It is the largest planet in our solar system, and one of the brightest objects in Earth’s night sky. An enormous gaseous ball blanketed by reddish whorls and stripes, it has captivated humanity for centuries, featuring prominently in the astrology and mythology of ancient Babylonian, Chinese, Germanic, and Hindu cultures. Astronomers have watched Jupiter for as long as they have had telescopes: Galileo discovered its four largest moons in 1610. Modern technology has since revealed 59 more Jovian moons, and we now know that Jupiter’s whorls are a cloud layer that includes ammonia, hydrogen, and helium. Some scientists suspect that Jupiter’s strong gravitational field may protect the Earth from passing meteors and comets. According to Mike Wong, a research scientist in the Department of Astronomy at UC Berkeley, Jupiter also serves as a kind of “gravitational gas station for spacecraft,” since by passing close to the planet, outward bound spacecraft can accelerate to even greater speeds.
Despite the progress we have made toward understanding some of Jupiter’s mysteries, the great gas giant continues to baffle and inspire scientists. For example, Jupiter’s Great Red Spot is a storm that has raged for at least three hundred years-how does Jupiter’s weather allow for such a phenomenon? How do elements commonly found on Earth like hydrogen and helium behave under Jupiter’s extreme heat and pressure? How does Jupiter’s atmosphere react to the impact of comets and meteors? Understanding these and other Jupiter-related phenomena is not only fascinating in itself, but may also yield important insights into the Universe, the Milky Way, our solar system, and the Earth itself. Work from many labs, including that of Geoff Marcy, professor of astronomy at UC Berkeley and adjunct professor of physics and astronomy at San Francisco State University, has revealed hundreds of Jupiter-like planets in the universe beyond our solar system (see “Strange new worlds,” BSR Spring 2003). It thus appears that Jovian planets are fairly common in the galaxy-what remains to be seen is whether small rocky planets like Earth are as well. In addition, “Jupiter’s sheer size, being far more massive than all the other planets of our solar system combined, means that understanding Jupiter and how it formed is a huge part of understanding the formation of our solar system, and of planetary systems in general,” says Hugh Wilson, postdoctoral researcher in the Department of Earth and Planetary Science. According to Philip Marcus, professor in the mechanical engineering department, studying physical phenomena on Jupiter is valuable because it provides additional data points that help us understand these phenomena on Earth. “Granted, we have historical information for the Earth which provides some additional data,” says Marcus, “but the more planets we look at, the better.”
No Man’s Land
While studying Jupiter is conceptually appealing, in practice it poses significant challenges. As a gas giant, Jupiter has no solid surface-just layers of mixed elements in different phases. “Jupiter has a cold gaseous outer atmosphere, but as you go inwards it gets hotter and denser, eventually reaching a phase we just call fluid because the distinction between liquid and gas is lost at high pressures,” says Wilson. “Eventually, you might hit a solid core made of rock and ice, but it will be tiny; probably only around five percent of the mass of the planet, and the pressure and temperature at that point would be ginormous.” Indeed, Jupiter’s interior is a pressure cooker that defies imagination. With temperatures of up to 20,000 Kelvin (approximately 35,000 degrees Fahrenheit) and pressures up to four terapascals (approximately 40,000,000 atmospheres-the Earth’s surface is at about one atmosphere), no probe would survive a mission to Jupiter’s deep interior. Instead, scientists must work with data such as telescopic observations, measurements of planetary emissions, and samples of Jupiter’s atmosphere.
Centuries of storms
One of the planet’s most striking features, Jupiter’s Great Red Spot (GRS), was discovered in 1665. An incredibly stable storm, the GRS is perhaps the most famous example of Jupiter’s baffling weather patterns. Marcus, trained as a physicist in fluid dynamics, was struck by Jupiter’s weather early in his career. “Jupiter has 12 westward jet streams and 12 eastward jet streams,” says Marcus, “and all along those jets there are rows of vortices,” or swirling storms. From far away, the jet streams appear as distinctive stripes that cover the planet from pole to pole, and some storms appear as ovals of various sizes. The vortices are referred to as cyclones or anticyclones, depending on whether they spin in the same or opposite direction as the planet’s spin (see sidebar). The GRS, for example, spins counter clockwise in Jupiter’s southern hemisphere, making it a large anticyclone.
