NASA

NASA

NASA

NASA

NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

NASA

NASA

NASA

NASA



Water map: ESA/Herschel/T. Cavali et al.; Jupiter image: NASA/ESA/Reta Beebe

NASA

At first blush Jupiter may seem like a rather dull planet. A failed star. A ball of gas. A large, red storm. Sure, it's big, but what more is there? And we’ve been there before—lots of times, in fact. Beginning with Pioneer 10 and 11, NASA has flown seven probes by the gas world. One mission, Galileo, studied the Jovian system for nearly a decade from 1995 to 2003. So why is the space agency sending yet another probe, Juno, to once again visit the Solar System's giant?

We're still studying Jupiter because despite all of these missions, NASA has literally only scratched the surface. Its inner structure remains largely a mystery—and an intriguing one, too. The planet is essentially made of gas piled upon more gas. And like a big stack of pillows, as more gas is added on top, the bottom layers become more and more compressed as gravity pulls down on the gas. This creates extremely dense conditions inside the planet. Jupiter, after all, is only a little bit larger than Saturn—but it has three times the mass. Scientists have almost no idea how hydrogen will behave at the extreme pressures deep beneath Jupiter's outer layers toward its core. Indeed, does Jupiter even have a core? We simply don’t know.

The Juno spacecraft, launched in 2011 and arriving at Jupiter on July 4 this year, will be able to scrutinize the planet's gravity field and peer beneath its upper cloud layer. This should help offer some clarity about Jupiter’s interior. During a presentation in May at the Lunar and Planetary Institute in Houston, mission scientist Fran Bagenal discussed just how strange that interior might be.

Metallic hydrogen

The exterior of Jupiter is gaseous and cold, with a temperature of about -150 degrees Celsius. The temperature and density increase rapidly as one goes into the planet, and when the temperature reaches about 1,700 degrees Celsius, hydrogen is forced into a liquid because of the accompanying pressure at that depth. Just a little bit further down—still only about 20 percent of the distance from the surface to the center—the temperature becomes high enough that the bonds of the hydrogen molecules begin to break down. And from there, down to the center of the planet where temperatures reach about 20,000 degrees Celsius, things get really weird.

It’s not clear what the combination of pressure and temperature (four or five times hotter than the surface of the Sun) will do to hydrogen. To get a sense of the pressure on atoms at the center of Jupiter, Bagenal conjured an image of an elephant standing on one foot. Now imagine 1,000 elephants standing on top of that one elephant. And even that doesn’t approximate the pressure in the interior of Jupiter, which is 100 million times the atmospheric pressure on Earth. That one elephant would need to be standing in a stiletto heel.

“We don’t really know how hydrogen behaves at those pressures,” she said. “We are sort of making it up. We are working at a realm where we don’t have experimental evidence. So we have to rely on theorists to tell us what happens.”

Scientists have taken hydrogen to a couple million times the pressure of the planet’s surface in Earth-based experiments, and at these high pressures the bonds of molecular hydrogen break to create a conductive fluid of protons and electrons known as metallic hydrogen. This fluid might be something like the liquid mercury in thermometers. But what happens at the much higher pressures inside Jupiter?

Bagenal said this truly represents the cutting edge of science. When Juno launched five years ago, theorists generally believed Jupiter had a core of heavier elements such as oxygen and carbon beneath a layer of metallic hydrogen. However, physicists working with quantum mechanical models of hydrogen say they now expect the heavier elements to be all mixed up in the metallic hydrogen soup. Bagenal hopes the answers to this mystery lie in studying the gravitational field of Jupiter.

The spacecraft

Named after the Roman goddess who was both sister and wife to Jupiter, the $1.1 billion Juno mission launched aboard an Atlas V rocket with five solid rocket boosters attached. The spacecraft now approaching Jupiter weighs about 3,600kg and is 3.5 meters long and 3.5 meters wide. It features huge solar panels, measuring a total of 60 square meters, because the solar output is about 25 times weaker at Jupiter than Earth.

In addition to the gravity science experiment mentioned by Bagenal, the mission carries eight other instruments, including a magnetometer and spectrograph. Aside from understanding the planet's mysterious interior, scientists also hope to gather data that will allow them to piece together the formation and evolution of Jupiter as the largest planet in the Solar System and understand the precise mechanism that generates its large magnetic field. Understanding Jupiter’s development should lead to a better overall picture of the Solar System’s formation.

Scientists are not sure how long the spacecraft will survive in the harsh radiation environment generated by Jupiter. Most of Juno’s electronics are housed inside a vault with a thick titanium casing, because the spacecraft will pass repeatedly through the planet’s lethal radiation belts as it travels over the poles. Mission scientists hope to get 33 orbits out of the spacecraft, but Bagenal said they’re not sure its sensitive equipment will last that long. At the end of its mission Juno will maneuver into an orbit that will slowly degrade and eventually allow the spacecraft to be swallowed by Jupiter’s gaseous maw. It will collect no more data in this death spiral, but at least Juno will be kept away from any Jovian moons that might harbor life–such as Europa. At least for the time being, we will continue to attempt no landing there.

For NASA the most critical date in the mission is coming soon: on July 4, Juno will commence its orbital insertion maneuver. Right now, Juno is approaching Jupiter at about 6 km/s relative to the planet. At approximately 10:30pm Pacific time on July 4, the spacecraft will begin a 35-minute burn to slow down and—hopefully—slide into orbit around Jupiter. If all goes well, the spacecraft will begin peering into the clouds of the planet shortly thereafter.