The Juno spacecraft had barely arrived at the planet Jupiter when it began picking up a sound that some described as monstrous roar. Is this a natural phenomenon? A Jupiterian burglar alarm? A bad sign?

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What WAS that?

According to NASA reports, the sound was first heard by Juno’s auditory sensors as the spacecraft was sliding into home on June 24th after completing its five-year trip to the giant planet. The Waves instrument - a radio/plasma wave experiment – began picking up radio signals as Juno entered the outer edge of Jupiter’s magnetic field. When NASA scientists converted the signals to audio, a strange sound emerged.

Aliens? That’s what some immediately suspected, of course. NASA had a more logical explanation (also of course) – a bow shock. A what?

The bow shock is analogous to a sonic boom. The solar wind blows past all the planets at a speed of about a million miles per hour, and where it hits an obstacle, there's all this turbulence.

William Kurth, lead co-investigator for the Waves experiment, further explains that the particles and fields around Juno changed as it moved from the space dominated by interplanetary solar wind, which contained one particle per cubic centimeter (16 per cubic inch) to Jupiter’s magnetosphere, with a density less that one-hundredth of solar wind. This magnetosphere is the largest structure in the solar system, says Kurth.

If Jupiter's magnetosphere glowed in visible light, it would be twice the size of the full moon as seen from Earth. And that's the shorter dimension of the teardrop-shaped structure; the dimension extending outward behind Jupiter has a length about five times the distance between Earth and the sun.

So all of Juno’s exploratory activity won’t be occurring just on the surface of Jupiter, according to Barry Mauk, the instrument lead for the Jupiter Energetic-Particle Detector Instrument (JEDI) on Juno.

This unusual boundary structure will itself be the subject of scientific investigation.

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Abundant Jovian biota?

Will that include a search for aliens possibly creating the strange sound? Carl Sagan speculated about "abundant biota" in Jupiter's atmosphere and Arthur C. Clarke wrote about aerial life-forms in his short story, "A Meeting with Medusa."

The roar encountered by Juno may be a bow shock or it could be millions of ancient Jovian life-forms yelling:

Hey, you kids … get off of our magnetosphere!

Paul Seaburn

Paul Seaburn is the editor at Mysterious Universe and its most prolific writer. He’s written for TV shows such as "The Tonight Show", "Politically Incorrect" and an award-winning children’s program. He's been published in “The New York Times" and "Huffington Post” and has co-authored numerous collections of trivia, puzzles and humor. His “What in the World!” podcast is a fun look at the latest weird and paranormal news, strange sports stories and odd trivia. Paul likes to add a bit of humor to each MU post he crafts. After all, the mysterious doesn't always have to be serious.

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