Glowing footprints Ganymede's journey around Jupiter causes spots to appear in the giant planet's spectacular electric blue auroral lights, scientists say.
The discovery, reported in Geophysical Research Letters, confirms the footprints are caused by Jupiter's moons passing through plasma in the planet's magnetic bubble, also known as magnetosphere.
"Aurorae are caused by charged particles, electrons, impacting atoms and molecules in the atmosphere," says the study's lead author, Dr Bertrand Bonfond of the Université de Liège, in Belgium, and the Southwest Research Institute, in Boulder Colorado.
"On Earth these particles come from the Sun. But they also occur on Jupiter and Saturn, where the phenomenon is the same, but the cause is totally different."
Aurorae on Jupiter are generated by particles spewed into the planet's magnetosphere by eruptions from its volcanic moon Io.
The particles in the magnetosphere of Saturn originate from geysers shooting out of the ice moon Escalades.
But astronomers have recently detected unusually bright regions in the Jovian auroral rings.
These regions called satellite footprints, are caused by orbiting moons affecting the charged particles travelling along the planet's magnetic field lines.
Scientists studying Jupiter's aurorae found the footprints would move and change their distance in relation to each other, as each of the moons orbit around the planet.
"We initially found there was one big footprint for each satellite," says Bonfond.
"But when we looked with more detail on the Io footprint, we found that it was actually made up of three spots, not one."
According to Bonfond, one spot is caused by charged particles erupting from Io's volcanoes, the second is generated by the acceleration of electrons along magnetic field lines, and the third is caused by particles reflecting off Io.
"Then the question was, 'why don't the other satellite footprints also have multiple spots?'" says Bonfond.
Using far ultra-violet images taken by NASA's Hubble Space Telescope, Bonfond and colleagues have been able to detect at least two spots in the Ganymede footprint.
They also found that the distance between the two spots varies as Ganymede orbits around Jupiter.
"Looking at Ganymede we see two spots, but there should be others that are too faint to be seen because we are at the limit of our instruments detection ability," says Bonfond.
Bonford is looking forward to the arrival of NASA's Juno spacecraft at Jupiter in 2016.
"Then we should see multiple spots for all the other moons," he says. "I'm pretty sure Callisto will have footprints as well, the hunt is on, but we haven't found it yet."
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