Neptune's Great Dark Spot

An elliptical atmospheric feature in Neptune’s southern hemisphere—one large enough to contain Earth—was named the Great Dark Spot in 1989. It was interpreted as an anticyclonic storm. This dark spot eventually disappeared, but others subsequently appeared in the high northern latitudes. Given our understanding of terrestrial storms as driven by solar heating and that Uranus, which is larger and closer to the Sun, did not show such activity until the early twenty-first century, such phenomena on distant Neptune were very surprising.

Overview

Images taken through filters that permit the passage of light at wavelengths of 467 nanometers were taken by the Voyager 2spacecraft in 1989 at a distance of 2.8 million kilometers. Those images revealed an elliptic feature darker in albedo by about 10 percent. Covering between thirty and forty-five degrees of longitude and eight to seventeen degrees of latitude, the Great Dark Spot (GDS) was about the size of Earth. It was initially positioned at twenty-seven degrees south latitude, drifting toward the equator at about 1.2 degrees per month. Given the nomenclature GDS89, the Great Dark Spot was tracked from twenty-seven to seventeen degrees south latitude over eight months and interpreted as a depression in Neptune’s atmosphere, indicating a strong vortex such as a hurricane. GDS89 had a companion bright cloud band on one edge. This surprised astronomers after receiving featureless images of Uranus, a planet larger and closer to the Sun, during the Voyager 2 flyby in January 1986.

To understand the concept of the Great Dark Spot, it is helpful to understand the vortex dynamics of storms. When gas heats up in the lower atmosphere and becomes less dense, it forms a “sinklike” flow, moving inward and then rising. The plume starts rotating, since the conservation of angular momentum amplifies any difference in tangential speed as the radius decreases. In hurricanes that cover many degrees of latitude on a planet, the direction of rotation is predictable because of the Coriolis effect, whereby wind velocity interacts with the planet’s rotation. The core remains clear of clouds and has rising warm air inside, while clouds revolve fastest around its periphery. As the plume reaches levels with lower density, the flow spreads out, and the rotation slows outward. The warm, moist air (above Earth) condenses into clouds. Outside the core, cold air sinks. Thus, from observed cloud-top movement and temperature gradients, the strength of the storm and its axial upflow can be determined, along with the density differences and energy input that drive the storm. Prevailing regional winds and the Coriolis effect drive the storm across longitudes and toward the equator.

Near GDS89, wind speeds up to 2,400 kilometers per hour were recorded, but prevailing regional winds were on the order of 900 to 1,500 kilometers per hour. In addition to its drift, GDS89 showed oscillations of about fourteen-degree amplitude about the horizontal, with an eight-day period. Its aspect ratio fluctuated between 0.35 and 0.55, the longitudinal size varying between thirty and forty-five degrees, and the latitudinal extent between twelve and seventeen degrees. Over a 225-day period, GDS89 became increasingly circular. It also showed “tadpole-like tails,” dark features off each of the smaller sides. A 1990 paper reported a dynamic model for such oscillations based on a single isolated vortex embedded in a shearing flow and derived lower limits for the Rossby radius of deformation relating Coriolis forces and buoyancy forces.

In 1994, the Hubble Space Telescope (HST) observed that GDS89 had disappeared, but other dark spots had appeared in the northern hemisphere. The Northern Great Dark Spot (NGDS32), a very stable dark spot, stayed near thirty-two degrees north from around 1994 to 1996 and perhaps until 2000. Another dark spot, NGDS15, stayed near fifteen degrees north from March 1996 to June 1997. NGDS32 drifted across longitudes very steadily at about thirty-six degrees per day. The drift rate of NGDS15, being much closer to the pole, proved harder to estimate. Bright methane clouds were associated with specific latitudes in any given period, but the active latitudes changed from negative twenty-five degrees and negative thirty degrees in 1989 to negative thirty degrees and negative forty-six degrees from 1994 to 1996. Astronomers Heidi Hammel and G. Wesley Lockwood noted that GDS appeared in images where the dominant radiation was at 467 nanometers (blue), close to the brightest spots in the red (619 nanometers) and infrared (889 nanometers) existing on the planet at the time.

More detailed three-dimensional vortex simulations in 1998 provided explanations for the appearance of overlapping ellipses and correlated the drift rate of the spots with the prevailing wind speeds in the region. They predicted the breakup of these anticyclones near the equator, with Rossby waves propagating out over a few weeks. A 2001 paper in Icarus by P. W. Stratman et al. used simulations of storms, along with measurements of the bright accompanying clouds of the GDS, to estimate the atmospheric level where the top of the GDS should occur. If the top were in the stratosphere, the GDS would drift too quickly toward the equator and disperse. If the top were deep inside the troposphere, the clouds would be much larger than what was seen. Hence, the top of a GDS should be near the tropopause. Based on this result, the pressure drop along a streamline threaded through a companion cloud was on the order of three millibars, and the temperature change was on the order of one kelvin, indicating a lifting on the order of half a kilometer and relative wind speeds between the dark spot and the surrounding winds of forty-five meters per second eastward.

