For the first time ever, NASA’s Juno spacecraft has spotted electrons being fired down into Jupiter’s atmosphere at up to 400,000 volts. That’s an enormous amount of energy that gives rise to the planet’s glowing auroras. These incredibly high voltages, however, are only spotted occasionally — and that’s raising questions about what exactly is behind some of the planet’s most vivid glows at the poles.
The discovery, detailed in a study published today in Nature, was made possible by the instruments on board Juno, which has been orbiting Jupiter for a little over a year, passing by the poles closer than any other spacecraft has before. It confirms, in part, what astronomers expected, but it also shows that Jupiter’s auroras behave differently than auroras on Earth — through processes that we don’t fully understand yet.
Auroras, on both Earth and Jupiter, are formed when charged particles like electrons spiral down a planet’s magnetic field lines, entering the atmosphere and creating a glow. On Earth, the most intense auroras are caused by solar storms, which occur when high-energy particles ejected from the Sun rain down on our planet. When these particles enter the atmosphere, they interact with gases and make the sky glow red, green, and blue at the poles. On Jupiter, auroras are formed by particles ejected mostly from the Io, the planet’s moon. Io’s volcanoes spew huge amounts of sulfur and oxygen into space, loading Jupiter’s magnetic field with particles.
On both planets, electrons are accelerated along the magnetic field lines by electric currents — similar to the electric current that goes through the socket when you plug in your phone charger. On Earth, the solar wind is the power source, firing electrons at up to 30,000 volts. (In comparison, your socket is 110–120 volts.) On Jupiter, it’s the planet’s superfast rotation that acts as a gigantic electric generator, so astronomers expected electrons to be fired by very high voltages on Jupiter as well. But they had never observed this before, so Juno gave astronomers that opportunity for the first time.
“We’ve never flown right over the poles of Jupiter before,” says Jonathan Nichols, a professor in the Department of Physics and Astronomy at the University of Leicester, who did not take part in the study. “So Juno is telling us about those particles for the first time.”
The spacecraft is in an extremely elliptical orbit around Jupiter, passing very close to the poles every 53 days. To study Jupiter’s auroras, the probe was equipped with several instruments, including the Juno Energetic Particle Detector Instrument (JEDI). The probe is traveling at about 30 miles per second over the poles, so the measurements have to happen in a matter of seconds, says study co-author Barry Mauk, lead for JEDI and a scientist at the Johns Hopkins University Applied Physics Laboratory, which made the instrument. “That was a very substantial challenge,” Mauk tells The Verge. “We’re very proud of the fact that we were able to pull that off.”
On its first flyby over the auroras, however, Juno didn’t detect the high voltages astronomers expected. “We were very surprised,” Mauk says. Then, in following flybys, the spacecraft finally detected the signature of electrons being fired down the atmosphere all at about the same energy — as high as 400,000 volts.
The curious thing, though, is that these high voltages aren’t always there, Mauk says. They’re only spotted occasionally. And sometimes, Juno is spotting electrons being fired down the atmosphere with all different energies, in a seemingly random way. What’s causing this random acceleration of electrons at different energies — which create very bright auroras — is a mystery, Mauk says.
“It seems that the picture is not quite as clear as we thought,” Nichols tells The Verge. “I’m not quite sure how you drive auroras so bright with that particular mechanism. But that’s something that Juno will be looking at in the future.”
The probe is going to keep flying by Jupiter’s poles, and every time it does so, it collects data. “Every time we have an encounter, we see different things,” Mauk says. So Mauk is hoping that the next observations will help astronomers answer the questions of why the auroras are so variable, and why they are sometimes strong and sometimes weak.
The goal is not to only understand the physical processes behind auroras on the Solar System’s largest planet. Other objects around the Universe — like pulsars, exoplanets, and white dwarfs — also have magnetic fields, and they also accelerate particles in a way that can resemble Jupiter’s. But Jupiter is in our backyard, so it’s actually accessible. “Jupiter is not only interested in its own right, but it also tells us a great deal about similar astrophysical bodies that we can’t reach with spacecraft,” says Nichols.
And Juno is already revealing that there are unknown processes at work on the gas giant. “That to me is very exciting because it means we’ve got a lot more work to do to figure out what is exactly going on,” Nichols says. “Jupiter isn’t going to give up its secrets as lightly, it seems.”