Until the early part of this century, data suggested that all the storms in a given jet stream spin in the same direction and are remarkably stable, sometimes lasting for hundreds of years. Beginning in 1998, however, several previously stable anticyclones merged to form “Oval BA,” affectionately nicknamed Red Spot Jr. after it slowly changed from white to red in 2006. This sudden merging of previously separate storms caused a flurry of excitement in the Jupiter research community. Marcus’s focus on fluid dynamics gave him a unique perspective. “In studying fluids, vortices of the same sign merge like crazy all the time. So, to me, the question was why they were stable for a century-why didn’t they merge in the first place?” says Marcus. He began asking how anticyclones could be relatively close to each other without merging, and found, surprisingly, that the answer lies in the presence of vortices spinning in the opposite direction: cyclones.
Long-lived cyclones were not previously known to exist on Jupiter. In part, this was because their presence is obscured by cloud patterns. “Using direct observation of clouds to try to understand what is going on can be really confusing,” says Marcus. “Just because you can’t see a storm doesn’t mean it isn’t there.” Instead, Marcus used simulations to show that the presence of cyclone-anticyclone pairs could produce stable storms. Specifically, if you drop dye into a liquid with vortices that exist in a plus-minus configuration, with an anticyclone flanking each cyclone, the resulting “cloud” of color mimics the patterns seen on Jupiter. “These plus-minus pairs of vortices engage in a chaotic dance in which they are drawn together and then repel each other,” says Marcus. This act hides the cyclones under chaotic clouds, keeps the anticyclones from merging, and causes Jupiter’s cloud cover to be filled with its distinctive twists and whorls.
Global warming Jupiter style
While working on storm stability, it occurred to Marcus and his collaborators that the interaction between cyclones and anticyclones might be directly involved in temperature dynamics on Jupiter. Measurements taken by spacecraft show that at the elevation of visible clouds, Jovian temperature is roughly the same across the planet. Uniform planetary temperature like Jupiter’s is unusual-because the Sun’s rays hit Jupiter’s poles at a different angle than along its equator, the temperature at the equator should be higher than at the poles. To explain this temperature homogeneity, Marcus suggests that the movement of cyclone-anticyclone pairs toward and away from each other results in a steady disruption of the cloud layer, acting like a giant mixing device that leads to an even temperature across the planet.
Based on the mixing hypothesis, Marcus and his collaborators made a number of predictions about what would happen after the 1998 merging of cyclones that created Oval BA. This merging meant a local loss of the special interaction between paired vortices-a loss of chaos, and therefore less mixing, which over time should lead to local differences in temperature. Indeed, in 2005, the color of Oval BA began changing from white to red. Although the cause of this color change remains unknown, Marcus, along with Wong and Imke de Pater, professor in the astronomy department, believe that it may be caused by a temperature increase which uncovers the reddish particles that some scientists believe inhabit Jupiter’s atmosphere. These red specks, of unknown composition, may serve as nuclei around which droplets of ammonia ice and other particles condense to form white Jovian clouds. As temperature increases, the droplets may no longer be able to coalesce, revealing the red particles and changing the color of a given storm from white to red. Marcus and his collaborators believe that events like the merging of cyclones and the changing of storm color may be part of a climate change cycle that has a period of hundreds of years.
Cloudy with a chance of helium
Jupiter’s bizarre climate is not limited to surface patterns-on Jupiter, it rains helium. Helium and hydrogen are the most abundant substances in the universe, and appear in Earth’s atmosphere as colorless, electrically insulating gases. Take Earth’s atmospheric pressure and multiply by 40,000,000, however, and helium and hydrogen atoms behave very differently. Toward Jupiter’s interior, helium forms a liquid while hydrogen forms a substance that is liquid and capable of conducting electricity-a liquid metal. Recently, Wilson and Burkhard Militzer, professor of Earth and planetary sciences at UC Berkeley, showed that Jupiter’s atmospheric helium forms liquid droplets that rain through a layer of metallic hydrogen.
Wilson and Militzer arrived at this insight by pursuing an unexpected result from the Galileo probe. Atmospheric data from the probe suggested that all noble gases were present in the amounts expected according to popular models except for two that were depleted-helium and neon. In particular, neon was depleted by a factor of 10. “Enrichment in a given element would not be strange because you can argue that Jupiter may have captured some comets or other objects,” says Militzer. “A depletion, however, is unusual.” Scientists struggled to explain the neon depletion. One controversial theory came from Caltech scientist David Stevenson, who suggested “helium rain.”
