Ganymede’s auroral ovals are interpreted as a moon orbiting Jupiter in this scientist’s theory. Credit: Illustration Credit: NASA, ESA, and G. Bacon (STScI); Science Credit: NASA, ESA, and J. Saur (University of Cologne, Germany)
On June 8, 2021, researchers used the Keck I telescope on the summit of Mauna Kea in Hawaii to see Jupiter’s moon Ganymede in the shadow of the gas giant. During the three-hour eclipse, they were able to take pictures of the aurorae visible in the lunar sky every five minutes, showing how they move and swell.
The team has now analyzed the data to look for changing conditions in Ganymede’s atmosphere, including whether water vapor was present. This question is very interesting because researchers have recently found aurorae in ultraviolet wavelengths from the water in your area – the first evidence that not only oxygen is present in the atmosphere of Ganymede, but also water molecules.
The new results were announced last month on The Planetary Science Journal.
Lighting up the atmosphere
Ganymede’s aurorae occur when electrons in Jupiter’s magnetosphere accelerate through gravitational waves and collide with molecules in the moon’s dense atmosphere. Strong impacts break the bonds, and the resulting particles emit light as they return to their original energy levels.
Based on the data collected by the team, most gases were detected as red and green light from oxygen atoms.
“The aurora we saw is the same kind of bright aurora we see on Earth,” says lead author Zachariah Milby, a PhD candidate in planetary science at the California Institute of Technology.
The researchers also measured the gases every few minutes to look for changes, marking the first time Ganymede’s aurorae had been studied in such detail.
One of the main results was that the emission of oxygen was almost twice as bright on the back side of the moon compared to the front side. The team used a computer model to check if the difference could be caused by more oxygen in the dusky side, but the results showed that such asymmetry could not explain the difference in emissions.
Milby says: “It’s possible that there are more complex processes going on, involving the flow of electrons that would be particularly exciting molecules at dusk than at dawn.
The researchers were also able to see the brightest change of the aurorae from the northern to the southern hemispheres.
Carl Schmidt, a planetary scientist at Boston University, says: “Imagine the northern lights of the Earth and the aurora australis flashing on and off every five hours.
A wet atmosphere?
Scientists first detected Ganymede’s aurorae from oxygen about 20 years ago. Oxygen molecules come from the surface of the moon’s ice and are released when ions from Jupiter’s moon Io hit the ice. Other molecules are also released by ice reduction and electron impact.
It was only in 2021 that scientists began to see gases coming from water molecules. The results showed that there was a large layer of water around the center of the side of the moon where the Sun was almost directly overhead.
But why the new results did not suggest the importance of water vapor in the region remains a mystery.
“There is strong evidence that ultraviolet emissions can come from water vapor,” says Schmidt, referring to the 2021 study.
He adds that perhaps the areas they saw in the new study were not warm enough to cover enough ice for water vapor to collect in Ganymede’s atmosphere.
Or maybe water vapor was present during the eclipse, but the lack of sunlight and freezing temperatures caused it to hold its own before the team made the first measurement.
Lorenz Roth, a planetary scientist at Sweden’s KTH Royal Institute of Technology, who led the 2021 study, said he wouldn’t be surprised if the liquid atmosphere froze in less than 10 minutes.
He says: “The knowledge of this unfortunate situation is still limited, because it is very difficult to take care of it.
Another possibility is that the image resolution of the Keck I telescope was not high enough to see regions in space where gravity might accelerate colliding electrons. and water molecules to produce such auroras.
Aspire
Roth and a team of researchers plan to use the Hubble Space Telescope this fall to see Ganymede in Jupiter’s shadow.
They also plan to include models of gas giants and the moon’s magnetospheres, which are the driving force behind the aurorae.
“That’s what they did well in their paper,” says Roth.
The biggest advantage of using Hubble is that it can observe radio waves. Thus, they will be able to detect aurorae before and after Ganymede is in shadow, enabling researchers to better compare the results and reconcile the differences.
“That will be exciting,” says Roth.
Meanwhile, “these two sides really fall on different wavelengths,” says Schmidt.
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