The not-so-great red spot: after shrinking in size for decades, is Jupiter's iconic Great Red Spot facing an identity crisis?
Generations of amateur and professional astronomers have viewed and studied Jupiter's Great Red Spot (GRS), one of the giant planet's most easily recognized features. It joins Saturn's rings and the Martian polar caps as "must-see" features for solar system enthusiasts. But the GRS is an inconstant icon. In fact, in recent years observers have become increasingly alarmed about how small (relatively speaking) the spot has become. Visually it's far less impressive than it was two or three decades ago.So planetary scientists have begun to ask whether this huge cloud system is really shrinking and, if so, might it someday disappear completely? The answer to the first question is unequivocally "yes," but the second one remains a mystery. Given that Jupiter has other small, enduring, red-hued ovals, perhaps the real question should be whether the GRS no longer qualifies as a "great" red spot.
Let's examine the history of this iconic storm and what we do--and don't--know about its evolution.
GIGANTIC STORM Arguably no planetary feature in the solar system is better known than Jupiter's Great Red Spot, or GRS, seen here in a contrast-enhanced composite of Voyager images from 1979. Note the vast regions of disturbed cloud flow to its north and south, along with the Earth-size white oval (designated BC) gliding nearby.
The Past
Historical records reveal that observers regularly measured the GRS from the 1870s onward. It became a popular target for study starting about 1878, in part because it was extremely red and prominent at the time. The earliest accurate size measurements employed transit timings of its red edges, often with finely scored micrometers in the eyepiece. These consistently showed that the GRS had an east-west length of about 34[degrees] in longitude. On Earth, this longitudinal extent would be something not quite the size of Australia. But on far larger Jupiter, this expanse corresponds to about 40,000 km (25,000 miles), three times Earth's diameter.
As early as the 1900s, observers clearly saw that the GRS was shrinking. Their measurements often varied, sometimes due to how much the spot blended into the cloudy background around it, but overall a consistent trend emerged. Many years of measurements showed that the spot's length was decreasing in longitude by about 0.14[degrees] per year. More recently, high-resolution measurements have refined this rate to an even more rapid 0.19[degrees] per year. Thus, the GRS has been steadily shrinking for more than a century. It currently spans about 14[degrees] in longitude, less than half its size a century ago.
This begs the question: could the GRS have been observed even earlier by telescopic observers--and, if so, was it even larger then? Some of the earliest telescopic observations by Gian Domenico Cassini and Robert Hooke in the mid-1660s note a "permanent Spot" on Jupiter, which many have since interpreted as being the GRS. It's not clear from the original papers themselves if this is the case, however, because the descriptions are somewhat vague as to size, color, and location. Also, observers provided only intermittent reports of such a feature during the two centuries that followed.
Moreover, the spot was of interest not so much for its color and size but because astronomers could use it to determine the rotation rate of Jupiter to a remarkably accurate 9 hours 56 minutes. Beyond that, it apparently warranted little mention other than as a "small spot in the biggest of Jupiter's three belts" spanning about 12[degrees], as shown in sketches over those years.
Notably, if the recent trend in size were to hold true farther back in time, the GRS would have been 30[degrees] to 40[degrees] larger in 1665 than in 1878 and thus occupied most of a hemisphere! This clearly would not be a small distinct spot seen rotating across the planet's disk--with that enormous size, it could have certainly stood out and even been seen through the low-resolution telescopes of that era.
Conceivably, the spot reported by Cassini and Hooke might have dissipated and been replaced by the current GRS over the intervening years. Another possibility is that the storm experiences episodes of growth, after which it becomes smaller. For example, despite evidence that the spot clearly was shrinking from the 1870s onward, occasional reports (for example, during 1915-25) gave a larger size--though only by a few degrees of longitude.
It's also hard to tell whether the larger measurements corresponded to growth of the actual oval of the GRS or to an extended region around it. As many Sky & Telescope readers know, the storm itself is sometimes very hard to distinguish from the surrounding clouds. These reports sometimes described the edges as pointy, rather than round, which gives us a clue. Hurricane-like vortices typically have rounded edges, even when very oblong, while the region around the GRS, shaped and bounded by regional winds, can have more angular shapes. Sketches of Jupiter, when compared with modern, high-resolution images, show that, indeed, sometimes the pointed features are not actually part of the storm itself.
This discrepancy shows one downside of using color to delineate storm size. The Great Red Spot's internal structure consists of a high-velocity collar surrounding a somewhat stagnant core. A cloud in this collar can complete a full circuit around the spot's perimeter in about 3 days. Some have suggested that using the location of the high-velocity collar is a better measure of the GRS's size. Although this radius is smaller than the colored region, it too is shrinking over time.
The Present
We have more than 20 years of Hubble Space Telescope observations of the GRS. Throughout HST's mission, the storm has gotten noticeably smaller and rounder. Composite images also reveal that the spot does actually change color quite dramatically. This is one key to understanding why the storm is changing.
Jupiter's cloudtops are dominated by bands of alternating winds that flow roughly along the boundaries between dark belts and bright zones. Storms typically "roll" between these counterflowing jets like ball bearings in their races. In fact, we suspect that large vortices such as the GRS help to maintain the adjacent wind jets by adding convective energy. Conversely, vortices might draw some energy from the wind jets by ingesting small eddies.
