spot is held in the same latitudinal region by global east-west wind currents. Around the edges of the storm, winds were measured at up to 2,100 kilometers per hour (1,300 mph), the fastest recorded in the Solar System. The Great Dark Spot is thought to be a hole in the
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
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
The tropopause is the atmospheric boundary that demarcates the troposphere from the stratosphere; which are two of the five layers of the atmosphere of Earth. The tropopause is a thermodynamic gradient-stratification layer, that marks the end of ...
layer similar to high-altitude
cirrus clouds
Cirrus ( cloud classification symbol: Ci) is a genus of high cloud made of ice crystals. Cirrus clouds typically appear delicate and wispy with white strands. Cirrus are usually formed when warm, dry air rises, causing water vapor deposition on ...
found on
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
. Unlike the clouds on Earth, however, which are composed of
crystals of water ice, Neptune's cirrus clouds are made up of crystals of frozen
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
. These high altitude clouds are located somewhere between 50-100 km (30-60 miles) above the main cloud deck. 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
The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
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
![Dark spot on Neptune](https://upload.wikimedia.org/wikipedia/commons/b/b5/Dark_spot_on_Neptune.jpg)
When the spot was to be photographed again in November 1994 by the
Hubble Space Telescope
The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...
, it had disappeared completely, leaving astronomers to believe that it has either been covered up or had 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.
Other dark spots observed
Following the Great Dark Spot, several other dark spots have been observed. In 1989, when the Voyager 2 observed the Great Dark Spot (GDS), a second dark spot, Dark Spot 2 (DS2) was found. Dark Spot 2 fully dissipated prior to the year 1994.
Beginning in 1994, the Hubble became the only operating facility to detect the presence and observe dark spots on Neptune and is still used in the present day. Hubble is able to view images at blue wavelength which is the only way the features are visible. In 1994, a Northern Dark Spot (NGDS-1994) formed in the northern hemisphere and disappeared between 1998 and 2000. The storm for its duration showed to be stable in latitude.
In 1996, a separate Northern Dark Spot (NGDS-1996) formed and was observed until its disappearance which occurred prior to 1998. Similarly to the prior dark spot, this one exhibited little to no meridional drift.
In 2015, a Southern Dark Spot (SDS) was discovered by the
Hubble Outer Planet Atmosphere Legacy (OPAL) program.
The Southern Dark Spot exhibited a poleward drift before its disappearance in 2017. In 2016, an almost identical spot as the Great Dark Spot (GDS) emerged in Neptune's northern hemisphere. This new spot, called the Northern Great Dark Spot (NGDS), has remained visible for several years. It is unknown whether this spot is still present on the planet, as observations using the Hubble telescope are limited.
More recently, in 2018, a newer main dark spot and smaller dark spot were identified and studied. This discovery of the dark spot in Neptune's northern hemisphere was monumental in that it was the first dark spot that the Hubble Telescope was able to document from birth. The storm is much smaller in comparison than the one discovered by NASA's Voyager 2, but was found to be larger in diameter than the Atlantic Ocean at approximately 4,600 miles across.
In August 2020, the new Great Dark Spot suddenly stopped its southward motion and reversed direction, contrary to projections that the storm would continue to the equator, where it would have met its likely demise. It is believed that the storms remain stable in the northern hemisphere due to the effect of Coriolis forces. However, as the storms moved towards the equator, the Coriolis forces weakens causing the storms to dissipated.
Around the same time, a smaller "Dark Spot Jr." was found near the larger storm, before disappearing later on. Dark Spot Jr. as the name suggests was smaller than the prior dark spot only measuring 3,900 is diameter. The coincidental appearance of this storm led astronomers to believe that the prior storm's reversal of motion may have been related to the birth of the smaller storm.
While the formation of the storms is still under investigation, it had been concluded from observations regarding the Southern Dark Spot (SDS-2015) and Northern Great Dark Spot (NGDS-2018) that their origins are preceded by an increase in cloud activity in the given region 2-3 years prior to becoming visible.
The storms from 1989-2018 have exhibited different movement patterns and are generally only visible for a few years. Furthermore, the disappearance of dark spots including the Southern Dark Spot can be linked companion clouds reaching the center of the storm and blocking the view of the blue wavelengths that are used to track the vortex, prior to their disappearance.
Proposed missions
Two mission ideas have been proposed to NASA to visit Neptune in the coming years.
Trident
A trident is a three- pronged spear. It is used for spear fishing and historically as a polearm.
The trident is the weapon of Poseidon, or Neptune, the God of the Sea in classical mythology. The trident may occasionally be held by other mari ...
was proposed in 2021 as a discovery mission to visit Neptune and its moon Triton in the year, but two missions to Venus (
DAVINCI+ and
VERITAS
Veritas is the name given to the Roman virtue of truthfulness, which was considered one of the main virtues any good Roman should possess. The Greek goddess of truth is Aletheia (Ancient Greek: ). The German philosopher Martin Heidegger argues ...
) were selected over it.
Neptune Odyssey
Neptune Odyssey is an orbiter mission concept to study Neptune and its moons, particularly Triton. The orbiter would enter into a retrograde orbit of Neptune to facilitate simultaneous study of Triton and would launch an atmospheric probe to ...
is a flagship orbiter mission concept with similar goals as Trident and is targeted to launch date of 2033. These missions have a high focus on learning more about Neptune's largest moon Triton, but also aim to gain more information about atmosphere of Neptune. New discoveries about the composition and evolution of Neptune's atmosphere, in turn could aid in explaining how the dark spots on the planet form.
See also
*
Dragon Storm (astronomy)
The Dragon Storm is a giant thunderstorm located in Saturn's Southern hemisphere, which is labeled as the "storm alley" region. The storm could have a range of or more, and can be compared to the electric thunderstorms of Earth. It acquires its e ...
*
Extraterrestrial cyclone
An extraterrestrial vortex is a vortex that occurs on planets and natural satellites other than Earth that have sufficient atmospheres. Most observed extraterrestrial vortices have been seen in large cyclones, or anticyclones. However, occasional d ...
*
Great Red Spot, a similar storm on Jupiter
*
Great White Spot
Saturn's Great White Spot in 2011.
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 from Earth by telescope by their character ...
, a similar storm on Saturn
*
Oval BA
*
Small Dark Spot
The Small Dark Spot, sometimes also called Dark Spot 2 or The Wizard's Eye, was an extraterrestrial vortex on the planet Neptune. It was the second largest southern cyclonic storm on the planet in 1989, when ''Voyager 2'' flew by the planet. Whe ...
*
Hypercane
A hypercane is a hypothetical class of extreme tropical cyclone that could form if sea surface temperatures reached approximately , which is warmer than the warmest ocean temperature ever recorded. Such an increase could be caused by a large ast ...
References
External links
Neptune's Great Dark Spot of 1989 ''Windows to the Universe''
''
Astronomy Picture of the Day
Astronomy Picture of the Day (APOD) is a website provided by NASA and Michigan Technological University (MTU). According to the website, "Each day a different image or photograph of our universe is featured, along with a brief explanation writt ...
'', May 8, 1996
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