Jupiter's Great Red Spot

The largest and longest-lived known weather system in the solar system, with horizontal dimensions comparable to the diameter of Earth and monitored behavior that has spanned centuries.

Overview

Excluding Jupiter’s general east-west belt and zone pattern, the Great Red Spot is the most obvious, persistent, and continuously observed feature of Jupiter’s visible cloud deck. Centered at about 20° south latitude, it spans about 12,000 kilometers in the north-south direction and about 20,000 kilometers in the east-west direction. Compared with the diameter of Earth, the Red Spot is a huge cloud structure large enough to span two Earths.

This well-defined oval feature has raised considerable curiosity ever since its discovery, which many credit to original observations made by Giovanni Cassini or Robert Hooke in the late seventeenth century. Coordinated reports by the British Astronomical Society and original drawings maintained in the Royal Astronomical Society library collections in London definitively established that the Red Spot has been observed since at least 1830. Earlier scattered reports of pink spots in the atmosphere of Jupiter extend back several centuries to the era of the earliest efforts to improve the resolution of simple telescopes.

Realizing that they were seeing an opaque cloud deck and that the surface of Jupiter was never visible, some observers suggested that the Red Spot was caused by interference between an elevated surface feature and the prevailing winds. In the early 1960s, Raymond Hide proposed a model for the driving mechanism. If this model had withstood scrutiny, it would have permitted scientists to calculate the planet’s surface rotation rate and to interpret other cloud motions by extrapolating from this rate. Careful examination of measured periods of rotation indicated, however, that Hide’s model was inconsistent with available data. The Red Spot circles Jupiter at an almost unvarying speed for a period of twenty to fifty years. At the end of each period, it suffers an acceleration or deceleration, which occurs over a period of weeks. After this adjustment period, the Red Spot continues to circle at its new speed. This behavior indicates that it cannot be the result of an upwardly propagating disturbance above an elevated region on a hidden surface. Measured positions indicate that there is no rate at which the interior of the planet could rotate that would not force the Red Spot to drift freely either east or west within the atmosphere, yet historical observations indicate that the spot is trapped in the prevailing east-west wind pattern and is not free to move north or south like weather systems in Earth’s midlatitudinal regions.

Along the southern edge of Jupiter’s equatorial zone, winds blow eastward at 150 meters per second. West-to-east zonal winds decrease poleward until at 17.5° south latitude, they are moving westward at 70 meters per second. From 17.5 to 24.5° latitude, winds increase eastward to a maximum of nearly 60 meters per second. From 24.5° to 50°, winds alternate eastward and westward. This alternating east-west wind pattern, with four cycles between the equator and 50° south latitude, generates significant latitudinal wind shear. If local heating occurs below the cloud deck, causing the atmosphere to rise and clouds to form, maintaining a long-lived cloud system in the presence of strong horizontal shear would require the cloud to rotate about its center. If the cloud rotates in the same sense as the local horizontal shear, it can deflect the prevailing winds about its perimeter. The Red Spot displays this behavior. Not only is it trapped between westward winds at 17.5° and eastward winds at 24.5°, but it also deflects westward wind flow around its equatorward perimeter, creating a large indentation, or hollow, in the poleward side of the dark adjacent belt. Other, smaller oval cloud systems are associated with the more poleward wind-shear regions. Three white oval cloud systems, noted in 1938, are located near 29° south latitude. The east-west dimension of each of these systems is about 12,000 kilometers. A series of smaller ovals circle the planet near 37° south latitude. Morphologically, the Red Spot is not unique. However, it is the largest example of a type of cloud system common to the southern hemisphere of the planet.

The Red Spot is notable not only in size but also in coloration. Jupiter’s other oval clouds are white, indicating that their cloud decks are composed of highly reflective ammonia ices. When visible red and infrared reflection from the Red Spot is analyzed, data indicate ammonia ice is present there as well. The Red Spot has additional trace constituents in its cloud deck that are strong absorbers of ultraviolet, violet, and blue wavelengths. Small, short-lived ovals that form at similar latitudes in the northern hemisphere also absorb ultraviolet and blue light. This suggests that these absorbers are carried upward from below. In addition, the rate of vertical motion or the depth to which the convective motion reaches permits transport not present at the top of the cloud deck in storms located at more poleward latitudes.

A trip to a mountaintop on Earth’s surface makes it clear that lower elevations of Earth’s atmosphere are compressed. Jupiter’s atmosphere must behave similarly. For the Red Spot to behave as an isolated system, its vertical dimension must be small in relation to its horizontal extent. Comparisons of the Red Spot with a hurricane are inappropriate. The Red Spot is a giant rotating cloud system, trapped in the prevailing winds. Reflectivity and degree of redness vary with time; still, the deflection of the westward jet around the equatorward side of the Red Spot is always visible.

