Jupiter’s Moons Make Ghostly Auroral “Footprints”

UV image of Jupiter taken with the Hubble Space Telescope Imaging Spectrograph (STIS) on November 26, 1998. (NASA/ESA, John Clarke (University of Michigan)) — *Aurorae seen in a UV image of Jupiter taken with the Hubble Space Telescope Imaging Spectrograph (STIS) on November 26, 1998. (Credit: NASA and the Hubble Heritage Team)*

We have all marveled at incredible photos and time-lapse videos of Earth’s auroral displays, captured by talented photographers that have braved the frigid nighttime temperatures of remote high-latitude locations as well as by those privileged few living in orbit aboard the International Space Station. But our planet isn’t the only one with curtains of light crowning its poles – aurorae have been observed on Jupiter, Saturn, Uranus, and Neptune as well (and Venus may even have them too.) While we know that these light shows are caused by interactions between atoms in planets’ upper atmospheres and charged particles from the Sun that get caught up in magnetic fields focusing out from around the poles, Jupiter in particular is known to have a peculiar additional type of auroral feature, created by the moons that orbit it.

The image above, captured by the Hubble Space Telescope Imaging Spectrograph (STIS) on November 26, 1998, shows Jupiter’s north polar region in ultraviolet light. The planet’s energetic aurorae can be seen wrapping around its pole in wavy circular arcs, just like Earth’s does. But there are also several bright spots that aren’t due to solar activity but are instead the “footprints” of three of its largest moons: Ganymede, Europa, and Io.

The moons’ auroral footprints can be seen in the image from Io (along the left hand limb), Ganymede (near the center), and Europa (just below and to the right of Ganymede’s auroral footprint). These emissions produced by electric currents generated by the satellites flow along Jupiter’s magnetic field, bouncing in and out of the upper atmosphere. They are unlike anything seen on Earth.

Io's erupting volcanoes seen by New Horizons on March 2, 2007. (Source). Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute. — Io’s erupting volcanoes seen by New Horizons on March 2, 2007. (Source). Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Io’s footprint in particular is exceptionally bright because of the material ejected into orbit from its volcanic eruptions, caught up in huge plasma loops that connect the moon with Jupiter’s magnetic field and atmosphere.

And the footprints aren’t just made up of one spot, but rather multiple areas of emission for each moon located both “downstream” (and in the case of Io, “upstream“) of the main spot.

“We initially found there was one big footprint for each satellite,” said Dr. Bertrand Bonfond of the Université de Liège in Belgium and the Southwest Research Institute in Boulder Colorado. “But when we looked with more detail on the Io footprint, we found that it was actually made up of three spots, not one.” (Source)

(Io has even been observed by the Galileo spacecraft to have its own type of glowing aurorae! Read more here.)

When NASA’s Juno spacecraft arrives at Jupiter in 2016 it will make much more detailed investigations of its aurorae, magnetic field, plasma environment, and the effects of its moons on each.