NASA Detects Highest Energy Light From Jupiter

Scientists have been studying Jupiter up close since the 1970s, but the gas giant is still full of mysteries. New observations by NASA’s NuSTAR space observatory have revealed the highest-energy light ever detected from Jupiter. The light, in the form of X-rays that NuSTAR 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.

X-rays are a form of light, but with much higher energies and shorter wavelengths than the visible light human eyes can see. NASA’s Chandra X-ray Observatory and the ESA (European Space Agency) 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.

Electrons from Io are also accelerated by the planet’s magnetic field, according to observations by NASA’s Juno spacecraft, which arrived at Jupiter in 2016. Researchers suspected that those particles should produce even higher-energy X-rays than what Chandra and XMM-Newton observed, and NuSTAR (short for Nuclear Spectroscopic Telescope Array) is the first observatory to confirm that hypothesis.

“It’s quite challenging for planets to generate X-rays in the range that NuSTAR detects,” said Kaya Mori, an astrophysicist at Columbia University and lead author of the new study. “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 example, the higher-energy emissions are significantly fainter than the lower-energy ones. But none of the challenges could explain the nondetection by Ulysses, a joint mission between NASA and ESA that was capable of sensing higher-energy X-rays than NuSTAR. The Ulysses spacecraft launched in 1990 and, after multiple mission extensions, operated until 2009.

The solution to that puzzle, according to the new study, 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. 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. It’s like how a fast-moving car would transfer energy to its braking system to slow down; in fact, bremsstrahlung means “braking radiation” in German. (The ions that produce the lower-energy X-rays emit light through a process called atomic line emission.)

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.