Stevenson’s hypothesis is based on two lines of reasoning. First, it depends on the theory that at certain pressures and temperatures, helium and hydrogen become immiscible. Several groups have confirmed that helium and hydrogen, when mixed as liquids, separate from each other. Liquid helium is denser than liquid hydrogen, so, when present in smaller quantities than hydrogen, helium should form tiny droplets and “rain” out of the mix. “It has been speculated for 30 years that in giant planets like Jupiter and Saturn, there might be a region where this process actually occurs,” says Wilson.
The second important component of Stevenson’s hypothesis is the solubility of neon in helium versus hydrogen. If helium rain is responsible for neon depletion, the expectation is that neon gas energetically “prefers” to be dissolved in helium over hydrogen. Then, as the helium droplets descend through the hydrogen soup, they would take neon out of the atmosphere with them. Until recently, scientists lacked the computational power to run accurate simulations testing neon’s preferential solubility at the pressures and temperatures seen on Jupiter.
Now, technology has advanced such that it is possible to run such simulations. Using “very expensive computers,” says Militzer, he and Wilson calculated the thermodynamic stability of dissolving neon into helium versus hydrogen given the conditions in Jupiter’s interior. “Sure enough, it turned out that neon is more stable in helium than hydrogen, and the magnitude of this preference is exactly enough to explain the level of neon depletion that we observe,” says Wilson. The calculated magnitude closely matches the observed difference in actual versus expected levels of neon, leading Wilson and Militzer to surmise that helium rain is occurring in Jupiter’s interior.
Beware falling rocks
Aside from weather patterns and alien precipitation, Jupiter’s eccentricities extend to how well it can take a hit. In the past 18 months, three objects have slammed into Jupiter-a surprisingly high number, given that the collision of objects in space is extremely rare. The 1994 collision of the fragmented Shoemaker-Levy 9 comet into Jupiter, for example, is described on NASA’s website as “extraordinary,” and “millennial.” Unlike Shoemaker-Levy 9, which astronomers expected to impact Jupiter, no one expected any of these three recent impacts. “Everyone thought Shoemaker-Levy 9 was a once-in-a-lifetime event, but now, in just over a year, we have three more impacts,” says de Pater.
De Pater and Wong are part of a large group of scientists studying these impacts. De Pater specializes in using radio emissions, both thermal emissions from the planet’s deep atmosphere and X-rays emitted from the acceleration of charged particles (primarily electrons), to gain insight into planetary structure. As radiation moves through different substances, it is absorbed or scattered depending on the characteristics of the environment. These effects can be described mathematically to provide a picture of the radiation’s path. De Pater and Wong also capture and analyze different kinds of optical data. Infrared images from large land-based telescopes like the Keck telescope atop Mount Mauna Kea, Hawaii, complement images taken in the visible spectrum by telescopes like the Hubble space telescope. By combining data gathered from radio emissions and optical images, scientists deduce details about Jupiter’s atmosphere and interior. In the case of collision events, data like these can reveal the components and size of the impacting object, the object’s angle of entry, and any short- or long-term effects on Jupiter’s atmosphere.
With the recent impacts, de Pater and Wong have an additional data set to consider-one from amateur astronomers. Remarkably, all three impacts were discovered by amateurs, not by professionals. The first impact was a comet strike that left an earth-sized scar-Anthony Wesley spotted the gash in July 2009. Since then, Wesley and another amateur astronomer, Christopher Go, independently witnessed and filmed another impact on June 3, 2010 (the videos can be seen on http://www.youtube.com, search: Jupiter impact). Finally, Masayuki Tachikawa and Aoki Kazuo witnessed and filmed a third event in August 2010. “Amateur astronomers now have access to extremely advanced equipment. They are wonderful collaborators,” says de Pater.
Using data from the amateur videos, as well as from several techniques de Pater invented and refined during her extensive research on the Shoemaker Levy-9 impact, de Pater, Wong, and their collaborators are contributing to growing scientific knowledge about impact events. Not only does this work inform a better understanding of Jupiter’s composition, it also has potential ramifications for Earth. “The serendipitous recordings of these optical flashes by amateur astronomers may help us to quantify the number of bodies tens of meters in size in the outer solar system,” says de Pater. “Such numbers will ultimately help to determine the threat of impacts on our own planet.”
