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A New Red Spot for Jupiter

—Philip Marcus, Imke de Pater, Xyler Asay-Davis, UC Berkeley

Jupiter's colored bands, spots, and filigreed detail have fascinated both astrophysicists and amateur astronomers since 1662, when Robert Hooke (of Hooke's law fame) first observed the Great Red Spot. Since then, thousands of pictures have been taken of the patterns and motions of the jovian clouds which are presumably indicators of the planet's winds. Unlike Earth, Jupiter is nearly all atmosphere - in the sense that the planet is almost entirely fluid. If there is a solid core (which is still debatable), it is tiny compared to the rest of the planet. The main features of the fluid motion at cloud level are the stripes, or horizontal, multicolored bands, which indicate the planet's 12 east-west jet streams and the spots, which are the cyclonic and anticyclonic storms that last from a few days to at least 344 years (the duration over which the Great Spot has been observed). The storms range in size from 50 Km (the limit of our spatial resolution) to 26,000 Km. The longevity of these features, their enormous sizes, and the fact that they persist alongside intense turbulence, as indicated by the swirling motions at all scales, makes them a classic example of "order from chaos" and therefore of great interest to fluid dynamicists, applied mathematicians, physicists, and those who study nonlinear dynamics and dynamical systems.

Jupiter's atmosphere is a laboratory where theories of weather, atmospheric circulation, and climate change in a different physical and chemical regime than Earth can be tested. This laboratory sometimes produces interesting and unexpected events. A new red spot, which first appeared this spring at 34^oS, is the most recent example.

No other feature on Jupiter is as well known as the Great Red Spot with its distinctive red clouds. All of the other jovian, vortical storms have white clouds. Now, there is a second storm with red clouds. In addition, this storm is now Jupiter's second largest storm. It is so new that its name is still unsettled - "Redspot Jr." (favored by the popular press); "Oval BA," and "Red Oval" are current contenders.

The antecedent of the Red Oval (the name favored by us) was a large anticyclone that was formed between 1998 and 2000, when 3 anticyclones known as the White Ovals merged. Unlike the Great Red Spot, whose origins are lost in astronomical prehistory, the origins of the new red spot's parents -- the White Ovals were well documented. In the mid-1930's, astronomers observed the break-up of a horizontal band of white clouds into three distinct, highly-elongated clouds that quickly contracted into three white oval storms that were all located at 34^oS, just south of the Great Red Spot. For 60 years after their formation, the three White Ovals were usually widely separated in longitude oscillating back-and-forth in the east-west direction and appeared to repel each other. During that same period, their areas slowly shrank. After 1996, their behavior changed. Instead of repelling each other, a pair of anticyclonic White Ovals bunched closely together trapping a cyclonic storm between them. The trio of vortices paraded eastward as a single, seemingly bound, unit. Then, in 1998, their behavior changed again. The pair of White Ovals in that unit merged, and in 2000 this super storm merged with the remaining White Oval to create the storm that would eventually turn into a new red spot.

Using SDSC facilities to carry out large-scale computations, we simulated all of these behaviors of the White Ovals, including oscillations and repulsion, slow decrease in area, bunching together, and mergers. Moreover, by using the simulations as an empirical laboratory in which physics could be added, removed, or changed at will, we were able to build simplified models of the White Ovals dynamics to understand their behavior.

For example, many astronomers assumed, based on cloud morphology, that only anticyclones (in the southern hemisphere, these are the counter-clockwise rotating storms) were long-lived. However, observations showed that anticyclones, such as the White Ovals, rarely occurred by themselves (with the Great Red Spot, and now, the Red Oval, as the only exceptions) but rather appeared in rows that contained from 3 to 12 anticyclones, all at the same latitude. The anticyclones are always embedded in a shearing zonal flow, in which the ambient shear has the same sign as the vorticity of the storm. The shear is due to westward-going jet stream on the northern side of the row and an eastward-going jet stream on its southern side, as schematically shown in Figure 2.

Computations showed that the configuration in Figure 2 is unstable. In general, jovian storms will move with the local prevailing jet velocity, which is zero midway between the jets and the initial latitude of the storms. Left undisturbed, the initial situation will not change - the storms will maintain their separations and will not merge. The situation is fundamentally unstable, however, because the smallest perturbation will lead to a merger. For example, if the storm A2 moves slightly upward in latitude, it is carried to the left in longitude by the jet stream above it. In computer simulations, the separation between A2 and A1 becomes less than the storms' diameters within weeks. After that, the A2 and A1 quickly merge into a single storm. This result is contrary to the observations that show that anticyclones on Jupiter can persist for many decades, so we must add to the model.

Mergers are prevented if a row of cyclones is added, alternating with the anticyclones, as in Figure 3. This configuration is known as a Kármán vortex street, where anticyclones are longitudinally staggered with cyclones. Now, if anticyclone A2 is perturbed upward, the jet stream above it moves it left, as before, but then it is carried downward by the clockwise flow around C1, until the jet stream below it moves it right again, back to its original position. The cyclone has repelled the anticyclone, and the merger is averted. Simulations show that this configuration is stable for long periods of time, with storms oscillating back and forth in longitude. The periods of the oscillations as well as the velocities of the storms were computed and shown to be in good agreement with the observations.

