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| Artist’s depiction of oxygen aurora on Jupiter’s moon Ganymede, the largest moon in the solar system, as observed from Maunakea on Hawaiʻi Island using the twin Keck Observatory telescopes. Credit: Julie Inglis |
Astronomers using W. M. Keck Observatory on Maunakea in Hawaiʻi have
discovered that aurorae at visible wavelengths appear on all 4 major moons
of Jupiter: Io, Europa, Ganymede, and Callisto.
Using Keck Observatory's High-Resolution Echelle Spectrometer (HIRES) 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 so that their faint aurorae, which
are caused by the gas giant's strong magnetic field, could be spotted
without competition from bright sunlight reflected off of their surfaces.
"These observations are tricky because in Jupiter's shadow the moons are
nearly invisible. The light emitted by their faint aurorae is the only
confirmation that we've even pointed the telescope at the right place," says
Katherine de Kleer, Caltech professor and lead author of one of two new
research papers published today in The Planetary Science Journal describing
the discovery.
All four of the Galilean moons show the same oxygen aurora we see in skies
near the Earth's poles, but gases on Jupiter's moons are much thinner,
allowing a deep red color to glow nearly 15 times brighter than the familiar
green light.
At Europa and Ganymede, oxygen also lights up infrared wavelengths, just a
little redder than the human eye can see—the first occurrence of this
phenomenon seen in the atmosphere of a body other than Earth.
At Io, Jupiter's innermost moon, volcanic plumes of gas and dust are vast in
size, reaching hundreds of kilometers in height. These plumes contain salts
like sodium chloride and potassium chloride, which break down to produce
additional colors. Sodium gives Io's aurora the same yellowy-orange glow
that we see in urban streetlamps. The new measurements also show potassium
aurora at Io in infrared light, which has not been detected anywhere else
previously.
"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."
The new measurements show minimal evidence for water, fueling an active
scientific debate over whether the atmospheres of Jupiter's moons feature
significant water vapor. It's currently believed that the outer 3 Galilean
moons of Jupiter contain oceans of liquid water beneath their thick icy
surfaces, and there's tentative evidence that water in Europa's atmosphere
may sometimes be sourced from its ocean or liquid reservoirs within its ice
shell.
Since Jupiter's strong magnetic field is tilted, aurorae on these moons
change in brightness as the planet rotates. Additionally, the atmospheres
can respond to the rapid transition from warm sunlight to the cold shadow of
Jupiter.
"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," explains Carl Schmidt, Astronomy Professor at Boston University
and lead author of the second paper. "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."
New types of aurora on the four moons add an exciting aspect to what is
already a golden age for fans of Jupiter thanks to NASA's Juno mission and
the James Webb Space Telescope. If you're lucky enough to see the aurora
here on Earth, pause to consider how amazing the show might appear if you
were looking up from one of Jupiter's moons.
The first paper about this research, led by de Kleer, is titled "The Optical
Aurorae of Europa, Ganymede, and Callisto." The second paper, led by
Schmidt, is titled "Io's Optical Aurorae in Jupiter's Shadow."
Reference:
Katherine de Kleer et al, The Optical Aurorae of Europa, Ganymede, and
Callisto, The Planetary Science Journal (2023).
DOI: 10.3847/PSJ/acb53c
Carl Schmidt et al, Io's Optical Aurorae in Jupiter's Shadow, The Planetary
Science Journal (2023).
DOI: 10.3847/PSJ/ac85b0
Tags:
Space & Astrophysics