Richard A Lovett
Planetary scientists studying the upper atmosphere of Jupiter have discovered unexpected details in bright spots in its aurorae, created by two of its largest moons, Io, and Ganymede.
Earth’s aurorae are its southern and northern lights, visible as shimmering bands of colour, particularly in the polar winters. But Earth isn’t the only planet to have them.
“Aurorae are quite common in the solar system,” says Alessandro Mura, a planetary scientist and astrophysicist at Italy’s National Institute for Astrophysics.
Earth, Jupiter, Saturn, Uranus, and Neptune are all known to have them. They are created when charged particles hit an upper part of a planet’s atmosphere known as its ionosphere.
“It’s not very different from a neon light,” Mura says. “Electrons hit the gas and excite it, then the gas shows luminescence.”{%recommended 6763%}
Aurorae are visible not only from the ground, but also from space, from which NASA’s Jupiter-orbiting Juno spacecraft carries an instrument called the Jovian Infrared Auroral Mapper (JIRAM), specifically designed to study them.
Earth’s moon is outside its magnetic field, far enough away to have no impact on our aurorae. But Jupiter’s field is so large that several of its moons lie within it. This means that they can affect the motion of electrons within that zone, Mura explains, focusing them into patterns that create auroral “footprints” visible to Juno.
Using JIRAM to look for these footprints was something Mura and his colleagues had long wanted to do, although they didn’t think they would have much luck. “They are really small,” he says.
But they got lucky and found them on their first try. In the process, they got a surprise. They’d expected each footprint just to be “a big dot” — exciting to scientists trying to understand Jupiter’s magnetic field, but hardly the stuff of late-night conversations.
Instead, they found something more complex. Io, for example, produced a trail of evenly spaced bright spots, each about the size of the moon itself.
“This was really unexpected, and we don’t have a clear theoretical model to explain it yet,” Mura says. He notes that it resembles the type of turbulence created downstream from an obstacle in a moving fluid, such as the pylons of a wind turbine in an ocean current.
“The shape of the footprint is clearly linked to the interaction that causes it,” he says.
But that was just the beginning. Sometimes the “tail” of Io’s spot had a forked, or double-winged shape. Ganymede, on the other hand, cast two footprints, one following the other as it swept across Jupiter.
All of this, Mura says, is telling us something about the the moons that created them. In the case of Io, he says, that moon is moving through Jupiter’s magnetosphere, spinning off vortices of charged particles that eventually hit the giant planet’s upper atmosphere.
Ganymede, on the other hand, is the largest moon in the solar system, with a diameter of nearly 5,300 kilometres is big enough for it to have its own magnetic field. “It’s the only satellite with its own magnetic field,” Mura notes.
Because that field lies within Jupiter’s, he says, Ganymede’s footprint reflects the shape of its magnetosphere.
“Basically we have a remote image of the magnetosphere of Ganymede,” he explains.
Lucyna Kedziora-Chudcze, a planetary astrophysicist at the University of New South Wales, in Sydney, Australia, says that this finding is a testament to Juno’s ability to study Jupiter with unprecedented resolution.
Her own work involves studying Jupiter’s aurorae with ground-based telescopes. “This detailed picture allows me to put my observations in context,” she says.
The new study appears in the journal Science.