In 1878, the Red Spot underwent a deceleration. The surrounding cloud deck became highly reflective and white; however, the Red Spot remained dark and red. This sharp contrast made many casual observers aware of the phenomenon. In 1901, a disturbance occurred in the South Tropical Zone, the white band south of the Red Spot. This event appeared to be a major weather disturbance that moved eastward and caught up with and then passed the Red Spot, thereby accelerating the Spot. This continued until 1938. Then, the belt just south of the South Tropical Zone underwent a major disruption, resulting in greatly increased reflectivity of the belt and the formation of three white ovals. In the early 1930s, the Red Spot drifted at a rate similar to that seen prior to 1878. After the formation of these ovals, the Red Spot decelerated to its slowest drift rate ever observed. Since 1962, the Red Spot has been drifting at a rate similar to that of the 1878–1901 period.

This constitutes evidence that the Red Spot interacts with its surroundings and that variations in local temperature, pressure, and wind patterns occur. Even so, the entire range of variation in average Red Spot motion, with the average velocity derived from the annual longitudinal displacement of the Spot relative to the rotation rate of radio noise, lies between –4.4 and –0.6 meters per second. Although this variation is small when compared to daily wind speeds at midlatitudes on Earth, an annual increase of 2 meters per second in wind speed results in an eastward displacement of about 63,000 kilometers, or about two and a half times the Spot’s length.

That the Spot’s recovery from a given acceleration or deceleration takes years is expected. A body’s heat loss rate depends heavily on the relative temperatures of the body and its surroundings. The Jovian cloud deck temperature is approximately 153 kelvins. Thus, the rate of heat loss to black sky is relatively slow. It is logical that once an excess amount of heat has been inserted into the atmosphere, it will be several years before the atmosphere returns to its previous state. One basic question that atmospheric scientists wanted to answer concerned the nature of acceleration mechanisms. Ground-based observations indicated that these events occurred over short time intervals.

During the 1960s and early 1970s, Elmer Reese made many detailed measurements using photographs of Jupiter. One result of this work was a measurement of Red Spot rotation. The Red Spot rotates counterclockwise, completing one rotation every six days. A feature in Earth’s atmosphere with this behavior would be air rising in the center, flowing outward at the cloud top, and descending around the perimeter. Measurement of divergent flow was one goal of the two Voyager spacecraft. Superimposed on the drift is an oscillatory motion of the whole feature, speeding up and slowing down so that its velocity oscillates every ninety days, causing the Spot to shift back and forth about 900 kilometers relative to its average path. This behavior apparently results from some natural period of response of the system to its surroundings.

Because the Red Spot had already been subjected to much scrutiny, planetary scientists eagerly looked forward to obtaining high-resolution data from Pioneer and Voyager flybys in the 1970s and subsequently from the Galileo orbiter between December 1995 and September 2003. The Galileo spacecraft’s science mission centered on investigations of the planet’s particles and fields, various satellites, and general atmospheric dynamics; the latter included an atmospheric probe, but that heavily instrumented payload was not flown into the Red Spot. Additional high-resolution images of the Red Spot were obtained from the Earth-orbiting Hubble Space Telescope and during flybys of probes passing through the Jovian system to gain a gravity asset from Jupiter. Those flybys provided opportunities for controllers to test scientific packages on those spacecraft. The Cassini orbiter in December 2000 on its way to the Saturn system, obtained images of the Red Spot superior to those from the Voyager spacecraft, even though its closest approach to Jupiter was considerably farther out. New Horizons in February 2007, on its way to the Pluto-Charon system, obtained high-resolution images of Jupiter. It also verified that observations from ground-based telescopes and the Hubble Space Telescope had indeed recorded a lightening phase of the Red Spot beginning in 2006. In 2021, NASA’s Juno spacecraft provided scientists with three-dimensional images of Jupiter’s Great Red Spot, which showed the Spot to be far deeper than expected. Further, the James T. Webb Space Telescope has also provided valuable images of Jupiter’s Great Red Spot. In addition, New Horizons studied a relatively new storm farther south of the Great Red Spot, one referred to as the Little Red Spot, a much smaller version of its long-lived sibling.

The Little Red Spot started as one of three white storms that formed in the 1940s. Two of those merged in 1998, and the resulting white spot merged with the third in 2000. This feature then demonstrated a trend toward reddening in late 2005, and after 2006, it was referred to as the Little Red Spot. This storm continued to grow in wind speed and in reddish hue, providing a marvelous opportunity for in-depth study by the contemporary technology of the passing New Horizons spacecraft.

Knowledge Gained

Both Pioneerspacecraft were spin-stabilized so that the planet swept past their instruments’ fields of view. This design feature placed strong constraints on the type of instrumentation that could be implemented. The imaging experiment utilized a scanning camera that sampled one point on the planet at a time. Images were constructed by scanning the planet row by row as it passed the field of view. This method limited the ultimate resolution of the data and severely curtailed the number of images that could be recorded. Nevertheless, Pioneer data were highly useful to astronomers. Tom Gehrels and his team observed Jupiter at a time when the belt adjacent to the Red Spot was highly reflective and white. The Red Spot was quite dark. Comparison with descriptions of the Red Spot in 1878 indicated that the reflectivity of the Spot and its surroundings at the time of both Pioneer encounters was highly similar to its condition approximately a century earlier.

