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."

A close-up view of the weather systems that give Jupiter its distinctive look. Red Spot Jr. is visible to the left of and below the Great Red Spot.

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.

Schematic of helium rain and the resulting differences in helium and neon concentration on Jupiter. Inset shows the layer at which helium and hydrogen separate. Jupiter’s exterior is to the top, interior to the bottom.