One of the Earth’s great natural wonders is the Northern Lights or aurora borealis; delicate waves of colors that appear in the sky over polar regions when energetic particles from the sun interact with Earth’s magnetic field. But Earth isn’t the only planet to experience aurorae, as the strong magnetic fields of planets like Jupiter and Saturn mean that aurorae are observed there as well.
Aurorae aren’t just limited to Jupiter though but are also present on four of its moons: Io, Europa, Ganymede, and Callisto. Astronomers recently used the High-Resolution Echelle Spectrometer (HIRES) instrument at the W. M. Keck Observatory in Hawai’i to observe the moons while they were in the shadow of Jupiter, allowing them to see the faint aurorae.
“These observations are tricky because in Jupiter’s shadow, the moons are nearly invisible,” said one of the lead researchers, Katherine de Kleer of Caltech, in a statement. “The light emitted by their faint aurorae is the only confirmation that we’ve even pointed the telescope at the right place.”
Different colors of aurorae are created by different elements, and the researchers were able to see some green aurorae created by oxygen similar to those we see on Earth. But at low concentrations, oxygen produces a red aurora, and as these moons have extremely thin atmospheres they show aurorae that are 15 times more red than green.
And on Io, which has plumes of sodium chloride and potassium chloride coming from its volcanoes, its aurorae can have a yellow-orange color.
“The brightness of the different colors of aurora tell us what these moons’ atmospheres are likely made up of,” said de Kleer. “We find that molecular oxygen, just like what we breathe here on Earth, is likely the main constituent of the icy moon atmospheres.”
As aurorae occur when particles from the sun interact with a magnetosphere, you might expect that a moon would need a magnetic field of its own to experience these phenomena. But three of the moons in question — other than Ganymede — don’t have their own magnetic fields. However, the magnetic field of Jupiter is so strong that its effects reach out to its moons.
The magnetic field of Jupiter is also titled, so the field on the moons varies as the planet rotates, and that means their aurorae change in brightness over time.
Another change that can happen to the aurorae is when the atmospheres warm or cool as they exit or enter the shadow of Jupiter, an effect which was seen on Io.
“Io’s sodium becomes very faint within 15 minutes of entering Jupiter’s shadow, but it takes several hours to recover after it emerges into sunlight,” said another of the lead researchers, Carl Schmidt of Boston University. “These new characteristics are really insightful for understanding Io’s atmospheric chemistry. It’s neat that eclipses by Jupiter offer a natural experiment to learn how sunlight affects its atmosphere.”
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