At least three centuries after astronomers first observed features on Jupiter’s surface, mankind remains fascinated by its enormous neighbor. The United States has sent eight spacecraft to Jupiter and will send another, Juno, in 2011. Like other planets in our solar system, Jupiter is named for a Roman god. In the mythology, Jupiter was a philanderer and his wife, Juno, was always trying to unravel his deceptions to reveal the truth. NASA’s Juno probe will attempt to do the same with planet Jupiter. Juno’s primary goals are to take accurate measurements of Jupiter’s gravitational and magnetic fields and to probe the composition of its atmosphere. In addition to learning more from direct observation, these data will provide the basis for distinguishing between different models. Militzer is working with NASA scientists to generate solid theories for the behavior of hydrogen and helium to help interpret data from the probe. “You want to have the best models available,” says Militzer. “Ideally, you have two well-tested competing models that data from the probe can resolve into right and wrong.” Juno is set to launch in August of 2011, and will begin to send data after it reaches Jupiter in 2016. As they have for centuries, scientists in 2016 will excitedly gather to analyze the new data, discovering more mysteries even as they provide answers for others. Jupiter, in its aloof majesty, will likely continue to inspire mankind to ask questions about the Universe, the solar system, and the Earth for centuries to come.
The following are inset boxes with supplemental background information. They are also found in the print edition of the article.
The first orbiting spacecraft sent to Jupiter, Galileo began its 14-year mission in October of 1989. After its launch from the space shuttle Atlantis, a booster rocket propelled Galileo into interplanetary space, where it borrowed energy from the gravity of Venus (one flyby) and Earth (two flybys) to slingshot it to Jupiter. On the way, it passed through the asteroid belt separating Mars and Jupiter, and completed the first close studies of two asteroids, Gaspara and Ida. It spotted Dactyl, a tiny moon orbiting Ida, which was the first sighting of an asteroid moon. Already, Galileo was making discoveries.
Five years after launch, Galileo finally reached Jupiter, just in time to capture the only direct observations of the fragmented Shoemaker Levy-9 comet as it crashed into the planet. In July of 1995, it released a probe into Jupiter’s atmosphere, which managed to make a few precious measurements before being destroyed by the planet’s hostile environment. Researchers at UC Berkeley, including Mike Wong, worked with NASA on Galileo. “The probe entered a dry region,” says Wong, “the equivalent of a desert in Jupiter’s atmosphere so we never got to measure the abundance of water on the planet. However, Galileo made accurate measurements of other cloud-forming gases such as ammonia and hydrogen sulfide.”
Meanwhile, the main spacecraft continued its orbital tour of the Jovian system, making discoveries that caused NASA to extend its stay an extra six years until 2003. In particular, NASA approved an extension to tour two large Jupiter moons-icy Europa and fiery Io. Data from this mission supports the idea that an ocean exists under Europa’s icy crust. Io, on the other hand, remains the most volcanic body known, and Galileo flew close enough to photograph a lava fountain. Finally, in 2003, NASA crashed Galileo into Jupiter to avoid the possibility of contaminating Europa’s ocean via a chance collision. The first probe of the Jovian system, Galileo revealed moons, winds, oceans, and fire. Surely, Galileo Galilei would be proud.
The difference between cyclones and anticyclones, the swirling storms that give Jupiter its distinctive oval spots, can be tricky. In simplest terms cyclones spin in the same direction as planetary spin, while the reverse is true of anticyclones. This seemingly simple definition, however, is complicated by the fact that apparent planetary spin depends on location: a planet that appears to spin counterclockwise from the northern hemisphere will appear to rotate clockwise in the southern hemisphere. Imagine a wheel rolling downhill. A person watching the wheel from one side, parallel to the wheel with his or her right hand uphill, would say that the wheel was rotating to the left-counterclockwise. By contrast, a person watching the same wheel from the opposite side would say that it was rotating to the right-clockwise. This difference also applies to rotating spheres, in this case, planets. As a result, cyclones in the northern hemisphere spin counterclockwise, whereas those in the southern hemisphere spin clockwise.