JOVIAN GENESIS

When Galileo first saw Jupiter with his homemade telescope 400 years ago, he saw a dazzling planet with colorful clouds and bands, surrounded by its own moons like a miniature solar system. Ever since, we’ve been fascinated by Jupiter, snapping pictures with advanced telescopes and sending spacecraft to explore the gas giant. One spacecraft, named Galileo in honor of the scientist, spent eight years learning about Jupiter and its moons.

But we still don’t understand a few basic areas: we don’t know if the planet has a solid core; we don’t know how and where its magnetic field is produced; and we don’t know how much oxygen there is. Some theories about Jupiter’s formation predict that the planet’s oxygen weighs as much as 20 Earths. The abundance of oxygen isn’t just a major mystery about Jupiter, it’s the most important unanswered question about how our solar system formed.

As we’ve learned more about Jupiter, we’ve realized that we don’t understand planetary formation as well as we thought. In 1995, NASA’s Galileo spacecraft sent a probe into the thick clouds of Jupiter. From the information gleaned by the probe and from ideas developed over the years, we now know that Jupiter has a lot more heavy elements – that is, elements heavier than hydrogen and helium, such as nitrogen and carbon – than the sun. But if Jupiter formed from the same cloud of gas and dust as the sun, it should be made of the same stuff. How did it become so enriched with heavy elements?

We think that for Jupiter to be enriched, it must have been somehow assembled from many gas-containing icy bodies. But such objects would only have been able to exist far beyond Jupiter’s present orbit, where it would be much colder. The question, then, is whether Jupiter formed at its present orbit, somehow attracting these distant icy bodies, or whether Jupiter formed farther away from the Sun and then migrated inward.

In the last decade and a half, we’ve discovered hundreds of planets orbiting other stars. Many of these planets are gas giants bigger than Jupiter – and they are much closer to their stars than Jupiter is to the Sun. Why do these planetary systems seem so different from our own? We have to know how our own gas giants formed to under- stand whether our solar system formed in the same way as these other systems – or whether the solar system is a special case. Maybe the gas giants orbiting other stars weren’t born from the same processes as Jupiter. Maybe they’re failed stars – balls of gas that just aren’t massive enough for nuclear fusion to ignite.

One thing’s for sure, however. Although Jupiter’s colorful clouds get all the attention, the most enticing scientific mysteries are hiding deep inside the planet.

There are many ideas as to how Jupiter might have formed. Some scientists think that it began as a solid chunk of heavy material (a “planetesimal”), such as rock and ice. As its gravity gathered debris, it grew, increasing its gravitational pull. Eventually, it became so big that it had enough gravity to capture the light gases – hydrogen, for example – in the nebula around it.

Another possibility is that Jupiter formed when a small region of the gas disk that swirled around the young Sun suddenly collapsed on its own.

One of the most powerful aspects of science is its ability to make predictions about how the universe works. Various theories of Jupiter’s formation say different things about what the planet should be like. For example, one theory predicts that Jupiter should have a core of heavy, solid material – made out of elements like carbon, oxygen, nitrogen, and silicon – that’s as massive as three Earths. Another theory suggests that there should be nine Earths’ worth of material. Yet another idea says the core should weigh as much as 20 Earths. An entirely different group of theories makes predictions about how much of Jupiter is made of water.

By determining the nature of Jupiter’s core and how much water the planet has, scientists can narrow down the many ideas as to how and where Jupiter formed. Since Jupiter’s formation and orbital evolution is inextricably tied to Earth’s, Juno’s mission is, in essence, about understanding our own origin.

Juno is equipped with tools that allow us to learn about Jupiter’s interior – even if we can’t directly see inside the planet. Movement and density variations under the clouds – caused by a thick, churning blob of gas, for example – can subtly alter the gravitational field directly above the surface. By observing these slight effects, Juno can help decide what’s inside and how it’s moving. The spacecraft can also measure Jupiter’s magnetic field and, in doing so, determine where the magnetic field is generated, revealing fluid motion in the deep interior.

Even though several spacecraft have visited Jupiter, and despite having the best telescopes on Earth and in space, there’s still a lot we don’t know about the gas giant. We know that 99 percent of the planet is hydrogen and helium, but the remaining one percent remains a mystery. We’re also not sure whether there’s a solid core at the center or how Jupiter generates its powerful magnetic field.

We’ve come up with many possible answers to these questions, and many of our ideas are well supported by experiments and other space missions. With the help of Juno, we are making dramatic progress in solving these mysteries, allowing us to understand how Jupiter formed and became the planet we know today.

THE SEARCH FOR WATER
One of the biggest questions we have about Jupiter is how much of it is made of heavy elements – elements that are heavier than hydrogen and helium. By far the most important heavy element is oxygen, whose most common form is in water. Oxygen is the third most abundant element in the universe, and we expect that it should account for more than half of Jupiter’s heavy-element composition. In fact, the amount of oxygen in Jupiter could weigh as much as 20 Earths.

PLUNGING INTO JUPITER’S DEPTHS
According to data taken by spacecraft and telescopes, Jupiter must be made of materials heavier than hydrogen and helium. Most theories about what Jupiter looks like inside suggest that there’s a solid core at its center. But so far, we have never been able to verify its existence. We’re also unsure whether the core is like a solid ball with a surface, or whether the core’s interface with the rest of the planet is more gradual, with Jupiter’s interior gas becoming denser until it becomes solid at the center.

