(ORDO NEWS) — Astronomers using the W.M. Keck Observatory on Mauna Kea have found that auroras at visible wavelengths appear on all 4 of Jupiter’s major moons: Io, Europa, Ganymede, and Callisto.
Using the HIRES spectrometer, as well as high-resolution spectrographs at the Large Binocular Telescope and Apache Point Observatory, a team led by Caltech and Boston University observed the moons in Jupiter’s shadow.
Their faint auroras, caused by the strong magnetic field of the gas giant, were clearly visible.
“These observations are difficult because in the shadow of Jupiter, the moons are almost invisible.
The light emitted by their faint auroras is the only confirmation that we have pointed the telescope in the right place,” says Katherine de Cleer, professor at the California Institute of Technology and lead author of one of two new research papers published in the journal Planetary Science.
All four of the Galilean moons show auroras. The gases on Jupiter’s moons are much thinner than those on Earth, allowing the dark red color to shine nearly 15 times brighter than the green light we are used to.
On Europa and Ganymede, oxygen emits infrared waves. Io, Jupiter’s innermost moon, is home to volcanic columns of gas and dust hundreds of kilometers high.
They contain salts such as sodium chloride and potassium chloride. Sodium gives Io’s aurora a yellow-orange glow.
The new measurements also reveal a potassium aurora on Io in infrared light that has not previously been detected anywhere else.
“The brightness of the different colors of the aurora tells us what the atmospheres of these moons are likely to be made of,” de Clier said.
“We found that molecular oxygen, exactly like the one we breathe here on Earth, is probably the main component of the moon’s icy atmosphere.”
The new measurements show minimal signs of water, fueling a vigorous scientific debate about whether the atmospheres of Jupiter’s moons contain significant amounts of water vapor.
Jupiter’s outer 3 Galilean moons are currently thought to contain oceans of liquid water beneath their thick icy surfaces.
Because Jupiter’s strong magnetic field is tilted, the auroras on these moons change in brightness as the planet rotates.
In addition, the atmosphere may be responding to the rapid transition from Jupiter’s warm sunlight to Jupiter’s cold shadow.
“Sodium Io becomes very weak within 15 minutes of entering Jupiter’s shadow, but takes several hours to recover after entering sunlight,” explains Carl Schmidt, professor of astronomy at Boston University and lead author of the second paper.
“These new characteristics are really important for understanding the chemical composition of Io’s atmosphere.
Remarkably, Jupiter’s eclipses are a natural experiment to learn how sunlight affects its atmosphere.”
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