A sightseeing alien touring our solar system would do well to check out the emerald and blood orange-red ribbons of Earth’s auroras. But our world isn’t the only one with spectacular light shows in its atmosphere. New research shows auroras can also be seen on the Galilean moons of Jupiter: hypervolcanic Io, icy Europa, quirky Callisto and gigantic Ganymede.
Auroras exist throughout the cosmos, but often in wavelengths that human eyes cannot see. Astronomers in two papers published Thursday in The Planetary Science Journal are the first to report that if you were to stand on the surface of Ganymede and Callisto, visible auroras would be dancing above. The researchers also describe the existence of new types of visible auroras on Io and Europa.
The researchers captured the new auroras by peering through telescopes in Hawaii, Arizona and New Mexico. The team spent years watching the satellites repeatedly get swallowed by Jupiter’s shadow, then re-emerge.
Catching elusive and ephemeral glows on distant, diminutive and fast-moving moons can be difficult. But “the aurorae are always there when you observe an eclipse,” said Katherine R. de Kleer, a planetary astronomer at the California Institute of Technology and one of the authors of both studies. The researchers tracked down the darkness-smothered moons frequently enough to reveal the novel skyglows.
Galilean auroras are generated differently than those on Earth. Magnetic fields and energetic particles ejected from the sun reach Earth and are ensnared by our planet’s magnetic bubble. Those particles plunge toward the north and south magnetic poles and crash into gas molecules in the upper atmosphere, briefly energizing them and liberating various hues of visible light.
But other than Ganymede, the big moons of Jupiter lack magnetic bubbles. Instead, their auroras owe their existence to Io. Its noxious atmosphere — partly supplied by the moon’s epic volcanic eruptions — regularly sheds into space. The castoffs mingle with sunlight and become electrically excited. Plenty gets captured by Jupiter’s colossal magnetic bubble, but some of it slams back into Io’s atmosphere, or into the other three moons’ gassy sheaths. Those impacts are what ignite the moons’ auroral lights.
The researchers confirmed that all four moons’ skies glow in green and red hues similar to those seen on Earth. Although less luminous than our own world’s auroras, these otherworldly displays have their own beguiling features.
The moons’ extremely thin atmospheres make their red auroras shine far more intensely than their green ones do. Io, the oddest Galilean satellite, has a streetlamp-like yellow-orange radiance. This light show dims shortly after Io enters Jupiter’s shadow. After re-emerging, and bathing for a few hours in sunlight, this sickly amber glow burns brightly again.
As on Earth, the emergence of specific colors depends in part on an aurora’s altitude. But Io has an unusual mix of auroral pigments. “You would get a rainbow effect,” said Carl A. Schmidt, a planetary astronomer at Boston University and an author of both studies. Orange and red hues may be everywhere, like different paint tinctures smeared together, but green would be found only when an aurora occurs very high up.
These exotic fireworks are impressive. But the underlying scientific goal of this work was to reveal the compositions and behaviors of these moons’ atmospheres.
The colors in the moons’ auroras provide clues to the ingredients in each world’s atmosphere. The dominant atmospheric component of Callisto, Europa and Ganymede is molecular oxygen, which explains their newly discovered green and red auroral tints. Io’s sickly orange comes from sodium compounds, while a crimson light described for the first time in one of the new papers comes from potassium.
These visible auroras also suggest that little water vapor exists in the atmospheres of Europa and Ganymede, a finding that is surprising considering that Europa, and probably Ganymede, contain vast liquid water oceans beneath their frigid carapaces.
The Jupiter Icy Moons Explorer, a European Space Agency spacecraft that may launch in April, could help resolve this enigma. “Perhaps with a closer look, we can definitively figure out how much, if any, water is in the atmosphere of Europa,” said James O’Donoghue, a planetary astronomer at the Japan Aerospace Exploration Agency who is not involved with the studies.
Confounding chemistries aside, any astronaut photographing these Galilean glows — backgrounded by tempestuous Jupiter and the gas giant’s own kaleidoscopic auroras — would capture a memorable sight. Above all else, these studies demonstrate that beauty is not unique to Earth. If we are willing to search for it, enchanting scenes can be found anywhere in the cosmos.
