Our solar system contains four terrestrial planets and numerous terrestrial-like moons. Do any of these planets or moons display the consequences of plate tectonics? Planetary geologists, who have studied images of these planets taken through telescopes or from exploratory satellites, say no. The Earth appears to be the only body in the solar system to experience plate tectonics. Why? Because Earth’s interior has remained warm enough for flow to take place in its mantle. For this reason, asthenosphere can rise at mid-ocean ridges, and can move out of the way of subducting plates. The mantles of Mercury, Mars, the Moon, and Venus apparently cannot flow like that of Earth, so these planets have no active volcanoes, no continental drift, and no sea-floor spreading. Earlier in solar-system history, however, when they were warmer, Mars and Venus did have volcanoes, perhaps comparable to hot-spot volcanoes on Earth. Indeed, the pattern of faults on Venus suggests that plate-tectonics-like movement struggled to begin but never quite succeeded.

The Jovian planets consist mostly of gas, so they cannot possibly have a rigid lithosphere. Curiously, though, some of the moons of Jupiter and Saturn appear to exhibit tectonic features. In fact, the Galileo satellite in 1996 photographed a volcano in the act of erupting on Io, a moon of Jupiter. But these volcanoes reflect some other type of melting process, not plate tectonics.

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