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.