July 5, 2016: Jupiter’s swirling clouds can be seen through any department store telescope. With no more effort than it takes to bend over an eyepiece, you can witness storm systems bigger than Earth navigating ruddy belts that stretch hundreds of thousands of kilometers around Jupiter’s vast equator. It’s fascinating.
It’s also vexing. According to many researchers, the really interesting things–from the roots of monster storms to stores of exotic matter–are located at depth. The clouds themselves hide the greatest mysteries from view.
NASA’s Juno probe, which went into orbit on July 4,2016, could change all that. The goal of the mission is to answer the question, What lies inside Jupiter?
“Our knowledge of Jupiter is truly skin deep,” says Juno’s principal investigator, Scott Bolton of the SouthWest Research Institute in San Antonio, TX. “Even the Galileo probe, which dived into the clouds in 1995, penetrated no more than about 0.2% of Jupiter’s radius.”
There are many basic things researchers would like to know—like how far down does the Great Red Spot go? How much water does Jupiter hold? And what is the exotic material near the planet’s core?
Juno will lift the veil without actually diving through the clouds. Bolton explains how: “Swooping as low as 5000 km above the cloudtops, Juno will spend a full year orbiting nearer to Jupiter than any previous spacecraft. The probe’s flight path will cover all latitudes and longitudes, allowing us to fully map Jupiter’s gravitational field and thus figure out how the interior is layered.”
Jupiter is made primarily of hydrogen, but only the outer layers may be in gaseous form. Deep inside Jupiter, researchers believe, high temperatures and crushing pressures transform the gas into an exotic form of matter known as liquid metallic hydrogen–a liquid form of hydrogen akin to the slippery mercury in an old-fashioned thermometer. Jupiter’s powerful magnetic field almost certainly springs from dynamo action inside this vast realm of electrically conducting fluid.
“Juno’s magnetometers will precisely map Jupiter’s magnetic field,” says Bolton. “This will tell us a great deal about the planet’s inner magnetic dynamo and the role liquid metallic hydrogen plays in it.”
Juno will also probe Jupiter’s atmosphere using a set of microwave radiometers.
“Our sensors can measure the temperature and water content at depths where the pressure is 50 times greater than what the Galileo probe experienced,” says Bolton.
Jupiter’s water content is of particular interest. There are two leading theories of Jupiter’s origin: One holds that Jupiter formed more or less where it is today, while the other suggests Jupiter formed at greater distances from the sun, later migrating to its current location. (Imagine the havoc a giant planet migrating through the solar system could cause.) The two theories predict different amounts of water in Jupiter’s interior, so Juno should be able to distinguish between them—or rule out both.
Finally, Juno will get a grand view of the most powerful Northern Lights in the Solar System.
“Juno’s polar orbit is ideal for studying Jupiter’s auroras,” explains Bolton. “They are really strong, and we don’t fully understand how they are created.”
Unlike Earth, which lights up in response to solar activity, Jupiter makes its own auroras. The power source is the giant planet’s own rotation. Although Jupiter is ten times wider than Earth, it manages to spin around 2.5 times as fast as our little planet. As any freshman engineering student knows, if you spin a magnet—and Jupiter is a very big magnet—you’ve got an electric generator. Induced electric fields accelerate particles toward Jupiter’s poles where the aurora action takes place. Remarkably, many of the particles that rain down on Jupiter’s poles appear to be ejecta from volcanoes on Io. How this complicated system actually works is a puzzle.
It’s a puzzle that members of the public will witness at close range thanks to JunoCam—a public outreach instrument modeled on the descent camera for Mars rover Curiosity. When Juno swoops low over the cloudtops, JunoCam will go to work, snapping pictures better than the best Hubble images of Jupiter.
“JunoCam will show us what you would see if you were an astronaut orbiting Jupiter,” says Bolton. “I am looking forward to that.”