The Great Red Spot

The GRS is no doubt the most recognizable feature on the planet. It is embedded in the southern edge of the SEB and is nestled between wind jets to the north and south [23]. It is an anti-cyclonic storm, rotating in a counter clock-wise direction, not too dissimilar from hurricanes on Earth, only incomprehensibly larger (Fig. 3.23). According to Schmude, the longitudinal length of the GRS was 23,000 km during the 1992-1993 apparition. Since Earth is 12,756 km in diameter, almost two of them could fit inside the GRS! The GRS varies in color and shape with time. Sometimes, the GRS is a vibrant red against a cream-colored background [24]. At other times, such as is apparent during South Equatorial Disturbances, the spot seems to fade and blend into its surroundings [25]. Although the GRS can be quite dark, since 1997 it has been rather faint, with a color of salmon-pink or light orange reported by most observers instead of deep red. As a consequence, the GRS has been difficult to observe in recent years, especially by amateurs new to Jupiter observing. Of late the GRS has been in the shape of an oval, or ellipse. During some past apparitions, the preceding and following ends of the GRS were more pointed.

A dark, oval spot near the latitude of the present day GRS was first observed by J-D Cassini and others beginning in 1665 [26]. Observed for a long, long time, there is some conjecture as to whether or not this weather system has persisted for over 350 years [27]. Indeed the system currently identified as the GRS can only be tentatively traced back to circa 1830, 120 years after the last sighting of Cassini's spot [28]. In 1878, the present day GRS was observed to be a deep, brick-red color [29]. Since that time, the GRS has often been very faint or totally faded. The mostly cloud-free area around the GRS is traditionally called the GRS "hollow" [30], or simply Red Spot Hollow. The Red Spot Hollow (RSH) marks the position of the GRS when the GRS is faded. The Hollow protrudes distinctly into the southern edge of the SEB.

From apparition to apparition, the color of the GRS can change noticeably. In ale et fact, past changes in the STrZ and SEB have demonstrated that the GRS and its icncan surroundings can change on very short time scales of less than five months [31]. ¡X E CL

Historical notes about the physical appearance of the GRS and its surroundings hare also indicate variability of both very short and long time-scales [32]. In general, ^

the GRS is never "geranium" red, nor completely white [33], but no obvious peri- ® q, ^

odic color trend is evident. It is observed, however, that when the GRS "ingests" ThAo smaller white vortices, the northern portion or the so-called "eyelid" of the GRS does whiten [34]. Occasional brightening in the northern part of the spot, is often associated with the ingestion of smaller eddies that are sheared apart inside the storm or with interaction with the South Tropical Zone [35].

Fig. 3.23. Jupiter's Great Red Spot. Note the darker condensation in the center of the GRS and the overall salmon-orange color, surrounded by a slightly darker collar of material. The GRS is nestled into the Red Spot Hollow. A bright gap can be seen north of the GRS, between the GRS and the southern edge of the SEB. Bright turbulence can be seen in the wake following the GRS. South is up. (Credit: Ed Grafton).

During the 2000-2001 apparition, the GRS was faint, with a darker condensation in its very center (Fig. 3.23). This dark center was difficult to observe visually, but it was easily made out in CCD images. The GRS was bordered on its southern edge by a thin gray line. Sometimes, small white spots can be seen entering the GRS. When this happens we have an opportunity to monitor the circulation inside the GRS itself. According to Schmude, during the 2001-2002 apparition, the GRS was reported to have no color or a faint orange-pink color [36]. By the beginning of the 2003-2004 apparition, many observers were reporting a slight darkening of the GRS with a slightly more intense pink color. This appearance continued through 2006, with the GRS displaying a moderately intense orange-salmon color, and a darker condensation in its center.

In the past, small dark condensations have also been observed to circulate around the GRS, leading to the conclusion that the GRS is a vortex. These observations helped to identify the circulating currents inside the GRS. During the apparition of 1965-1966 a dark spot was observed on the north edge of the STB that approached the GRS in decreasing longitude. Upon reaching the GRS it continued to pass the GRS on the south side of the GRS. After reaching the preceding side of the GRS it did not pass it, but continued around the GRS to the northern side of the GRS and returned to the following end of the GRS. The spot was photographed by Elmer Reese and B. A. Smith of the New Mexico State University Observatory, and observed by them from December 1965 to January 1966. The spot circulated completely around the GRS in 9 days [37]. This observation was a significant contribution by amateurs. The Voyager spacecraft also acquired images of spots that allowed scientists to further study these currents. Dark spots were observed as recently as the 2001-2002 apparition [38].

To say we do not know everything we would like to know about the GRS is an understatement! We do not possess a complete understanding of the cause of many of the phenomena seen on the planet nor do we know with certainty when events will occur or reoccur. However, we do know that certain events seem to follow others. For example, the GRS alternates between periods when it is dark and faint. When SEB Fades occur, the GRS often darkens, only to fade again during an SEB Revival as the SEB is restored to its normal appearance. As this is written, a complete SEB Fading is long overdue on Jupiter. The observation of such an event is of great importance and some diligent amateur will probably be the first to note the start of such an event. When it happens, will the GRS darken as it has done in the past? And how much time will pass before the SEB Revival begins? Any inconsistency from previous patterns might indicate a change in Jupiter's weather patterns. Regardless, any new behavior would be of extreme importance to the professional community.

