NASA’s Juno mission has already made mincemeat of precedents and expectations. When it arrived at Jupiter last July after a five year journey, it was further from Earth than any solar-powered craft had ever been, and traveling faster than any other human-crafted object had before. Its flight path skims closer to the storm-torn gas giant than any orbiter preceding it. And it’s the first spacecraft to pass over Jupiter’s mysterious poles—finding, counter to most assumptions, that they’re blue, and lack the planet’s characteristic stripes.
Juno isn’t done with firsts, or with sending scientists back to their whiteboards. Scientists have been poring over the data Juno collected in its first cloud-grazingly close pass over Jupiter last August, and today published two papers on what they’ve discovered about Jupiter’s auroras, atmosphere, and magnetic and gravity fields. And not only are Jupiter’s atmospheric dynamics less Earth-like than scientists thought, they’re also far more complex and variable. That means if scientists want to fully understand planets, a single probe might give incomplete, misleading information. Luckily for Jupiter scientists, Juno—with its many, closely spaced orbits designed to map the whole planet—is the right tool for the job.
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Let’s start at the top, in the upper atmosphere with Jupiter’s auroras. Scientists already knew that Jupiter’s auroras make the Northern Lights look like a flicker: They’re hundreds of times more energetic, and cover more area than the entire planet Earth. Juno uses several instruments to look at the auroras’ energized particles and the physics controlling their dynamics, and if the data from this first close pass is any indication, they’ll only continue to diverge from Earth’s light shows. “It’s very tempting to interpret what you see on another planet based on Earth,” says Jack Connerney, an astrophysicist at NASA’s Goddard Space Flight Center and author of one of the papers. “Up until last week, our models of Jupiter’s auroras had the electrons going in the wrong direction.” On Earth, electrons in the planet’s magnetic field get excited by solar wind and then funneled toward the poles, where they bang into other atoms and molecules and emit light. On Jupiter, Juno’s instruments have found that electrons actually get excited when pulled out of polar regions.
On top of that, it seems like planetary scientists had Jupiter’s atmospheric dynamics wrong in general. “Scientists thought the main energy source in the atmosphere would be the sun,” says Scott Bolton, Juno’s principal investigator and lead author of the other paper. “So they assumed that once we dropped below the sunlight that the particles would be simple and well mixed.” That turns out not to be the case: The particles in Jupiter’s atmosphere are just as diverse and banded as the planet’s famously stripey exterior. Particularly interesting to Juno’s team is a massive equatorial band of ammonia that extends hundreds of kilometers down toward the planet’s core—as far as Juno’s instruments can see. According to even the most up-to-date models of Jupiter’s atmosphere, there’s no reason it should do that.
Another area showing surprising amounts of activity? The deeps of Jupiter’s atmosphere: the magnetic and gravity fields Juno intends to map. “If Jupiter is just a big, rotating gas ball, it should not have any odd harmonics in its gravity field,” says Connerney. But Jupiter’s gravity isn’t uniform, which might suggest deep convection—density differentials deep inside Jupiter might drive gravity fluctuations the same way atmospheric pressure differentials drive weather on Earth. Juno’s readings of the planet’s magnetic field were also much more geographically variable than scientists expected.
Juno’s team is still a long ways from understanding why Jupiter’s atmosphere is so all over the place, though Connerney ventures that the fluctuations may all be connected, with the deep convection expressed in the gravity field also driving the uneven magnetic field strength. “In hindsight, it’s hard to imagine why would we have ever thought it would be simple and boring,” Bolton says.
Understanding Jupiter’s atmosphere in more detail may help scientists come to grips with some of Earth’s weirder traits. Bolton compares Jupiter’s equatorial ammonia to the tropical band around Earth’s own equator. “The concept we have on Earth is that that band gets developed because the air has an ocean to bounce off,” Bolton says. “Jupiter doesn’t, so why would it look the same? We may be learning something fundamental about atmospheres. Maybe our assumptions about Earth are wrong.”
The same information transfer could apply to Earth’s magnetic field—which is tough to study because it’s generated deep under the crust and somewhat occluded by random iron deposits. Jupiter has no crust, and no extra magnets to gunk up sensor data. “This will be our first time looking down on a real functioning magnetic dynamo,” Connerney says. “Maybe we should have started with Jupiter.”
All of these discoveries are challenging conventional space wisdom—and not just because of their results. Typically, scientists send a probe to a planet first, and follow it up with an orbiter equipped with all the doodads the probe data suggests they’ll need. “Our concept of how Jupiter and giant planets work that developed over the last few decades was probably oversimplified,” Bolton says. “Maybe we need to stop sticking in a probe and thinking we’re going to accurately sample a whole planet.”
Bolton’s answer to this problem—develop more Juno-style missions with lots of orbits designed to map a planet in its entirety—befits his role as, well, the leader of a Juno-style mission. But he’s probably right. The good news is that Juno has Jupiter covered. If scientists learned this much from the mission’s first close brush with the planet, imagine what will come of the next 36.
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