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Space Topics: Jupiter

Io

Jupiter's Moon Io
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Jupiter's Moon Io
Swirls of brightly colored sulfur-rich surface deposits make volcanically active Io one of the most colorful places in the solar system. Credit: NASA/JPL/DLR

Diameter: 3,630 kilometers -- 0.2856 Earth diameters -- 13th largest solar system body
Orbital distance: 421,600 kilometers from Jupiter
Orbital period: 1.769 days
Discovery: 1610 by Galileo Galilei

Pizza-faced Io is the most volcanically active body in the solar system.  Active eruptions were first observed by Voyager 1 and 2 in 1979.  When Galileo returned to the Jovian system in 1995, the face of Io had changed, with new and colorful deposits around many plumes.  These colorful deposits were once thought to result from exotic Ionian lavas made of sulfur compounds, which would melt at lower temperatures than rock does.  But when Galileo measured the temperature of actively erupting lava, it found some to be as high as 1,700 to 2,000 Kelvin (1,400 to 1,700 degrees Celsius, 2,600 to 3,100 degrees Fahrenheit) or higher.  These temperatures are even hotter than that found in typical Earth volcanic eruptions and indicate that the Ionian lavas must have compositions that are “ultramafic,” high in magnesium, and originating in the mantle rather than the upper crust of the moon.  The colorful deposits are probably sulfur compounds, frosts from the sulfur-rich gases that accompany the eruptions.

The volcanic activity is driven by the flexing and squeezing of Io in response to tidal forces.  One set of tides is created by the gravity of giant Jupiter.  As Io revolves around Jupiter in its slightly elliptical orbit, Jupiter excites a tidal bulge on Io that moves back and forth.  This action should have made Io’s orbit circular over time and fixed the Jupiter tidal bulge in one place, but it is counteracted by another force.  Io is in an orbital resonance with Europa and Ganymede such that Io revolves around Jupiter four times for every two Europa orbits and every one Ganymede orbit.  Because the moons tend to meet each other in the same places in their orbits, Io gets a repeated, synchronized tug from the other moons, which acts to pull its orbit into a slightly elliptical shape.  Caught between all these forces, Io is flexed and squashed.  The internal friction generated by these shape changes drives Io’s fantastic volcanic activity.