The Earth may be unique in our solar system because it appears to be the only planet that is still volcanically and tectonically active; our planet therefore remains very much alive, while the others apparently have long ceased activity. Volcanic activity requires a source of internal heat, and it is the escape of this heat that fuels plate tectonics. While volcanism played a major role in the early history of Mars, the Moon, and probably Mercury, their small sizes relative to Earth resulted in the loss of internal heat at a much faster rate. They have been inactive globes for the last billion years or so.
Venus may still be active, though the evidence is questionable. In 1979, the Pioneer-Venus spacecraft measured a high amount of sulfur in the upper atmosphere of the planet; the sulfur amount then decreased over the next few years. This observation suggested that the high sulfur concentration measured in 1979 may have resulted from a catastrophic event, perhaps a volcanic eruption. Beginning in 1990, radar images made by the Magellan spacecraft revealed dramatic volcanic features and long, deep valleys similar in size and shape to oceanic trenches on Earth.
Left: A computer-generated image of the Aleutian Trench (in violet); "warm" colors (yellow to red) indicate topographic highs, and "cool" colors (green to blue) represent lower elevations. Below: The topography of Artemis Corona, a trench-like feature on Venus, shown at the same vertical and horizontal scale as the Aleutian Trench. (Imagery courtesy of David T. Sandwell, Scripps Institution of Oceanography.)
The Voyager spacecraft discovered several volcanic plumes rising many hundreds of kilometers above the surface of Io, one of the moons of Jupiter and about the size of our Moon. Scientists speculate that large pools of liquid sulfur may exist on Io, possibly heated by tidal forces resulting from gravitational attraction between Io and Jupiter. The thermal energy generated by such tidal forces may be enough to produce convection in Io's interior, although no one has clearly recognized any surface feature that may have formed from such convection.
The surface of Ganymede, another moon of Jupiter and about the size of Mercury, is broken into many plate-like blocks, with long narrow depressions between some of them. Whether these surface features represent ancient "fossil" plate tectonics, or are actively forming, remains to be answered. Crucial to determining whether plate tectonics is occurring on Ganymede is the search for evidence of a deep ocean beneath its icy surface. Such a body of water, if it exists, might contribute to internal convection.
A
volcanic plume of sulfur dioxide (SO2) gas rising about 150 km
above the surface of Io. This computer-enhanced image was captured
"live" by the Voyager 2 spacecraft on 4 March 1979. (Imagery
courtesy of NASA.)
The rate of heat loss is critical to a planet's tectonic activity. Size is one determining factor: larger bodies lose heat more slowly and will therefore remain active longer. Another factor is composition, which influences the ability of a body to convect. For example, a liquid interior, such as may exist within Ganymede, is more likely to convect and drive plate tectonics than the "stony" interiors of the Moon, Mercury, Venus, and Mars. The amount of radioactive elements present in the planet's composition also affects the likelihood of internal convection, because the decay of these elements produces heat. Apparently, the interiors of the Moon, Mercury, and Mars are either too rigid or have lost too much of their internal heat to convect and drive plate tectonics.
Eventually the Earth, too, will lose so much heat that its interior will stop convecting. Earthquake and volcanic activity will then cease. No new mountains will form, and the geologic cycle of mountain building, erosion, sedimentation, and soil formation will be disrupted and also will cease. Exactly how a cooled-down Earth will change surface conditions -- and whether our planet will still be habitable -- nobody knows. Fortunately, these changes will not happen for many billions of years!
Information provided by: http://pubs.usgs.gov