However, as mentioned above, observations of Jupiter have shown that a trio of vortices, two anticyclones and one cyclone, moved together in a tight packet between 1996 and 1998, rather than oscillating back and forth in longitude. Something must have kept the three vortices bound together. Our numerical simulations at SDSC showed that such groups of vortices can move together as a single unit if they are trapped in the trough of a Rossby wave. Rossby waves commonly travel along the eastward-going jet streams on Jupiter and also on Earth. It is not uncommon for vortices to get trapped in the troughs of Rossby waves and ride along with them. The Great Red Spot is almost certainly trapped in such a trough. A casual glance at a weather map shows that this type of vortex trapping frequently occurs on Earth's jet streams. On average, our northern jet stream flows from west to east, but only rarely does it do so along a straight path. More often, it is sinuous with waves and troughs. Generally, on the northern side of the jet stream, seated comfortably in each of the troughs, are high pressure centers, a.k.a., anticyclones. Simulations show that the same effect occurs on Jupiter. However, simulations of Jupiter's jet streams show that instead of having just a single anticyclone seated in a trough, there can be a trio of vortices consisting of two anticyclones with a cyclone between them as in Figure 4. Figure 4 schematically shows the configuration of two of the White Ovals with the intervening cyclone as was observed just prior to their merger in 2000. The sides of the trough squeeze the three trapped vortices so hard that any small perturbation, for example, a collision with a vortex outside the trough will cause the cyclone to be pushed out of the trough leaving the two anticyclones in direct contact. Once contact is made between the two anticyclones, they merge within days. This scenario of vortex trapping followed by cyclone ejection and then merger is what we believe happened in 1998 and again in 2000.

Seven frames from a video made from a simulation of the mergers computed at SDSC are shown in Figure 5.

The first frame in Figure 5 shows a small blue cyclone approaching the trio from the left. It collides with and is repelled by the trio, but the rogue cyclone perturbs them just enough so that the trio's blue cyclone is ejected allowing the two red anticyclones to merge.

After the second merger of White Ovals, the resulting storm remained white but in late 2005, it became brown and on February 24, 2006, an amateur astronomer, Christopher Go, announced that it had changed to red.

Astronomers generally agree that the white color of the White Ovals was due to ammonia ice. Surprisingly, no one knows for certain what makes the Great Red Spot, or the new red spot, red. The color of a cloud can change if a coloring agent contaminates the ice particles. The contaminant or "chromophore" could be a minor constituent of the upper atmosphere only encountered by high clouds, or it could be associated with material dredged up by the hurricane-like winds in the vortices from the deeper layers of the planet atmosphere. One potential source for red chromophores is phosphine gas, PH3, a colorless, flammable, and poisonous gas that has been detected on Jupiter and is somewhat more abundant above the Great Red Spot than at other locations. Ultraviolet light from the Sun might catalyze its conversion to red phosphorus, P4, according to one theory. This particular theory has been challenged, however, by others who showed that phosphine would interact with chemicals such as methane or ammonia to form complex compounds such as methylphosphane or phosphaethyne, rather than P4.

The color change from White Oval to Red Oval may be a harbinger of climatic change on Jupiter. Even though the red chromophore is unknown, the chemical reactions producing it are likely to be highly dependent on the ambient temperature. So the change of color probably implies a change in temperature at the Oval's latitude. The only other likely causes of a color change of the Oval are either that the storm has changed its altitude in the atmosphere or its vertical thickness has changed such that part or all of the storm now lies at an elevation that contains a red chromophore. However, since the storm's altitude and vertical thickness are also sensitive to the ambient temperature, these scenarios also imply a temperature change.

Immediately after the mergers of the White Ovals in 1998-2000, we predicted based on a series of calculations computed as SDSC, that there would be a global temperature change on Jupiter starting in approximately 2006. The color change of the Red Oval may be indicative of that predicted temperature change. The argument for the temperature change was that prior to 1998, Jupiter's equator and south pole had nearly the same temperature. Since the equator received much more heat from the Sun than the pole, it could be assumed that there was a transport mechanism that moved heat from the equator to the pole. Numerical calculations showed that the stirring action (and in particular the chaotic mixing) caused by the 3 large, and vigorously spinning, White Ovals could easily account for the heat transport, but that a solitary White (or Red) Oval could not. The radiative time for the jovian atmosphere where the Ovals are located is approximately 7 years due to the composition, pressure and density of the gas at that location. (That is, it takes about 7 years for the jovian atmosphere to cool down or warm up in response to a change in its heating.) Therefore, if the mechanism that transported heat from the jovian equator to its south pole was destroyed in 1998-2000, we would start to observe effects of the blocked heat transport and a temperature change in 2006. If this is the correct explanation for the color change of the Oval, then there will be other consequences of the change in temperature, such as huge waves on the jet streams and the formation of new storms. These, and other consequences, should be observable by astronomers and computable with SDSC computers.

In Spring 2006, we used the Hubble Space Telescope to take a series of pictures of the Red Oval and Great Red Spot (including Figure 1). Employing a synthesis of methods including data assimilation, animation, morphing, and Coherent Image Velocimetry, we used SDSC computers to compute velocity fields of the new storms with unprecedented accuracy, despite the fact that the spatial resolution of the Hubble Space Telescope is approximately 8 times worse than the resolution of the Voyager satellites that originally mapped the Great Red Spot in 1979. With these new velocity fields, we plan to quantify the changes in the jovian atmosphere that have taken place in the last 27 years and determine if Jupiter is on the path to a global temperature change.

Acknowledgement: We thank the NASA Planetary Atmospheres Program (NNG06GA09G) and NSF Astronomical Sciences (AST-0607836) for support. Computations were carried out at the San Diego Supercomputer Center (SDSC, supported by NSF). One of us (PSM) also thanks the Miller Institute for Basic Research in Science for support. The observations were obtained via Director's Discretionary (DD) time at the Hubble Space Telescope (HST). The analysis was supported in part by HST-GO-10782.