Astronomers from the WM Keck Observatory on Maunakea, Hawaii, discovered that auroras in the visible part of the wave appear on Jupiter’s 4 main moons, Io, Europa, Ganymede and Callisto. A team led by Caltech and Boston University has observed moons in Jupiter’s shadow so that faint auroras caused by the gas giant’s strong magnetic field can be seen without competing with the bright sunlight reflecting off their surfaces. They used the Keck Observatory’s High Resolution Echelle Spectrometer (HIRES), the Transverse Scattering Grid Echelle Spectrograph, and high-resolution spectrographs from the Large Binocular Telescope and Apache Point Observatory.

“These observations are difficult because the moons are almost invisible in Jupiter’s shadow. The light emitted by their faint aurora is the only proof that we’re pointing the telescope in the right place,” said Catherine de Clair, a professor at Cal Institute of Technology and lead author of one of the two new science papers published in February. says. 16, 2023 Planetary Scientific magazine describes the discovery.

All four of the Galilean moons exhibit the same oxygen aurora we see in the sky near Earth’s poles, but the gases on Jupiter’s moons are much more subtle, allowing the dark red to shine about 15 times brighter than normal green light.

At Europa and Ganymede, oxygen also emits infrared wavelengths slightly redder than what the human eye can see – the first phenomenon seen in the atmosphere of an extraterrestrial body.

On Io, Jupiter’s innermost moon, clouds of volcanic gas and dust are huge and reach hundreds of kilometers high. These hairs contain salts such as sodium chloride and potassium chloride, which decompose to produce additional colors. Sodium gives Io’s aurora the same yellow-orange glow we see in city streetlights. The new measurements also show a never-before-seen potassium aurora over Io in infrared light.

“The brightness of the different colors of the aurora tells us what these moons’ atmospheres are likely made of,” De Clair said. “We found that molecular oxygen, as we breathe on Earth, is a key component of the icy moon’s atmosphere.”

The new measurements show little evidence for water and spark an active scientific debate over whether the atmospheres of Jupiter’s moons contain significant amounts of water vapor. Jupiter’s 3 outer Galilean moons are now believed to contain oceans of liquid water beneath their thick icy surfaces, and there is preliminary evidence that water in Europa’s atmosphere may sometimes come from the ocean or from liquid reservoirs in its icy crust.

Because Jupiter’s strong magnetic field is inclined, the brightness of the auroras on these moons changes as the planet rotates. Also, the atmosphere may react to the rapid transition from warm sunlight to Jupiter’s cold shadow.

“Sodium Io becomes very faint within 15 minutes of entering Jupiter’s shadow, but takes several hours to recover after exposure to sunlight,” explains Carl Schmidt, professor of astronomy at Boston University and lead author of the second paper. “These new features are really useful for understanding the chemistry of Io’s atmosphere. It’s great that Jupiter’s eclipses provide a natural experiment to learn how sunlight affects its atmosphere.”

Thanks to NASA’s Juno mission and the James Webb Space Telescope, new types of aurora on four moons add an exciting dimension to what was already a golden age for Jupiter fans. If you’re lucky enough to see the aurora borealis here on Earth, just think how amazing the spectacle will look when you look up from one of Jupiter’s moons.