NASA says radio transmission from Jupiter moon Ganymede isn't aliens

An FM radio signal has been picked up coming from one of Jupiter's moons, but - as usual - scientists are playing down the likelihood it's aliens. 

The five-second radio burst was detected last year when NASA's Juno probe "crossed a polar region of the giant planet where the magnetic field lines are connected to Ganymede" said Andrew Yau, editor of Geophysical Research Letters, the journal which published the findings.

Ganymede is bigger than Mercury and Pluto, and almost the size of Mars - but as it orbits Jupiter, it's considered a moon, not a planet. 

"Juno was travelling at a speed of approximately 50 kilometres per second, and it spent at least about five seconds crossing the source region of the emission, which was therefore at least about 250 kilometres in size," said Yau. 

"It's not ET," NASA's Patrick Wiggins told Fox affiliate KTVX in Salt Lake City. "It's more of a natural function."

Yau explained exactly what Juno detected.

"The radio emissions were produced by electrons... in a region where the electron's oscillation frequency ('plasma frequency') is much lower than its gyration frequency ('cyclotron frequency'). Such electrons can amplify radio waves very close to the electron cyclotron frequency very rapidly, via a physical process called electron cyclotron maser instability (CMI). 

"They can as well produce aurora in the far-ultraviolet - which was also observed by the camera on Juno."

Radio waves in the Jupiter system aren't new to science - similar decametric radio emissions (decametric meaning wavelengths in the tens of metres) were detected coming from another of Jupiter's moons, Io, in the 1960s. Ganymede's transmissions had been picked up from afar before, but this is the first time they've been measured in close proximity. 

Ganymede and Jupiter.
Ganymede and Jupiter. Photo credit: Getty

Though the Jovian radio transmissions have made headlines worldwide this week, the study which revealed the findings was actually published in Geophysical Research Letters in October last year. 

It's perhaps made headlines now after a wave of sightings of bizarre monoliths on planet Earth, resembling those featured in 1968 sci-fi movie classic 2001: A Space Odyssey - including one that was seen orbiting Jupiter.