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Jupiter’s Galilean Satellites: Io

by Dani Johnson

May 30, 2013

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Image Above: Io, the most volcanic body in the solar system, is seen in front of Jupiter's cloudy atmosphere in this image from NASA's Galileo spacecraft. Image credit: NASA/Galileo

Tug of War:

The weird-looking moon , Io, has been said to resemble the view of the toppings on a slice of pizza, and it really does. It also looks metallic because the white regions look like reflections. Io is the innermost of the Galilean satellites and it is about 422,000 kilometers from the planet. At this close proximity the gravitational pull from Jupiter and the other Galilean satellites play tug of war with Io. Ganymede, Europa and Io orbit their planet in a 1:2:4 resonance, meaning that for every time Ganymede orbits one time Europa orbits twice and Io orbits four times. They are also tidally locked, which means that each moon keeps the same hemisphere facing Jupiter during their orbit. This game of tug is pulling Io this way and that and is ultimately what causes Io's high volcanic activity. Linda A. Morabito, a Jet Propulsion Laboratory engineer, saw the very first extraterrestrial eruption caught on camera by Voyager 1 in March 1979. Since then we've learned that Io is the most volcanically active body in the entire Solar System.

Image Above: This photo was taken by Voyager 1 looking back 2.6 million miles (4.5 million km) at Io, three days after its historic encounter. Image Credit: NASA/Galileo

Plasma Torus and the Flux Tube: Jupiter has a massive magnetic field that scrapes charged particles right off of the surface of the moon. The particles hang out in a doughnut-shaped cloud called the Plasma Torus that orbits the planet in conjunction with Io. The cloud resembles a Hula Hoop wobbling around the planet's equator. The cloud wobbles because Jupiter's axis is tilted. The interaction with the magnetic field also creates the electrical current between Io and Jupiter called the Flux Tube. It's a little tricky to understand, but thankfully the team at University of Tennessee Knoxville's Department of Physics and Astronomy explains it incredibly well,

"The moon orbits within the field, and material ejected from its surface by volcanoes (and by the charged particle bombardment from the material already trapped in Jupiter's magnetic field) is a primary source of the charged particles in the Jovian magnetic field. These particles (primarily sulfur and oxygen) have a high concentration in a doughnut shaped region surrounding Io's orbit called the Io [plasma] Torus.

As Io moves around its orbit in the strong magnetic field of Jupiter and through this plasma torus, a huge electrical current is set up between Io and Jupiter in a cylinder of highly concentrated magnetic flux called theIo Flux Tube. The Flux Tube has a power output of about 2 trillion watts, comparable to the amount of all manmade [sic] power produced on Earth. It is responsible for bursts of radio frequency radiation long detected on Earth."

This video also offers a visual explanation:

For more information on Io visit:

For more information on Jupiter and its moons visit:

Extra Content:

Hear the sound of Voyager 1 passing through Jupiter's bow shock.
"All of the planets are bathed by a hot plasma called the solar wind which boils off the sun and moves outward at speeds of a million miles per hour. The planets are a little like supersonic aircraft in Earth's atmosphere. Should a supersonic jet fly over your house, you would hear a sonic boom caused by the jet moving faster than sound waves in the air. Since the solar wind is moving past the planets at supersonic speeds, a similar 'sonic boom' is created in the solar wind. The signals in this sound file were acquired as Voyager 1 was approaching the 'sonic boom' (or bow shock, as scientists refer to it) of Jupiter. The chirps heard at the beginning of the interval are waves generated by electrons coming from the shock and moving 'upstream' into the approaching solar wind. These soon die out and, except for a slight hum from one of the science instruments onboard and the firing of one of Voyager's thrusters (making a short thud) things become quiet. Then, suddenly, the spacecraft enters the bow shock and is enveloped by the turbulence in this planetary 'sonic boom'. The bow shock is nature's way of slowing, deflecting, and heating the solar wind as it runs into an object, in this case the Jovian magnetosphere. In fact, the waves you are hearing are at least partly responsible for heating the solar wind as it is slowed and deflected around the magnetosphere."

For more Sounds of Jupiter visit:


by Dani Johnson

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