NASA's Juno spacecraft, which reached the planet Jupiter in July 2016 after a five-year, 1.7-billion-mile (2.7-billion-kilometer) journey, is exploring our solar system's biggest planet. It's taking advantage of a polar orbit that allows it to swoop down within 3,100 miles (4,990 kilometers) of the immense world's cloud tops. Imagine it this way: If Jupiter were the size of a basketball, Juno would be only about a third of an inch away.
That's the closest that any spacecraft has gotten to the immense planet without smashing into it, and it's a chance to take a more detailed look than ever before possible at an immense, faraway object. And as scientists analyze the initial data from the $1.13 billion mission, they're discovering that the gas giant appears to be significantly different than anyone would have imagined.
"Jupiter is not as simple as we thought it was," says Scott Bolton, a scientist at the Southwest Research Institute (SWRI) who is Juno's principal investigator. He describes the planet as having much more structure, variability and motion than scientists had envisioned. "Nobody expected Jupiter to have so much complexity, [and] so deep."
As this SWRI press release details, Juno has eight scientific instruments designed to study the planet's magnetic field, atmosphere and interior structure, and they show that Jupiter is far more than just a giant ball of gases.
"Until Juno's arrival, our understanding of Jupiter's atmosphere was based on what we can see from the side, near the equator," says Steven M. Levin, a Juno project scientist at NASA's Jet Propulsion Laboratory, in an email. "From that angle, Jupiter appears dynamic, but organized. Large belts and zones form a consistent pattern of jet streams which give the planet its familiar striped appearance. The new polar images from Juno look nothing like that. Around the poles, Jupiter looks chaotic, without an obvious, stable structure, and certainly without the regular stripes we see near the equator."
"We don't really know what this means yet," Levin says. "It could tell us something important about how Jupiter's atmosphere circulates, about how the great depth of Jupiter's atmosphere affects its weather patterns, or about how the heat escapes from Jupiter's interior. "
Juno's observations also show that at Jupiter's equator, the planet has a narrow, ammonia-rich plume, which resembles a bigger version of the air currents rising from Earth's equator to generate the trade winds.
Additionally, as this NASA press release details, Juno's observations reveal that Jupiter's magnetic field — already known to be the most intense in the solar system — is even stronger than previously estimated, and uneven, with lumpy areas of relative strength and weakness. Bolton says that data, coupled with measurements of the planet's gravity field, differing from projections points toward a planet whose interior has more complexity and motion than anyone would have expected.
"The magnetic field does that at a higher level in the atmosphere, but the gravity field tells us that it's going on a lot deeper, in the middle of the planet, or even deeper than that," Bolton says.
"Our first magnetic field results suggest a magnetic dynamo region which is closer to the surface of the planet than anyone predicted, and perhaps more complicated than expected as well," Levin explains. "The first gravity measurements suggest the possibility of deep winds, reaching thousands of kilometers into the planet, and/or a central core which is larger and less distinct than expected."
Both Bolton and Levin say that the initial data already suggests that scientists may have to rethink their previous notions not just about Jupiter, but about how other gas giants work. They expect more insights to come from the information gathered by the satellite as it passes over Jupiter every 53 days, in a mission that Bolton says may go on for another three years.
"Along with the microwave radiometer measurements, which have also shown surprises in the deep atmosphere, these results demonstrate that if we want to understand giant planets, we will need to study all of Jupiter," Levin says. "That means from the top of its atmosphere all the way to the core at its center, from the north pole to the south pole and everything in between, and across the longitudes all around the planet. That's Juno's mission, and we're off to a fascinating start."