Astronomy — The volcanism of Io
Volcanism of Io, Jupiter’s largest moon
Io is Jupiter’s largest moon. Of all the bodies in our solar system, only Io and Earth have ongoing volcanic activity. What is the ultimate source of the heat needed to generate the volcanic activity? Astronomers generally consider two methods as ways for planets or moons to produce enough internal heat to drive surface geological activity. The first is by the decay of radioactive elements. Long-lived radioactive elements such as uranium, thorium, and potassium can generate heat for billions of years as they decay. This decay is what powers volcanoes and plate tectonics on Earth.
However, the smaller an object is, the less radioactive material it contains and the faster it radiates heat into space. Molten lava once flowed on the Moon’s surface, for example, but that was billions of years ago. The Moon has now cooled to the point where its solid crust is several hundred kilometers thick. Because Io has roughly the same size and mass as the Moon, radioactive heating plays little or no role in the ongoing volcanism there.
The second method of internal heating is the tidal heating that is induced by a neighboring body. This is the method at work on Io. Jupiter’s gravity raises a tidal bulge in the solid body of Io in the same way that the Moon produces ocean tides and a tidal bulge on Earth. The Earth also produces a tidal bulge on the Moon. As with our Moon, tidal forces caused Io long ago to fall into synchronous rotation about Jupiter. This means Io rotates once on its axis every time it revolves once around Jupiter and thus always keeps the same face pointing toward Jupiter.
These tidal forces would produce no heating if Io’s orbit around Jupiter was a perfect circle. But Io follows a slightly elliptical orbital path, and when it is closest to Jupiter it travels slightly faster than average, and when it is farthest from Jupiter it travels slightly slower. Io keeps spinning on it axis at the same rate regardless of where it lies in its orbit, forcing its tidal bulge to wobble back and forth. Jupiter’s gravity always works to pull Io’s tidal bulge into a straight line with Jupiter. This causes constant flexing of Io’s crust and generates Io’s internal heat.
If Io and Jupiter existed alone in space this action would eventually force Io’s orbit into a perfect circle. However the inner three Galilean moons orbit in resonance — each time Ganymede completes one orbit around Jupiter, Europa completes two and Io four. The periodic alignment keeps the elliptical orbit of Io from becoming circular and keeps the internal fires burning brightly.
Though tidal heating explains how Io generates so much volcanic activity, one troubling question remains. Detailed measurements show that the amount of heat flowing from Io during the Voyager 1 flyby is probably significantly greater than the average amount of tidal heating. Apparently Io either was anomalously active during the Voyager encounter or the amount of tidal heating varies with time.
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