That Red Spot behavior continued until July 1975, when a bright white cloud appeared west of the Spot that expanded rapidly and sheared out in the zonal winds. Considerable turbulence accompanied this event, and within a few weeks, the Red Spot and belt had changed significantly. Material from the disturbance encountered the Red Spot from the west and formed a large white mass of clouds to the west of the Spot. Turbulent cloud masses also spilled into the westward wind jet along the south side of the belt. This material was carried around the planet and approached the Red Spot from the east side. The contrast of the Red Spot decreased as it became whiter. Historically, Red Spot lightening has been fairly common. It appears that increased turbulence and vertical mixing in the belt that lies to the equatorward side of the Red Spot carry ammonia ices into the Spot. The Spot retained this appearance from 1975 to 1987 and was observed at high resolution in this condition by the Voyager spacecraft.

Voyagers1 and 2 arrived at Jupiter in March and July of 1979. The two spacecraft were equipped with two television cameras each; one had a wide field of view, and the other focused on a smaller field with higher resolution. The two were boresighted; thus, simultaneous views allowed detailed sampling while defining the direction that the cameras were pointing. Ultraviolet and infrared spectrometers and a photopolarimeter allowed observations as a function of wavelength. Red Spot images with higher spatial resolution than could be obtained from Earth were taken over a period of three months with each spacecraft. The Galileo probe began collecting visible light and near-infrared images in 1996. The Hubble Space Telescope, Cassini orbiter, and New Horizons spacecraft also supplied data. In the twenty-first century, the Webb telescope and the Juno orbiter have added to that data.

Ultimately, high-resolution sequences requiring as many as twenty-seven narrow-angle camera frames to map the Red Spot were executed. The resulting data revealed details of the flow pattern around the Red Spot. Winds were deflected around the Spot; small ammonia ice clouds, however, were observed to pass around the equatorward edge and to continue around the western cusp and along the feature’s southern edge. When these clouds reached the Spot’s southeast corner, they moved into the Red Spot and were sheared apart to form a high-velocity collar inside the Red Spot. Jim Mitchell, Reta Beebe, Andrew Ingersoll, and others analyzed the flow within the Red Spot and the white ovals. Velocities of rotation about the Spot’s center as high as 150 meters per second were measured in the outer third of the feature. In the inner half of the Spot, reflectivity was lower, and the motion of the cloud deck was small and random. No outward flow from the center toward the perimeter of the Spot was detected. When infrared data from the Red Spot and one of the white ovals were compared, no difference in absorption as a function of color could be detected; thus, the infrared data offered no clues to the identity of the ultraviolet absorber. This finding was not unexpected because it was known that the ammonia ice would tend to dominate in the infrared.

Context

High-resolution spacecraft imagery has been combined with long-term, lower-resolution ground-based photography in an effort to understand the Red Spot’s nature. The apparent motion of planetary atmospheric features can be attributed to mass motion when material is physically translated in the zonal wind or to wave motion. In the case of wave motion, variations in local pressure and temperature introduced by the wave cause local condensation or evaporation. Many of the small-scale patterns that add beauty to Earth’s water clouds are of the second type. Thus, in an effort to elucidate the Red Spot’s nature, models that consider different types of wave structures have been constructed. Not all wave structures are a series of oscillations with equal amplitudes traveling through space. By varying modeled environmental conditions within which the wave is formed, various researchers, including Tony Maxworthy, Andrew Ingersoll, and Gareth Williams, have investigated the characteristics of waves and related them to the morphology of the Red Spot.

To construct a realistic model of the Red Spot, information concerning the manner in which the zonal winds change with depth as well as with latitude is necessary. Because all required parameters are not available, a series of models must be constructed. Peter Read and others have attempted to shed light on atmospheric flow around the Red Spot by constructing cylindrical tanks of rotating fluids, within which they generate closed eddies that have characteristics similar to those of the Red Spot. High-resolution spacecraft data have provided astronomers with a wealth of information.

It is not clear why the well-formed oval clouds in Jupiter’s atmosphere preferentially form in the southern hemisphere. The fact that they are very long-lived is, however, consistent with their being large, closed eddies rotating in the local wind shear. Little is known concerning the rate of vertical motion associated with these features or the depth to which they extend below the cloud deck. Interplanetary spacecraft will continue to provide high-resolution data for researchers struggling to define the nature of Jupiter’s Great Red Spot.

The variability of Jupiter’s atmosphere, as well as its ability to sustain prolonged features, was illustrated when a number of smaller red spots broke out beginning in early 2006. The first small spot continued into 2008, when in May, a third spot appeared, this one located in the southern hemisphere farther south in longitude than the Great Red Spot. Both little red spots moved in a way that led scientists to expect them to merge. However, the influence of the Great Red Spot held the potential to push both little red spots to the side. Coordinated observations of these small storms from the Hubble Space Telescope and ground-based telescopes in visible and near-infrared light suggested that these little red spots started as white storms and then gradually assumed a reddish color.

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