NASA’s Juno spacecraft has been orbiting Jupiter for nearly a year now — and the space probe is revealing the gas giant to be more complex and surprising than we ever thought.
Juno’s instruments show that massive cyclones dominate the planet’s poles, while a deep tropical band of ammonia circles its equator. Meanwhile, the planet’s magnetic field is turning out to be much stronger than expected and the gravity field is indicating that Jupiter’s interior core may not be super dense. All these findings, published in two new studies today in Science, as well as 44 in the journal Geophysical Research Letters, will eventually help planetary scientists piece together the structure of Jupiter. And that could tell us a lot about how the planet formed billions of years ago.
These new tantalizing clues about Jupiter were gathered by Juno during the its first couple of passes by the planet. Right now, Juno is in an extremely elliptical orbit, which brings the probe screeching by Jupiter’s surface for a few hours at a time during each trip around the planet. These passes, known as Perijove passes, bring Juno over the planet’s poles — closer than any previous vehicle has gone before. And it’s during these swings by Jupiter that Juno gathers the bulk of its data.
Currently Juno does one Perijove pass every 53 days, though the original plan was one pass every 14 days. (Engine problems messed up the plan, so Juno is going to remain in its much longer orbit for the rest of its lifetime.) That just means it’s going to take longer to get answers about Jupiter than the mission team wanted. And the researchers are hesitant to make conclusions about Jupiter’s origins until the probe has passed by the planet a few more times.
“Juno is a mapping mission; we’re going to go over the planet 32 different times and each time at different longitudes,” Scott Bolton, of the Southwest Research Institute in San Antonio, Texas, and the principal investigator for NASA’s Juno mission, tells The Verge. “So we don’t want to make too many assumptions just yet. We need to finish that map.” However, Bolton is confident that the NASA spacecraft will be able to deliver some solid answers about Jupiter after a while. “Juno is the right tool to sort this out,” he says.
So here’s what Juno has found out so far about the largest planet in our Solar System:
Massive polar cyclones
During its first pass around Jupiter, Juno’s camera spotted immense storms and cyclones at the planet’s poles. The vehicle captured one giant storm right over the terminator — the moving line that separates day from night on a planet. That created shadows that allowed the mission team to calculate exactly where the storm sat, finding that it was actually sticking up out of the atmosphere. And this storm was massive, too, reaching 30 to 60 miles high with a diameter about half the width of Earth.
These cyclones make Jupiter’s poles a fairly unique place. Neither pole has the same number of storms or configurations, according to Bolton, and they look very different from the poles of Saturn. “When you get over the poles, it’s a very different looking planet,” he says. “I’m not sure you’d even recognize it as Jupiter if you didn’t know.”
An ammonia belt
Juno is specifically equipped with instruments that — for the first time — can peak underneath Jupiter’s cloud tops, to figure out the composition of the planet below. Measurements have revealed a super-deep band of ammonia around the planet’s equator — and it goes as deep into the planet as the spacecraft can see.
Bolton compares the ammonia band to the tropical band around Earth — the region around the equator that has its own distinct climate. “Earth’s band is driven by the fact that there’s a surface and ocean underneath, but Jupiter is all gas and yet we see the same kind of feature,” he says. And the ammonia isn’t just found at the equator either. Bolton says the concentration of ammonia is highly variable throughout Jupiter, making the planet’s composition incredibly diverse.
“The naive simple view of giant planets is that as soon as you scratched beneath the surface where the sunlight was no longer getting to, then everything would just be this boring well-mixed sphere of gas that all looked the same,” says Bolton. “And Jupiter is anything but that; it’s very complicated.”
A non-compact core?
A big goal of Juno has been to figure out if the planet has a compact core or not. So Juno has been mapping Jupiter’s gravity field to determine the distribution of materials inside the planet. And those measurements seem to indicate that Jupiter doesn’t have a smaller compact core like people expected, but instead the core is larger and more spread out. There’s also some indication that various regions of the core may be moving in different directions. “It may not be moving around like it’s one solid body,” says Bolton.
A variable magnetic field
Juno is also measuring Jupiter’s magnetic field, a charged current that surrounds the planet and deflects or captures particles coming from the Sun. In June, before Juno inserted itself into orbit, the spacecraft crossed through the boundary of the magnetic field — known as the bow shock. It was the only time that Juno ever encountered the bow shock, and that seems to suggest that the magnetic field was expanding when Juno encountered it.
Now that the spacecraft has been hanging out at Jupiter, it’s been taking measurements of the field from within. And one thing that Juno has noticed is that the closer it gets to Jupiter, the more variation it finds in the strength of the magnetic field. “When we get really close to the planet, we’re getting much higher highs and much lower lows than we would have anticipated based on our current knowledge,” Jack Connerney, an astrophysicist at NASA’s Goddard Space Flight Center and lead author on one of the Science papers, tells The Verge. And that provides some clues as to where inside the planet the magnetic field is being generated.
Earth’s magnetic field is thought to stem from the core. On our planet, hot liquid iron is constantly moving in the core, and this process — known as a dynamo — generates Earth’s massive magnetic field. Connerney says the closer you get to the dynamo’s movement, the more variation in the magnetic field you can see. From the surface of Earth, you don’t see much variation, though, because the dynamo is all the way down in the center of the planet. But since there seems to be a lot of variation near the surface of Jupiter, perhaps the planet’s isn’t super deep within the planet’s core. “The indication might be the dynamo core’s surface is much closer to the top than we had anticipated,” says Connerney.
A surprising aurora
During Juno’s trips over the poles, the spacecraft passes through one of the most stunning phenomena on Jupiter — its aurorae. The planet’s vivid aurorae are created when high-energy particles trapped in the magnetic field filter down to the poles and smash into the gas there.
Or at least that’s what everyone thought was happening. Connerney says Juno’s measurements indicate something else: it seems that the aurorae may be caused by high-energy electrons being sucked up out of the atmosphere. “What we’re find out is, guess what? Those electrons aren’t coming down; they’re going up,” says Connerney. “So it turned out our thinking was 180 degrees wrong, because we were never there to measure this stuff.”