Neptune’s Great Dark Spot is a feature on Neptune’s surface that was discovered in 1989 when Voyager 2 spacecraft flew by Neptune on its mission to study other outer planets. When the Hubble Space Telescope observed Neptune in 1994, the giant dark spot had disappeared while other spots had appeared in the northern atmosphere.¹ The motivation behind the study of Neptune's Great Dark Spots is because it is a subject that is not well known in the astronomic world today due to being overshadowed by Jupiter’s Great Red Spot. Neptune and its spots are unique to outer space because of how it was discovered and keeps disappearing when further observed. The discovery of Neptune’s Great Dark Spot is a more recent discovery compared to Jupiter’s spots which motivates researchers to find more information on the reasons behind Neptune’s spots and why it is only visible through Hubble’s Space Telescope.

What is it?

Neptune's Dark Spot captured in 1989, accompanied by the Dark Spot 2 to its south. https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA00049 Credit: NASA

Neptune’s Great Dark Spot is a huge spinning storm about the size of the earth that has winds with speeds estimated up to 223 miles per hour, these winds are similar to the wind speeds on Jupiter.² Since the Voyager 2 Spacecraft did a flyby in 1989, observing the first two mysterious spots on Neptune, the Hubble Space Telescope has discovered an additional four spots, realizing the spots disappear after a certain amount of time with new spots arising. Observations have been made that the Great Dark Spots on Neptune do not last very long compared to the size of the planet. Neptune’s Great Dark Spots are still being further observed to answer the unknown questions about the discovery and disappearance of the spots. Neptune’s Great Dark Spot is a more recent discovery compared to Jupiter’s Great Red Spot that has been observed continuously since the 1800s. The arising question about Neptune and its Great Dark Spots is if these spots are actually disappearing or are the characteristics of them changing causing them to increase or decrease in size. 

Initial Discovery

The first discovery of the great dark spot was in 1989 when NASA's Voyager 2 spacecraft flew by Neptune. Since Neptune is about 4.5 billion km from the sun, it was expected that Neptune is dark, dormant, and frigid. Astronomers were shocked after seeing the Voyager images. Among the images that surprised them was an image of the great dark spot on Neptune.

Since Neptune is a gas giant planet, its atmosphere undergoes differential rotation. As for the wide equatorial zone, it rotates with a period of about 18 hours. On the other hand, the rotation period around the polar regions is 12 hours. This differential rotation results in strong latitudinal wind shear and violent storms. In 1989, three storms were spotted by the Voyager 2 space probe, and they were named based on their appearances.²

The Great Dark Spot was the first storm to be spotted. It was a massive anticyclonic storm measuring 13,000 x 6,600 km and resembling the Great Red Spot of Jupiter. Another storm was a white cloud group located farther south than the Great Dark Spot. It was called the Scooter. The second-most-intense storm observed during the 1989 encounter was the Small Dark Spot, a southern cyclonic storm. It was initially completely dark; but a bright core developed when Voyager 2 approached the planet and could be seen in most of the highest-resolution images.³

On Nov. 2, 1994, when the Hubble Space Telescope took sharp images of Neptune, the Great Dark Spot and the Small Dark Spot disappeared, and new dark spots had appeared in the northern atmosphere of Neptune. It was certainly a surprise compared to Jupiter's Great Red Spot, which presumably had been there for more than a hundred years.⁴

Simon and her colleagues discovered small, bright white clouds while they were analyzing Hubble images of a smaller dark spot that appeared in 2015. In the region of this small, bright white clouds appeared a new Great Dark Spot nearly identical in size and shape to the Great Dark Spot that appeared in 1989. "We were so busy tracking this smaller storm from 2015, that we weren't necessarily expecting to see another big one so soon," Simon said.⁴

Characteristics

Being one in a series of storms on Neptune, the Great Dark Spot (GDS-89) was the first sighting of said storms, which provoked a series of observations into Neptune's atmosphere and the workings of its climate. The other storms that circulated around Neptune were of similar interest to scientists: GDS-94, SDS (Small Dark Spot), The Scooter, etc. These storms seemed to echo one another in their general composition, but there were telling differences between a few, while others were more comparable to sister storms. 

The GDS-89 was an anticyclonic counter clockwise storm on Neptune’s southern hemisphere that was as large as the Earth itself, a result of Neptune's winds moving opposite to the rotation of the planet itself.⁵ These winds regularly move opposite to the planet's rotation, causing Neptune's extreme climate, however, the SDS was discovered to have a cyclonic clockwise direction and was indicative that these storms can vary from one another in a multitude of different ways.⁶ All dark spots are extreme storms, but not all dark spots are the same. The GDS-89 was 13,000 x 6,600 kilometers in size, with brighter clouds that float above the darker areas of the great dark spots.7 It had a wavelength of 480 nm and a rotation period of 18.3 hours and had a bright companion near the equator that kept its placement for nearly a month.¹ A thin veil fills the interior of the GDS-89 as well. The winds that categorize these storms go at about approximately 328 feet a second, this being a key factor in why the great dark spots of Neptune are so fascinating: Neptune has the fastest winds on any planet in our system despite Neptune's energy being so small in comparison.⁸

Changes in Neptune's Great Dark Spot, taken in 1996
https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA00047 Credit: NASA

High altitude, infrared-bright clouds are composed of methane ice crystals and seemingly are companions to these dark spot storms, floating above them near the tropopause while the dark spots are thought to occur in the troposphere at low altitudes. Meanwhile, these darker spots are thought to be a hole in the cloud deck of Neptune.⁹ Computer based models of Neptune's atmosphere have yielded more important information about Neptune's storms, showing the deeper that the storm is, the brighter these high-altitude clouds are.¹⁰

The GDS-94 differs slightly from the GDS-89, however, it gave scientists further information on these great dark spots and their composition. Founded on August 24th, 1994, an indication of strong winds was present due to bands that drifted around the spot, meanwhile in the inner parts of the spot there seemed to be structures that, to the scientists, showed equal indications of a central rotation.¹¹ Compared to the GDS-89, GDS-94 had a rotation period of 18.4 hours with a wavelength of 467 nm.¹

There was another important comparison that was made to further the understanding of Neptune's storms. A difference was apparent between the GRS (Great Red Spot) of Jupiter and the GDS-89 of Neptune: The period of the GRS matches the planet's radio rotation rate of approximately 4 m/s westward, while the GDS-89 had a period of 18.3 hours westward compared to the interior period of 16.11 hours.¹² This communicates to us the possibility of the GDS-89 being deep while the GRS is much shallower. Further studies by Simon, Wong, and Hsu of Neptune's atmosphere also indicates that these high-altitude clouds appear two years before the great dark spot's formation and that when these great dark spots do make themselves known, the brightness of these clouds lower to some degree points to the idea that these dark spots may be deeper in the atmosphere.¹³