NASA’s Juno finds surprise radiation on Jupiter

Scientists using data from NASA’s Juno mission have found zones of unexpected radiation on Jupiter and have been able to peer deep into the planet’s Great Red Spot.

One of the newly discovered radiation zones lies at the equator, right above the top of the planet’s atmosphere. But it is not composed of the high-energy electrons that make up the bulk of Jupiter’s already identified radiation belts, Heidi Becker of NASA’s Jet Propulsion Laboratory, Pasadena, California said this week at a meeting of the American Geophysical Union in New Orleans, Louisiana. 

Instead, the newly discovered radiation zone is a separate belt of high-energy ions such as hydrogen, oxygen, and sulfur.

The ions were found by Juno’s Jupiter Energetic Particle Detector Instrument (JEDI), which was built to monitor radiation as the probe dives beneath the planet’s belts every 53 days. 

The find was a surprise, but Juno was designed for surprises, as its orbit carries it closer to Jupiter than any other spacecraft in history. “We are about 2,100 miles (3,400 kilometres) from the cloud tops at our closest approach,” says Becker.

Most likely, she says, these particles come from gas clouds near the icy moon of Europa and the sulfur-spewing volcanic moon of Io. They fall toward Jupiter, hit the atmosphere, and have their electrons stripped away, turning them into high-energy ions that then gather in the region where they were detected. 

”So, this gap between the clouds and the radiation belt isn’t a gap after all,” she says.

The other discovery came near the inner edge of the main radiation belts, which Juno grazes at the planet’s higher latitudes on each orbital pass. 

At the moment, all the scientists know is that the probe hit a belt of very energetic particles: even more energetic than those in the equatorial ion belt. What they are is isn’t clear because Juno’s instruments weren’t built to detect such things. Instead, the detections showed up when the radiation penetrated the shielding of the spacecraft’s star-tracker navigation instrument — the most heavily shielded device on the entire spacecraft. 

The Juno team was anticipating that the star tracker would experience occasional light flashes from high-energy electrons that got through its shielding. Jupiter’s radiation belts were known to be very potent.

What the scientists didn’t expect was that among these electrons would be particles hundreds of times more powerful. {%recommended 2040%}

The hits must be from some sort of heavy ion, but beyond that it’s a grab bag of guesses. “The species, and where they might come from, is something we’re still studying,” Becker says.

Meanwhile, on a close approach last July the spacecraft passed above the Great Red Spot — a giant storm larger than the Earth that is circled by winds moving 120 metres per second. It has persisted at least since telescopes were good enough to observe it, about 150 years ago, and is one of Jupiter’s great mysteries.

One of Juno’s goals, says Andy Ingersoll of California Institute of Technology, Pasadena, California, was to use microwave sensors to peer beneath the cloud tops to determine how deep the Great Red Spot’s roots descend. To do this, it uses six different microwave frequencies, each able to measure temperatures at a different depth. 

These sensors found a hot zone beneath the Great Red Spot that went to a depth of at least 350 kilometres, the deepest to which they could observe. “How deep it goes down beyond that is still to be determined,” Ingersoll says. He notes, however, that on future passes, Juno can peer even deeper via careful measurements of Jupiter’s gravity field. This, he says, will be affected by the density of the underlying gases, which in turn is affected by their temperature. 

The source of the Great Red Spot’s heat is not fully understood, although knowing how far down its roots extend will help scientists refine their models. Ingersoll’s favorite theory is that the Spot has been observed to “swallow” smaller storms, each of which contains energy. 

“It’s like a food chain,” he says. “A big fish eating little fish.” 

Another option is that the spot might be getting energy from its sides, one of which is closer to the equator, better warmed by sunlight, and the other closer to the pole. 

Meanwhile, the spacecraft is in good shape. It was designed to complete 32 orbits of Jupiter, then plunge into its atmosphere to avoid the risk of hitting one of Jupiter’s moons and contaminating it with Earth bacteria. Initially, these orbits were planned to take two weeks each, but an equipment problem caused NASA engineers to leave it in its current 53-day cycle, rather than risking an engine burn that might go wrong.

That slowed the science mission by a factor of about four but didn’t change much else, says Scott Bolton, the mission’s principal investigator. “All the science is enabled by the close passes,” he says, “so if you’re in a longer orbit, it’s pretty much the same.” 

Meanwhile, he says, Juno is in excellent health. “We haven’t seen any signs of degradation due to the radiation, which was one of our big fears.”

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