This is an active area of study, as we try to understand how waves and vortices are related to Jupiter's very stable wind pattern. The GRS is nestled between a westward-moving jet on the northern edge of the South Tropical Zone (STrZ) and an eastward jet on its southern edge. Since the GRS slightly overfills the STrZ region, those wind jets are deflected around the big oval, which causes turbulence and cloud mixing as the flows return to their normal latitudes after going by.
Small eddies on either of those deflected wind jets can be drawn into the flow of the GRS as they pass by and subsequently dragged into its interior. While it's unclear if these additions help sustain the storm, they do affect its color. As eddies are ingested and sheared apart, fresh, whiter material often appears within the GRS. Ingesting multiple eddies can turn the storm a very pale color, as occurred in 2012. Conversely, when no eddies accompany the jets, the spot's color can become quite intense--as it did during a widely observed fadeout of the South Equatorial Belt (SEB) in 2010.
In 2014, after a rapid decrease in the spot's length, observers noticed that its color had intensified, even though there were still eddies on the jets nearby. Perhaps these eddies could no longer enter the flow of the GRS for some reason. Why might this be?
First, the storm has gotten smaller not just in its longitudinal width but also in its north-south extent. In addition, it occasionally moves slightly northward or southward, affecting which wind jet it deflects more. HST's 2014 measurements show the spot located near its normal central latitude (22[degrees] south), but both its north and south edges contracted centerward as the entire system shrunk.
The westward jet still experiences significant deflection around the GRS's northern edge, though perhaps slightly less than previously. However, the eastward jet is now barely deflected southward by the GRS. This means that any entrained eddies in this region are not interacting closely enough to be caught in the spot's flow.
We now strongly suspect that these jet interactions are affecting not only the color but also possibly the size of the Great Red Spot. In time, our computer modeling of the GRS's interaction with the nearby wind jets should help us understand the energy exchange between them.
The Future
So what does the future hold for the most famous storm in the solar system? That's difficult to predict, as sudden outbreaks could always alter the energy balance and wind flow near and within the GRS and change our view completely. Atmospheric specialists describe the shape of Jupiter's anticyclonic storms (high-pressure systems like the GRS) by the ratio of their length to width, and it's not unusual for this key aspect ratio to decrease in such storms, particularly right after they form.
For example, the white ovals just south of the GRS first appeared in the late 1930s not as discrete spots but rather as high-altitude clouds and haze that suddenly inundated their latitude band, probably related to a large convective event. The brightened zone then coalesced into three distinct, oblong white storms, which observers designated BC, DE, and FA. By the Voyager flybys in 1979, each had an aspect ratio of about 2. They continued to shrink until merging into a single oval in 2000, with the new, combined oval BA having an aspect ratio of 1.25. In 2005, it turned red--earning the moniker "Red Spot Jr."--and by 2015 its aspect ratio was 1.3. Meanwhile, the GRS's aspect ratio has changed from about 3 during the Voyager flybys in 1979 to the 1.4 it has now.
Computer models suggest that Jupiter's anticyclonic vortices should exhibit preferred aspect ratios, but those depend somewhat on interactions with adjacent wind jets, especially for a large storm that can deflect the jets. So the GRS might eventually reach a shape that is "just right," allowing it to stabilize.
Studies of its internal dynamics will help us to model its fate, so we continue to make yearly Hubble observations. And amateurs' observations are critical for monitoring the GRS. Their regular reports help us to fill in the gaps between studies with HST and professional telescopes. They can also alert us to new happenings that require rapid follow-up.
All things considered, we expect that the Great Red Spot will continue to shrink for some time to come. We don't expect it to disappear completely--but for now what happens in the years ahead is anyone's guess.
Amy Simon, an atmospheric dynamicist at NASA's Goddard Space Flight Center, leads the observing team for HST's Outer Planet Atmospheres Legacy monitoring program.
GET TO KNOW JUPITER
For more information about the study of Jupiter's cloud features and the Great Red Spot in particular, see The Giant Planet Jupiter (Cambridge University Press, 1995), authored by John Rogers of the British Astronomical Association. Also see page 48 for a general guide to observing Jupiter and page 50 for a list of times when the GRS is positioned best for telecopic viewing.
Of a permanent Spot in Jupiter : by which is manifested the conversion of Jupiter about his own Axis.
Besides that Transient Shadow last mentioned, there hath been observed, by Mr. Hook, first (as is mentioned in Numb.I. of these Transact) and since by M. Cassini, a permanent Spot in the Disque of Jupiter; by the help whereof, they have been able to observe, not onely that Jupiter turns about upon his own Axis, but also the Time of such conversion which he estimate to be, 9 hours and 56 minutes.
HISTORICAL HINTS Early reports of a spot on Jupiter from Robert Hooke and Gian Domenico Cassini appear in the Royal Society's Philosophical Transactions (1665-66). Historians generally (but not universally) believe these observers saw something other than the Great Red Spot.
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| Title Annotation: | Jupiter's Shrinking Storm |
|---|---|
| Author: | Simon, Amy A. |
| Publication: | Sky & Telescope |
| Geographic Code: | 1USA |
| Date: | Mar 1, 2016 |
| Words: | 2019 |
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