However, a 2002 paper in Icarus by L. A. Sromovsky et al. showed that the steady latitudes maintained by NGDS32 and NGDS15 are not consistent with the model of anticyclonic storms, which should have moved strongly across latitudes. Computational fluid dynamic simulations by R. P. LeBeau and colleagues in 2006 and 2007 captured shape and oscillation phenomena similar to those of GDS89, but the size and oscillation amplitudes were off by as much as a factor of two. They calculated the shear profile in the background winds of Neptune that would be needed to explain the slow drift toward the equator of GDS89, but the much slower rates of the northern dark spots remain a challenge to explain.

In comparison, for more than three hundred years, the Great Red Spot (GRS) of Jupiter has circled that planet along a southern latitude with minimal longitudinal drift. Attempts to explain the GRS as the flow around a high solid surface peak have been abandoned and it is considered to be a shallow cloud system trapped between shearing layers of horizontal winds. The redness relative to the surrounding white ammonia clouds indicates some temperature difference. Efforts to model the GRS as a vortical upwelling of fluid from below have met with limited success. It is not certain that the dark spots of Neptune are shallow structures or that they extend to the cloud tops rather than being features lying below clear atmospheric regions.

In 2006, the Hubble Space Telescope detected a dark spot more than one thousand kilometers in extent, at twenty-seven degrees south latitude on Uranus. This observation was made as that planet began experiencing increased atmospheric activity with the coming of summer in its eighty-four-year orbit around the Sun. Scientists believe that Uranus is not as bland as Voyager 2’s images suggest. Instead, as the amount of solar radiation that the planet intercepts increases, the planet may develop features seen in the atmospheres of the other gas giant planets.

Knowledge Gained

What is known about the Great Dark Spots comes from images taken by the Voyager 2 spacecraft in 1989, by the Hubble Space Telescope from 1994 to 2000, and by ground-based optical and radio telescopes, including the Keck telescope, the Mauna Kea observatory, and the Very Large Array radio telescope in New Mexico. These are all passive observations, and the investigations for which they provide evidence depend on analyzing images taken with various filters that show specific wavelengths. Given the great distance, even the Hubble observations do not begin to approach the resolution achieved with Voyager 2’s 1970s-vintage cameras. The narrow-angle camera on Voyager 2 was the instrument used to capture cloud-top images, which were then used to calculate wind speeds at that level.

Radio telescopes capture signals that should indicate rotation of the magnetic field and the internal structure of the planet and hence give rotation rates based on those factors. With a planet composed mostly of fluid, there can be large differences between these rotation rates, which remain unexplained but are attributed to extremely high winds, which imply large frictional losses that require high energy input. The structure of the dark spots is derived mostly from simulations of fluid mechanics based on what is known of terrestrial storm systems and the limited data from these planets.

Researchers continue to model the dark spots using the fluid mechanics of hurricanes. Their speed of travel around the planet appears to match what is known or assumed of local wind speeds. However, their slowness in crossing latitudes requires fortuitous combinations of wind profiles to explain it. Their shorter persistence compared to the Great Red Spot of Jupiter appears consistent with the much higher “wind speed” and the shear between different zones prevailing on Neptune. The spectral contrast suggests the presence of different gases and different temperatures in the center of a spot, suggesting strong vertical motions of gas from the warmer depths of Neptune. The absence of features inside the dark spots, unlike the clouds seen above the GRS, frustrates efforts to derive their interior structure and vorticity directly.

Context

The driving engine for Neptune’s dark spots remains puzzling since the small changes in solar intensity associated with seasonal changes do not provide sufficient differences to explain these mysterious phenomena. If such strong weather activity can occur with such low intensity of sunlight, clearly much remains to be learned about why the Earth’s weather behaves as it does. Observations of summertime on Uranus may explain some of the mysteries, but Neptune’s great dark spots are unlikely to yield their secrets until spacecraft descend through the Neptunian atmosphere to probe the planet’s winds. New discoveries from such missions could greatly improve our ability to predict the course and evolution of killer storms on Earth.

In 2002 a study was convened to investigate the possibility of a Neptune orbiter probe, but the concept was not funded and was hampered by the need for nuclear propulsion technology, still to be developed at the time. The route of the New Horizons spacecraft included passing through the orbit of Neptune on its way to Pluto and the Kuiper belt (it arrived near Pluto in July 2015). However, the nuclear-powered spacecraft was only scheduled to take pictures of the planet after it has passed through its orbit and, therefore, from “behind.”

In 2020, the Hubble Space Telescope captured images of dark spots believed to be a piece of a giant storm on the planet. The James E. Webb Space Telescope, launched in 2021, has provided promise in better understanding Neptune by providing scientists with the clearest pictures of the planet in decades. Continuing study of Neptune and its atmospheric features will also be conducted using the Hubble Space Telescope and ground-based observatories like the Keck telescope.

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