Cracking the Mystery of Jupiter’s High-Energy ‘Northern Lights’
Both Earth and Jupiter throw off light when their magnetospheres funnel charged particles from space to both poles and those particles hit the atmosphere. On Earth, the night sky erupts with fluorescent colors in what’s known as the northern and southern lights, or aurorae. But no matter how spectacular the light shows here on Earth, Jupiter’s are arguably more dramatic: steady, where ours are intermittent, and far more powerful.
A fuller picture of Jupiter’s night sky is sketched out in a new Nature Astronomy study led by Gabriel Bridges, (CC'20), and Shifra Mandel, (GS'18), second-year PhD students in the lab of Columbia physics professor Charles Hailey and senior research scientist Kaya Mori, who are senior authors on the paper.
The data they use comes from the NuSTAR telescope launched a decade ago, and built at Columbia’s Nevis Laboratories. Though NuSTAR has spent most of the last decade tracking black holes and the remnants of exploded stars, it periodically recorded snapshots of Jupiter at Hailey and Mori's request.
Over a three-year period, NuSTAR captured the highest-energy radiation ever recorded on the gas giant. But this new picture of Jupiter’s aurorae puzzled the Columbia team. It seemed to contradict the Ulysses mission, which 30 years ago found no evidence of high-energy X-rays.
By comparing the NuSTAR data with electron data from a second probe, NASA’s Juno spacecraft, the Columbia team realized that Ulysses had missed the high-energy X-rays because the light fell beyond Ulysses' detection limit.
Through simulations, they further showed that the electrons flowing to Jupiter’s poles produced the X-rays that NuSTAR observed. Jupiter’s volcanically active moon, Io (pronounced “EYE oh”) is thought to be the source of those electrons.
“We now have the complete story of how high-energy X-rays produced by electrons on Io are accelerated by Jupiter’s magnetosphere and dispersed into its atmosphere,” said Mori, who led the NuSTAR observation campaign. “Such a detailed view of Jupiter’s night sky would have been unimaginable when Galileo discovered four moons orbiting the planet in the 17th century.”
The campaign was well on its way when Bridges and Mandel joined. They found the lab as undergraduates, but their interest in astronomy was sparked much earlier. In elementary school, Bridges remembers bringing a stack of photos shot from the Mars Rover to show-and-tell. For Mandel, it was the recycled rockets that SpaceX sent into space, and then recovered, that dazzled her. The mentoring and hands-on experience they received convinced them to stay on for their PhDs. Columbia News spoke with Bridges and Mandel about the Jupiter study, and tips they have for other astronomy enthusiasts.
What causes aurorae?
Gabriel Bridges: They arise when charged particles from space hit a planet’s magnetic field, and are diverted to the planet’s north and south poles. The magnetic field draws the particles in, and as they crash into the atmosphere, they slow dramatically and give off lots of electromagnetic radiation. On Earth, most of the radiation is visible light. If you’re far enough north (or south), you’ll see brilliant green curtains of light that are the result of electrons colliding with oxygen atoms in our atmosphere.
Shifra Mandel: The magnetic field that connects Earth’s north and south pole forms a protective barrier that prevents solar wind from eroding our atmosphere; it also protects life from potentially dangerous cosmic rays. The magnetosphere is constantly bombarded with charged particles; some penetrate the outer layers and are accelerated along the magnetic field lines toward both poles. As these energetic particles collide with Earth's atmosphere, they generate the aurorae.
What do aurorae look like from Jupiter?
GB: Interestingly, Jupiter’s northern lights are pretty unimpressive, at least to the human eye. The real light-show is happening at higher energies than we can see. The ultraviolet aurorae of Jupiter are brilliant, persistent features that can be seen in this visualization based on data from NASA’s Hubble Space Telescope. In this video, you can clearly see the magnetic footprint of Jupiter’s moon, Io. That bright point at the bottom right of the aurora is Io’s magnetic shadow.
SM: Io is constantly bombarding Jupiter with charged particles from volcanic eruptions at its surface. Those charged particles drive most of the X-ray emission we see. By contrast, Earth’s main source of ions comes from periodic solar storms. So, our northern lights are not continuous like Jupiter’s.
What’s so intriguing about Jupiter’s X-ray light?
GB: Jupiter’s magnetic field is 20 times stronger than Earth’s, and the most powerful in our solar system. If Jupiter’s magnetic field was visible at its widest point from Earth, it would appear three times bigger than our sun or moon. This means that Jupiter has unparalleled power to speed up and focus charged particles. So, you’d expect more energetic X-rays from Jupiter than Earth because its magnetic field generates so much energetic acceleration.
Jupiter’s closest moon also throws off a ton of ions and electrons each second. This gives Jupiter a constant supply of charged particles to power its aurorae. Back on Earth, we have to wait around for solar storms to drive charged particles into our atmosphere.
SM: If we stood on Jupiter observing Earth, we probably wouldn’t be able to see Earth’s northern lights. But Jupiter's aurorae are so much more powerful, that we’re able to observe them from the same vast distance.
What other planets in our solar system give off high-energy radiation?
GB: Planets emit high-energy radiation through aurorae and by reflecting X-rays thrown off by the sun. To produce an aurora you need three things: a magnetic field, an atmosphere, and a source of charged particles. Most planets in our solar system don’t meet these criteria. Mercury has no atmosphere. Mars and Venus have no magnetic field. Uranus and Neptune both have weak magnetic fields. That leaves Earth, Jupiter, and Saturn. We know Earth and Jupiter definitely have X-ray aurorae, but we’re not sure about Saturn — yet!
What’s the mystery at the heart of this paper?
SM: The Ulysses space probe flew by Jupiter in 1992 equipped with a detector for recording high-energy X-rays between 27 and 48 kilo-electronvolts (keV) but found nothing. This puzzled astrophysicists, who expected that the electrons producing ultraviolet radiation from Jupiter's aurorae would also produce energetic X-rays. The mystery deepened when the European Space Agency’s XMM-Newton telescope later recorded high-energy X-rays near the upper end of its detection limit, around 7 keV. The source of this X-ray emission wasn’t clear, but we were confident that NuSTAR, with its ability to detect radiation from 3 keV to 79 keV, could provide answers.
How did you solve it?
SM: The NuSTAR observations confirmed that Jupiter produces X-rays as high as 20 keV — far higher than what XMM-Newton is capable of detecting, but below Ulysses’ detection band, which is why Ulysses missed it. To test our suspicions that the X-rays detected by NuSTAR were generated by the electrons streaming into Jupiter's atmosphere, we looked at data from NASA's Juno space probe; as Juno orbits Jupiter, it records the levels of charged particles in its path. We simulated the effects of these particles traveling through, and colliding with, a Jupiter-like atmosphere. We discovered that the X-rays produced matched the radiation we saw with the NuSTAR telescope.
What should everyone know about astrophysics?
GB: It is not as pretentious and inaccessible as it may seem. Anyone can make an impact. All it takes is a lot of time and hard work. If you’re interested, find a researcher and ask to work with them.
SM: It's not easy, but if you love it, it's totally worth it!
Tips for other students debating a career in astronomy?
GB: I used to think that I couldn’t contribute to a research group until I knew X, Y, and Z about physics. If you’re seriously interested, the best time to start doing research is now. The second-best time is tomorrow! You won’t know what a career in astrophysics looks like until you try it.
SM: Find your niche—something you're passionate about, a topic that gets you excited to get out of bed in the morning.