Discovering a phenomenon that occurs on Jupiter's major moons, similar to that on Earth
Astronomers have discovered that the aurora borealis appear in visible wavelengths on all four of Jupiter's major moons: Io, Europa, Ganymede and Callisto.
Using the High-Resolution Eckel Spectrometer (HIRES) of the W.M. Keck Observatory in Maunakea, Hawaii, as well as the high-resolution spectrometers of the Large Binary Telescope and the Apache Point Observatory, a team led by Caltech and Boston University observed the moons in Jupiter's shadow, so that The faint aurora produced by the gas giant's strong magnetic field are uncompeted by the bright sunlight reflecting off their surfaces.
“These observations are difficult because in Jupiter’s shadow the moons are almost invisible,” says Catherine De Claire, a Caltech professor and lead author of one of two new papers published in The Planetary Science Journal describing the discovery. The light from the faint aurora is the only confirmation that we have pointed the telescope to the right place.”
All four of the Galilean moons show the same oxygen auroras we see in the sky near Earth's poles, but the gases on Jupiter's moons are much lighter, allowing the deep red color to glow about 15 times more than the familiar green light.
On the moons Europa and Ganymede, oxygen also glows at infrared wavelengths, which are slightly redder than what the human eye can see, the first occurrence of this phenomenon seen in the atmosphere of a body other than Earth.
In Io, the closest Galilean moon to Jupiter, large plumes of gas and volcanic dust reach hundreds of kilometers high. These columns contain salts such as sodium chloride and potassium chloride, which decompose to produce additional colors.
The sodium gives Io's aurora borealis the same orange-yellow glow we see in urban street lights. The new measurements also show potassium in Io's aurora borealis in infrared light, which has not been detected anywhere else before.
“The brightness of the different colors of the aurora borealis tells us what the atmospheric components of these moons are likely to be. "We found that molecular oxygen, just like what we breathe here on Earth, is probably the main component of the icy moon's atmosphere."
The new measurements show little evidence of water, fueling a lively scientific debate about whether the atmospheres of Jupiter's moons contain significant water vapor.
It is now believed that the three outer Galilean moons of Jupiter contain oceans of liquid water beneath their thick icy surfaces, and there is tentative evidence that water in Europa's atmosphere may sometimes be obtained from the ocean or liquid reservoirs within its icy cover.
Because Jupiter's strong magnetic field is tilted, the aurora borealis on these moons change in brightness as the planet rotates.
In addition, the atmospheres can respond to the rapid transition from the warm sunlight to the cool shadow of Jupiter.
“Sodium Io becomes very dim within 15 minutes of entering Jupiter’s shadow, but it takes several hours to recover after emerging into sunlight,” explains Carl Schmidt, professor of astronomy at Boston University and lead author of the second paper. These new properties are really insightful for understanding the chemistry of Io's atmosphere. It's great that the eclipse by Jupiter provides a natural experiment to see how sunlight affects its atmosphere.”
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