열람 Great Red Spot
Great Red Spot
The Great Red Spot (GRS) is a persistent anticyclonic vortex on the south border of the South Equatorial belt. It appears to be a remarkably stable feature, and most sources concur that it has been continually observed for 300 years.[64]
The GRS rotates counterclockwise, with a period of about six Earth days[65] or 14 Jovian days. Its dimensions are 24–40,000 km west–to–east and 12–14,000 km south–to–north. The spot is large enough to contain two or three planets the size of Earth. At the start of 2004, the Great Red Spot had approximately half the longitudinal extent it had a century ago, when it was 40,000 km in diameter. At the present rate of reduction it would become circular by 2040, although this is unlikely because of the distortion effect of the neighboring jet streams.[66] It is not known how long the spot will last, or whether the change is a result of normal fluctuations.[67]
Approximate size comparison of Earth and the GRS
According to a study by scientists at the University of California, Berkeley, between 1996 and 2006 the spot lost 15 percent of its diameter along its major axis. Xylar Asay-Davis, who was on the team that conducted the study, noted that the spot is not in danger of disappearing because “[v]elocity is a more robust measurement because the clouds associated with the Red Spot are also strongly influenced by numerous other phenomena in the surrounding atmosphere.”[68]
Infrared data have long indicated that the Great Red Spot is colder (and thus, higher in altitude) than most of the other clouds on the planet;[69] the cloudtops of the GRS are about 8 km above the surrounding clouds. Furthermore, careful tracking of atmospheric features revealed the spot’s counterclockwise circulation as far back as 1966—observations dramatically confirmed by the first time-lapse movies from the Voyager flybys.[70] The spot is spatially confined by a modest eastward jet stream (prograde) to its south and a very strong westward (retrograde) one to its north.[71] Though winds around the edge of the spot peak at about 120 m/s (430 km/h), currents inside it seem stagnant, with little inflow or outflow.[72] The rotation period of the spot has decreased with time, perhaps as a direct result of its steady reduction in size.[73]
The Great Red Spot’s latitude has been stable for the duration of good observational records, typically varying by about a degree. Its longitude, however, is subject to constant variation.[74][75] Because Jupiter does not rotate uniformly at all latitudes, astronomers have defined three different systems for defining the latitude. System II is used for latitudes of more than 10°, and was originally based on the average rotation rate of the Great Red Spot of 9h 55m 42s.[76][77] Despite this, the spot has “lapped” the planet in System II at least 10 times since the early nineteenth century. Its drift rate has changed dramatically over the years and has been linked to the brightness of the South Equatorial Belt, and the presence or absence of a South Tropical Disturbance.[78]
It is not known exactly what causes the Great Red Spot’s reddish color. Theories supported by laboratory experiments suppose that the color may be caused by complex organic molecules, red phosphorus, or yet another sulfur compound. The GRS varies greatly in hue, from almost brick-red to pale salmon, or even white. The spot occasionally disappears from the visible spectrum, becoming evident only through the Red Spot Hollow, which is its niche in the South Equatorial Belt. The visibility of GRS is apparently coupled to the appearance of the SEB; when the belt is bright white, the spot tends to be dark, and when it is dark, the spot is usually light. The periods when the spot is dark or light occur at irregular intervals; as of 1997, during the preceding 50 years, the spot was darkest in the periods 1961–66, 1968–75, 1989–90, and 1992–93.[64]
The Great Red Spot should not be confused with the Great Dark Spot, a feature observed near the northern pole of Jupiter in 2000 by the Cassini–Huygens spacecraft.[79] Note that a feature in the atmosphere of Neptune was also called the Great Dark Spot. The latter feature was imaged by Voyager 2 in 1989, and may have been an atmospheric hole rather than a storm and it was no longer present in 1994 (although a similar spot had appeared farther to the north).[8