Different theories, or models, of how Jupiter formed make different predictions about the size, mass, and composition of the core. In some models, Jupiter’s core could weigh as much as three, nine, or even twenty Earths. By determining what Jupiter’s core is like, Juno will help us narrow down the correct model. And if Juno finds that the core is nothing like what we expected, then we would be forced to rethink our ideas about how giant planets like Jupiter form.

Another mystery is the structure of Jupiter’s swirling clouds, bands, and storms. With Juno, we are able to see the structure and movement of material deep beneath Jupiter’s clouds for the first time. Jupiter’s most breathtaking surface features could be connected to the structure and motions of gas deep in its interior. Or, they could be shallow patterns on the outermost layer of the atmosphere, like drops of oil on a pool of water. Juno data indicate that the solar system’s most famous storm, the Great Red Spot, is almost one-and-a-half Earths wide, and has roots that penetrate at least 200 miles (300 kilometers) into the planet’s atmosphere.

EXPLORING THE MAGNETIC FIELD
 Deep inside Jupiter, the crushing weight of the planet creates extreme temperatures and pressures. Researchers have recreated similar conditions in the laboratory – but only for mere fractions of a second. Their experiments suggest that at some point inside Jupiter, maybe about a third of the way toward its center, the pressure and temperature become so intense that the planet’s hydrogen is squeezed into a metallic state, where electrons are freed from atomic nuclei, roaming effortlessly throughout this “superconductor.” It’s possible that Jupiter’s magnetic field is generated at depth, where hydrogen takes on this exotic form, but early Juno observations suggest that Jupiter’s huge magnetic field is produced at a shallower depth, with the molecular Hydrogen envelope sitting atop the metallic Hydrogen.

But the conditions here are so strange that we don’t have a good understanding of what exactly goes on. The magnetic field could be generated in a way that’s similar to how Earth’s field is generated. Or, the engine behind Jupiter’s magnetic field could more closely resemble how material flows inside the Sun. To improve our understanding of Jupiter’s magnetic field, Juno is mapping the field and monitoring how it changes over time.

Measurements of the massive planet’s magnetosphere, from Juno’s magnetometer investigation (MAG), indicate that Jupiter’s surface magnetic field is even stronger than models expected, and more irregular in shape. MAG data indicates the surface magnetic field, at about 20 Gauss, is roughly 30 times stronger than the strongest magnetic field found on Earth.

As tiny, charged particles fly through space, they get caught up in Jupiter’s magnetic field, which channels them toward the planet’s north and south poles. When these particles slam into the polar atmosphere, they create intense light shows – Jupiter’s aurorae, northern and southern lights just like on Earth. Juno is equipped with powerful instruments that measure the aurorae and detect these particles as they stream toward the atmosphere. These processes also produce radio signals that Juno listens to with its radio antennas. The data will help us understand the complex interactions between Jupiter’s rotation, its atmosphere and its magnetic field. Juno’s initial observations indicate that the process seems to work differently at Jupiter than at Earth.        

In Roman mythology, Jupiter, king of the gods, shrouded himself with clouds to hide his mischief. But Jupiter’s wife, the goddess Juno, was able to peer through the clouds and discover the truth. Likewise, the Juno spacecraft will look beneath Jupiter’s clouds to help us understand the planet’s structure and history.    

When the Galileo probe plunged into Jupiter’s clouds, it discovered something unexpected. It found that there were two to three times more volatile substances – materials that melt at low temperatures, such as the elements argon, krypton, xenon, carbon, and nitrogen – on Jupiter than on the Sun.

These elements could have come from small, asteroid- and comet-like bodies. For these bodies to contain the argon and nitrogen that’s found in Jupiter, they must have formed at temperatures lower than 30° Kelvin (-405 Fahrenheit). This means that either Jupiter was born far from the Sun, where it would have been cold enough for these elements to exist, and then migrated closer, or that comets and asteroids brought these elements with them when they crashed into Jupiter.

But there’s another puzzle. The Galileo probe also found that Jupiter has much less oxygen than the sun. Comets, which were leftover objects in the cold regions of the solar system that failed to grow large enough to become planets, have a lot of water. Oxygen is a main component of water, and if these comets delivered elements to Jupiter, why didn’t they also bring oxygen? One possibility is that the lack of oxygen is just an anomaly; the probe entered a “hot spot” on Jupiter, a region that just happened to be dry, like a desert. Or, the lack of water and oxygen could be a fundamental clue about the formation of gas giants.

By getting a more detailed measurement of Jupiter’s heavy elements with Juno, scientists will be able to determine how the planet became chemically enriched. If these elements were brought by comets from a distant, icy region of the solar system called the Kuiper Belt, then Jupiter should have roughly the equal amounts of oxygen and heavy elements. But if Juno finds that there’s more oxygen than heavy elements, then these elements must have originated locally, near Jupiter’s present location.    

Juno is currently in orbit around Jupiter. At its closest approach, the spacecraft passes within 2,200 miles (3,500 kilometers) of Jupiter’s cloud tops once during each 53-day orbit. At the high end of each orbit, Juno is about 5 million miles (8 million kilometers) from the planet.

See Juno’s current position, speed and more via NASA’s Eyes on the Solar System 3D interactive. Launch the Juno module or view Juno in the standard Eyes on the Solar System interface.