Scientists with the NASA Juno mission have released the findings from the spacecraft's first two orbits around Jupiter — and the results are unlike anything they expected.
Deep beneath its striped layers, Jupiter still has a surprising amount of structure that researchers had not predicted. The core, such as it may be, is not rocky and dense as they suspected, but large and diffuse. Even its auroras seem to work differently than Earth's.
The deluge of findings, described in two papers in Science and more than 40 more in Geophysical Research Letters, mark the mission's first major steps toward unlocking the mysteries of Jupiter, which lie at the heart of our understanding of planetary formation.
Launched in 2011, Juno entered orbit around the gas giant last year — the first satellite to do so since Galileo last circled it in 2003. The new spacecraft is bristling with instruments meant to probe the planet's magnetosphere, polar regions, composition and interior structure.
While Jupiter's iconic red storm, stripes and girth may loom large in the skies and in our minds, surprisingly little is known about it. Among the many questions Juno set out to answer: Does the gas giant have a solid metal core? Does it have any structure beneath its banded atmosphere, or are its depths well-mixed? How much water lies within its body? And what powers its auroras and its magnetic field?
Now, data from just the first two passes are already offering up surprising answers to some of those longstanding questions.
Researchers have largely fallen into two camps as to what lies at Jupiter's heart: a solid, metal-rich core, or mostly hydrogen. But the truth may lie somewhere in the middle.
"It does look like Jupiter has a core, but it's very large," said Scott Bolton, Juno's principal investigator based at the Southwest Research Institute. "It's diffuse; it's not as concentrated as we thought. We don't know exactly how it gets that way."
Many scientists had also supposed that Jupiter's beautiful bands were really only skin-deep, and that there was little structure deeper within the planet. That turned out to be off, too.
The Juno Microwave Radiometer, which probed hundreds of kilometers beneath the surface, found a remarkable level of organization — including a strange band of ammonia sitting right above the equator that extends deep into the planet. Just how deep remains unclear.
"What's always been assumed is that as soon as we drop below the cloud tops where the sunlight can't get to, that Jupiter inside would be kind of boring and all uniform and everything would be mixed together and it wouldn't matter where you looked, it would all look the same," Bolton said. "It turns out it's exactly the opposite."
In fact, this band of ammonia seems to have dynamics somewhat reminiscent of an atmospheric circulation pattern on Earth known as a Hadley Cell — even though that pattern requires a surface like Earth's land or oceans to work. Jupiter has no such known surfaces, as far as researchers know.
"You have to question now whether what we're seeing now is a general feature of all planetary atmospheres, or is it just a coincidence?" Bolton said. "Or does this equatorial band go so deep that it gets to some kind of transition region really far down in Jupiter?"
In some ways, the strange results from the interior are a validation of the probe Galileo, sent into Jupiter's depths back in 1995, whose surprising readings made scientists think the probe had landed in an anomalous "warm spot."
In reality, Bolton explained, if three probes had been sent to three different places, they'd probably all have ended up with three different measurements. Jupiter's complex internal life, it turns out, defies a single reading.
Juno's polar orbit has also allowed it to pick out flurries of whirling cyclones at the poles, unlike the orderly bands covering the rest of its body and in stark contrast to the elegant hexagon that sits atop Saturn. The scientists want to know whether these storms last weeks, or months, or even years.
"The north pole doesn't look like the south pole," Bolton said. "We don't completely understand that. Somehow all these cyclones get created and one of the questions is, are they stable over time?"
Even the auroras seemed to function differently than expected; the magnetic field, meanwhile showed an unprecedented level of structure when viewed up close.
Earth's magnetic field is powered by a dynamo created by the flow of liquid metal in its core. Jupiter's dynamo, scientists say, is produced by metallic hydrogen — hydrogen that's so deep in Jupiter that the pressure has squeezed its electrons off, allowing the compressed gas to behave like a metal.
But the results show that Jupiter's magnetic field might actually be generated higher up, in the shallower layer of molecular hydrogen (which is strange, since molecular hydrogen is a neutral compound).
"That's very significant," John Connerney, the mission's deputy principal investigator at NASA's Goddard Space Flight Center, and lead for Juno's magnetometer, said at a press briefing Thursday.
Scientists have discovered that Juno is more capable than they even planned. While staring endlessly at the stars, the spacecraft navigation camera caught high-speed dust particles smashing into its nearly 30-foot-long solar panels — allowing the researchers to study these tiny space missiles.
"It turns out that we can use this spacecraft … as the largest dust detector ever flown in space," Connerney said. "We're sensitive to particles that are larger and more infrequent than those that can be measured in space by a dedicated dust detector. So that's exciting."
These preliminary data highlight the need for scientific instruments that can get up close and personal with their subject, said F. Michael Flasar, a planetary scientist at NASA Goddard who was not involved in the paper.
"The lesson I take from this is, if you want to learn something about these complex systems, you have to look at them," Flasar said. "Because you're not going to figure it out from first principles. You have to have data that sort of constrains your imagination ... because you're going to see things you didn't expect."
The amount of water in the planet remains something of an outstanding question — and the answer could shed light on where and how Jupiter formed, and the early dynamic history of our solar system. That analysis may take a little longer, Bolton said.
In the meantime, he's already looking forward to the next flyby when Juno will be flying directly over the Great Red Spot.
"We're going to see what that thing looks like close up," he said.
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4:30 p.m.: This story was updated with additional information from John Connerney and F. Michael Flasar.