Great Dark Spot

• 1. Introduction

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  The Great Dark Spot appeared on Neptune in 1989, and for a brief window in time, it was the most violent weather system in the entire Solar System. Winds racing around its edges reached 2,100 kilometers per hour, nearly 1,300 miles per hour, the fastest ever recorded anywhere in the Solar System. The storm was roughly the size of Earth itself. Then, when astronomers pointed the Hubble Space Telescope at Neptune in November 1994 to photograph it again, it was simply gone. What was this churning void in Neptune's clouds? Where did it go? And what has Hubble found prowling Neptune's atmosphere in the decades since?

• 2. A Hole in the Clouds

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  NASA's Voyager 2 captured the Great Dark Spot in Neptune's southern hemisphere as the probe swept through the outer Solar System in 1989. The spot earned the designation GDS-89, and its initial dimensions made it roughly comparable in scale to Earth. Scientists believe the dark feature is not a solid storm system in the way most people picture one, but rather a hole punched through Neptune's methane cloud deck, exposing deeper atmospheric layers below.

  Neptune's dark spots form in the troposphere, at lower altitudes than the brighter, upper cloud features that sit above them. This places them in a different atmospheric tier entirely from the white, wispy structures sometimes seen nearby. Because dark spots can persist for several months, researchers classify them as vortex structures, stable rotating systems anchored in the deeper atmosphere.

  Unlike Jupiter's Great Red Spot, which holds its latitudinal position because east-west wind currents pin it in place, the Great Dark Spot showed the ability to shift north and south over time. This difference in behavior points to something fundamental about the way Neptune's atmosphere is organized compared to Jupiter's. The Great Red Spot has lasted for hundreds of years; Neptune's dark spots, by contrast, appear to form and dissipate over periods of just a few years.

• 3. Frozen Methane Skies

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  Around the edges and just above the Great Dark Spot, large white clouds formed near the tropopause layer, the boundary between two atmospheric zones. These clouds visually resemble high-altitude cirrus clouds on Earth, but the chemistry is completely different. On Earth, cirrus clouds are made of crystals of water ice. Neptune's companion clouds are made of crystals of frozen methane.

  Those companion clouds sit somewhere between 50 and 100 kilometers above the main cloud deck, suspended far above the dark vortex beneath them. Ordinary cirrus clouds on Earth typically form and disperse within a few hours. The clouds linked to the Great Dark Spot were still present after 36 hours, the equivalent of two full rotations of Neptune. That persistence was striking and suggested the vortex below was powerful enough to sustain them well beyond a typical cloud's lifespan.

  The relationship between companion clouds and the dark spots beneath them would later become a clue investigators used to track storm behavior. When a dark spot disappears from view, the companion clouds can sometimes linger, leading researchers to think that the vortex may still be active as a cyclone even after the dark feature is no longer visible. The clouds, in other words, sometimes outlast the visible signature of the storm that created them.

• 4. Disappearance and Discovery

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  In November 1994, the Hubble Space Telescope turned toward Neptune expecting to photograph the Great Dark Spot, and found nothing. The spot had disappeared completely, leaving astronomers uncertain whether it had dissipated entirely or had simply been covered by other cloud features. From 1994 onward, Hubble became the only operating facility capable of detecting dark spots on Neptune, because the telescope can observe the planet at blue wavelengths, the only wavelengths at which these features become visible.

  Dark Spot 2, a separate storm spotted by Voyager 2 in the same 1989 encounter, had already fully dissipated before 1994. A Northern Dark Spot designated NGDS-1994 appeared in Neptune's northern hemisphere in 1994 and vanished sometime between 1998 and 2000. Another northern storm, NGDS-1996, formed in 1996 and disappeared before 1998, exhibiting little to no drift in latitude during its brief existence.

  In 2015, the Hubble Outer Planet Atmosphere Legacy program, known as OPAL, found a Southern Dark Spot. That storm showed a poleward drift before disappearing in 2017. A year later, in 2016, a spot in Neptune's northern hemisphere closely resembling the original Great Dark Spot appeared; it has been called the Northern Great Dark Spot and remained visible for several years, though whether it still persists is uncertain because Hubble observations of Neptune are limited.

• 5. The Reversing Storm

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  In 2018, Hubble documented a new dark spot in Neptune's northern hemisphere in a way that had never been achieved before: it observed the storm from birth, the first time any telescope had captured a Neptune dark spot from its very beginning. The storm was smaller than the one Voyager 2 found in 1989, but at approximately 4,600 miles across, it was still wider than the Atlantic Ocean.

  Projections suggested the storm would drift toward the equator and likely dissipate there, because as storms move toward the equatorial zone, the Coriolis forces that stabilize them weaken. In August 2020, however, the storm suddenly stopped moving southward and reversed direction, defying those projections. Astronomers noted that around the same time, a smaller companion storm appeared nearby. This smaller storm, informally called Dark Spot Jr., measured approximately 3,900 miles in diameter. Its coincidental arrival led researchers to suspect that the reversal of the larger storm may have been connected to the birth of the smaller one.

  Dark Spot Jr. subsequently disappeared, and the dynamics between the two storms raised new questions about how these vortices interact. Observations of the Southern Dark Spot from 2015 and the Northern Great Dark Spot from 2018 have led to one concrete finding: both storms were preceded by a noticeable increase in cloud activity in their respective regions, occurring roughly two to three years before the dark spot itself became visible. That early cloud surge may be the earliest warning sign that a storm is forming.

• 6. Proposed Missions

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  Two mission concepts aimed at Neptune have been proposed to NASA in recent years. Trident was put forward in 2021 as a discovery-class mission to visit Neptune and its moon Triton, but NASA selected two Venus missions, DAVINCI and VERITAS, instead. Neptune Odyssey is a flagship-level orbiter concept with similar scientific goals and carries a targeted launch date of 2033.

  Both missions place a high priority on learning more about Triton, Neptune's largest moon, while also aiming to gather detailed atmospheric data about Neptune itself. The China National Space Administration has separately published an analysis of a nuclear-electric propulsion mission to Neptune, indicating that interest in reaching the planet extends beyond a single space agency.

  Given that Hubble remains the only instrument currently capable of monitoring Neptune's dark spots, and that its view is limited, a dedicated orbiter would fundamentally change what researchers can learn about these storms. The origins, movement, and disappearance of Neptune's dark spots remain poorly understood, even after more than three decades of observation since 1989.