Caltech’s NuSTAR Mission Records Highest-Energy Light Ever Detected from Jupiter

A ‘Small Explorer’ mission led by Caltech and managed by JPL in Pasadena called NuSTAR has reported observations that reveal the highest-energy light ever detected from Jupiter. The light, in the form of X-rays that only NuSTAR so far can detect, is also the highest-energy light ever detected from a solar system planet other than Earth.

A paper in the journal Nature Astronomy reports the finding and solves a decades-old mystery: Why the Ulysses mission saw no X-rays when it flew past Jupiter in 1992, even when the planet’s auroras have long been known to produce low-energy x-ray light.

NASA’s NuSTAR, which stands for Nuclear Spectroscopic Telescope Array, launched on June 13, 2012. Managed by JPL for NASA’s Science Mission Directorate in Washington, it was developed in partnership with the Danish Technical University and the Italian Space Agency (ASI). The telescope optics were built by Columbia University; NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and DTU, the Technical University of Denmark. The spacecraft was built by Orbital Sciences Corp. in Dulles, Virginia. NuSTAR’s mission operations center is at UC Berkeley, and the official data archive is at NASA’s High Energy Astrophysics Science Archive Research Center.

The NuSTAR mission studies the universe in high energy X-rays to better understand the dynamics of black holes, exploding stars and the most extreme active galaxies. NuSTAR is the first hard-focusing X-ray telescope to orbit Earth and is expected to greatly improve on observations from ground-based observatories.

Before NuSTAR, NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton observatory have both studied low-energy X-rays from Jupiter’s auroras – light shows near the planet’s north and south poles that are produced when volcanoes on Jupiter’s moon Io shower the planet with ions (atoms stripped of their electrons). Jupiter’s powerful magnetic field accelerates these particles and funnels them toward the planet’s poles, where they collide with its atmosphere and release energy in the form of light.

NASA’s Juno spacecraft, which arrived at Jupiter in 2016 and is also managed by JPL, have observed that electrons from Io are also accelerated by Jupiter’s magnetic field. Researchers suspected that those particles should produce even higher-energy X-rays than what Chandra and XMM-Newton observed. NuSTAR is the first observatory to confirm that hypothesis.

“It’s quite challenging for planets to generate X-rays in the range that NuSTAR detects,” Kaya Mori, an astrophysicist at Columbia University and lead author of the new study, said. “But Jupiter has an enormous magnetic field, and it’s spinning very quickly. Those two characteristics mean that the planet’s magnetosphere acts like a giant particle accelerator, and that’s what makes these higher-energy emissions possible.”

Researchers faced multiple hurdles to make the NuSTAR detection: For one, the higher-energy emissions are significantly fainter than the lower-energy ones. But none of the challenges could explain why Ulysses, a joint mission between NASA and the European Space Agency and launched in 1990, did not detect those. Ulysses operated until 2009 after multiple mission extensions.

According to the new study, the solution to that puzzle lies in the mechanism that produces the high-energy X-rays. The light comes from the energetic electrons that Juno can detect with its Jovian Auroral Distributions Experiment (JADE) and Jupiter Energetic-particle Detector Instrument (JEDI), but there are multiple mechanisms that can cause particles to produce light. Without a direct observation of the light that the particles emit, it’s almost impossible to know which mechanism is responsible.

In this case, the culprit is something called “bremsstrahlung” emission. “Bremsstrahlung” means “braking radiation” in German. When the fast-moving electrons encounter charged atoms in Jupiter’s atmosphere, they are attracted to the atoms like magnets. This causes the electrons to rapidly decelerate and lose energy in the form of high-energy X-rays, like how a fast-moving car would transfer energy to its braking system to slow down.

Each light-emission mechanism produces a slightly different light profile. Using established studies of bremsstrahlung light profiles, the researchers showed that the X-rays should get significantly fainter at higher energies, including in Ulysses’ detection range.

“If you did a simple extrapolation of the NuSTAR data, it would show you that Ulysses should have been able to detect X-rays at Jupiter,” said Shifra Mandel, a Ph.D. student in astrophysics at Columbia University and a co-author of the new study. “But we built a model that includes bremsstrahlung emission, and that model not only matches the NuSTAR observations, it shows us that at even higher energies, the X-rays would have been too faint for Ulysses to detect.”

The conclusions of the paper relied on simultaneous observations of Jupiter by NuSTAR, Juno, and XMM-Newton.

“The discovery of these emissions does not close the case; it’s opening a new chapter,” William Dunn, a researcher at the University College London and a co-author of the paper, said. “We still have so many questions about these emissions and their sources. We know that rotating magnetic fields can accelerate particles, but we don’t fully understand how they reach such high speeds at Jupiter. What fundamental processes naturally produce such energetic particles?”

The new study is the first example of scientists being able to compare NuSTAR observations with data taken at the source of the X-rays (by Juno). This enabled researchers to directly test their ideas about what creates these high-energy X-rays.

Jupiter also shares a number of physical similarities with other magnetic objects in the universe – magnetars, neutron stars, and white dwarfs – but researchers don’t fully understand how particles are accelerated in these objects’ magnetospheres and emit high-energy radiation. By studying Jupiter, researchers may unveil details of distant sources that any Earth-initiated mission cannot yet visit.