Using WM Keck Observatory on Maunakea in Hawai’i, astronomers have detected new aurorae on all four of Jupiter’s largest moons: Io, Europa, Ganymede and Callisto. These aurorae are caused by the gas giant’s strong magnetic field.

Astronomers used Keck Observatory’s High-Resolution Echelle Spectrometer (HIRES) and high-resolution spectrographs from the Large Binocular Telescope and Apache Point Observatory to observe the moons in Jupiter’s shadow. This allowed them to see faint aurorae without competition from bright sunlight reflected off their surfaces.

Because Jupiter’s strong magnetic field is tilted, the aurorae on these moons change in brightness as the planet rotates. In addition, the atmosphere may respond to the rapid transition from warm sunlight to Jupiter’s cold shadow.

Katherine de Kleer, Caltech professor and lead author of one of the two new research papers, said: “These observations are tricky because in Jupiter’s shadow, the moons are almost invisible. The light emitted by their faint aurorae only confirms that we even pointed the telescope in the right place.”

The oxygen aurora we see in the sky near Earth’s poles is present on all four of the Galilean moons. But because the gases on Jupiter’s moons are significantly thinner, a deep red glows about 15 times brighter than typical green light.

At Europa and Ganymede, oxygen also emits a slightly redder glow at infrared wavelengths than is visible to the human eye. At Io, Jupiter’s innermost moon, volcanic plumes of gas and dust are huge, hundreds of miles high. These plumes contain salts such as sodium chloride and potassium chloride. Sodium gives Io’s aurora the yellow-orange glow seen in urban streetlights. The new measurements also show potassium aurora at Io in infrared light, which was not detected anywhere else.

de Kleer said, “The brightness of the different colors of aurora tell us what the atmospheres of these moons are likely made of. We are finding that molecular oxygen, like what we breathe here on Earth, is probably the main component of the lunar icy atmosphere.”

The latest observations provide little evidence for water, sparking a heated scientific debate about the presence of significant water vapor in the atmospheres of Jupiter’s moons. Jupiter’s outer three Galilean moons are believed to have oceans of liquid water beneath their thick icy coverings. There is tentative evidence that Europa’s atmosphere occasionally gets its water from an ocean or from fluid reserves in its ice sheet.

Carl Schmidt, professor of astronomy at Boston University and lead author of the second paper, said: “Io’s sodium becomes very weak within 15 minutes of entering Jupiter’s shadow, but takes several hours to recover after it appears in sunlight. These new features are insightful for understanding Io’s atmospheric chemistry. It’s nice that Jupiter eclipses provide a natural experiment to learn how sunlight affects the atmosphere.”

Magazine references:

  1. Katherine de Kleer et al. The optical aurorae of Europa, Ganymede and Callisto. The journal Planetary Science. DOI 10.3847/PSJ/acb53c
  2. Carl Schmidt et al. Io’s optic aurorae in Jupiter’s shadow. The journal Planetary Science. DOI 10.3847/PSJ/ac85b0