The size of the GRS has been changing over the years. According to SimonMiller et al. the latitudinal extent, or north-south distance, of the GRS has remained constant, with ground based observations from 1880 to 1970 indicating a latitudinal length of 11° ± 1°. This was consistent with Voyager data indicating a measured latitudinal extent of ~12,000 km [39]. However, the length of the GRS, longitudinally, has been diminishing over the years. In 1882, the GRS had a length of 34°. Until 1920, its length remained 30°, but since then it has never been as long again [40]. It has been known since the early 1900s [41] that the GRS is shrinking in apparent longitudinal extent.

If the latitudinal extent of the storm is thus assumed to be constant, the aspect ratio can be plotted for the older observations, indicating that the GRS is indeed becoming more round in appearance [42]. The system currently has an aspect ratio of about 1.6 and, obeying the historical trend, the GRS should become round by the year 2080, although that is not a stable configuration [43]. Simon-Miller, et al. found a longitudinal shrinkage rate for the GRS of -0.193° per year. At this present shrinkage rate, the GRS would be approximately round by the year 2040 [44].

The accuracy of any measurement of the length of the GRS is dependent upon the observer's ability to accurately identify the preceding and following edge of the system. This may seem straightforward but is not necessarily so. Poor seeing can undermine ones ability to see fine detail and transit timings under poor conditions can induce error into the timing. Likewise, measurements of longitude taken from CCD or webcam images can also suffer. Crisp, focused images of high resolution are needed to get accurate and reliable measurements. Sometimes the preceding and following edge of the GRS can be mistaken for the preceding and following edge of the Red Spot Hollow, even when measuring CCD or webcam images. Prior to the advent of CCD cameras and webcams, measurements of the GRS could only be made with the use of filar micrometers or central meridian transit timings, to be discussed later in this book. In spite of the inherent inaccuracies, multiple measurements by a large number of observers are desirable, and can average out the errors of measurement.

To illustrate the shrinking of the GRS over the years, we can review the historical record. The difficulty of measurement is illustrated by the results reported by various observers and researchers. All longitudes are given in System 2. Well known Jupiter observer Elmer J. Reese determined a length of 24.4° ± 1° during the 1962-1963 apparition [45]. During the 1968-1969 apparition Phillip Budine reported a length of 23° [46]. Giancarlo Favero et al.. reported 22.9° ± 0.5° in October 1975 [47]. Phillip Budine reported a length of 20° during the apparition of 1967-1968 [48]. A length of 23° was reported by Phillip Budine in 1983 [49]. A length of 19° was reported by Phillip Budine for the 1985-1986 apparition [50]. For the 1986-1987 apparition Budine determined a length of 22° early in the apparition, and 25° late in the apparition

[51]. Richard Schmude reported 23.6° ± 1° during February 1989 [52], while Lehman — q q et al. determined a mean length of 20° for the 1989-1990 apparition [53]. Schmude g C

determined a length of 25.5° ± 3° for the 1989-1990 apparition [54]. According to g J3

Lehman and McAnally, during the apparition of 1998-1999, the GRS presented itself Jj q ^

as a much smaller feature inside the RSH. The GRS proper was determined to be ^ <D

only 10° in length, while the RSH was 25° in length [55]. Using only high quality CCD V ^

and webcam images in which the preceding and following edge of the GRS could be ¡£ ^

accurately identified, I measured the length of the GRS to be 19° in November 1999, 17° in September 2000, 20° in September 2001, 17.5° in January 2002, and 17° in late January 2003. During the 2003-2004 apparition I determined the mean length of the GRS to be 17° in longitude. During 2006, I measured the length of the GRS to be 16°. Observations and measurements of the GRS by astronomers, including amateurs, will continue to be very important.

Sometimes the GRS presents a curious appearance, not quite filling the entire space occupied by the red spot hollow (RSH). This appearance has been noted during several recent apparitions. One such apparition, the 1998-1999 one, has already been mentioned above. In a CCD image taken by Donald Parker on September 04, 2000 a salmon-orange colored GRS was nestled in the southern two-thirds of a bright RSH. The bay of the RSH protruded significantly into the SEB, with bright material filling a large gap between the southern edge of the GRS and the northern edge of the SEB. The GRS and RSH again presented this kind of appearance in a CCD image taken by Maurizio Di Sciullo on January 12, 2002 in an image taken by Eric Ng on January 28, 2003 in an image taken by Tan Wei

Leong on February 23, 2003 an image by Cristian Fattinnanzi on April 02, 2004 and on webcam images taken by Parker on May 06, 2004 and 22. Sometimes the color in the GRS can appear to be concentrated into a small area inside the GRS itself, giving the GRS an even smaller appearance. This was the case on a CCD image taken by G. Kiss on November 18, 2001 when the color of the GRS was concentrated into an ellipse in the southern half of the GRS. This effect was again seen in a CCD image taken by Parker on January 21, 2002. In all the images cited above, an even darker concentration of color was seen in the center of the GRS, giving the appearance of a dark spot inside the GRS! This has been a common appearance in recent years. By April 2004, images by Fattinnanzi, Parker, and others revealed a GRS in which the color again filled the entire ellipse of the spot; however, the dark center remained.

The source of color for the GRS is not understood. However, it appears that Jupiter's visible color is contained in the tropospheric haze [56], which is an extended, colored, tropospheric haze denser than that seen on the rest of the planet, and reaching higher in altitude than most other locations on the planet [57]. Spacecraft missions have confirmed that the GRS has the vertical characteristic of a tilted pancake [58].

Much about the GRS is left unanswered, such as what created it, what maintains it, what colors it, and why there is no counterpart to it in Jupiter's northern hemisphere. And so, we do not know everything we would like to know about the GRS by a lot! One of my favorite endeavors is to keep a record of the changing appearance of the GRS. Certainly the GRS will continue to attract the attention of amateurs and professionals alike.

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