Maintaining Geologic Activity
For a moon to have an active dynamo producing a magnetic field, it must have a source of internal heat. But even more internal heat is required to drive another process that may be essential: the geologic activity needed to keep the carbonate-silicate weathering cycle going, which regulates the global atmospheric temperature on geologic timescales.
This important cycle maintains a roughly constant level of carbon dioxide in an atmosphere across many millions of years, due to a feedback effect between temperature, the weathering of rocks on continents, the deposition of weathered carbonates in ocean sediments, and the release of CO2 from buried sediments by volcanic activity. Without this self-regulating cycle, an otherwise habitable world will fall into a perpetual ice age, much as Mars is in today.
In solar system's terrestrial planets, the most important source of internal heat today is the decay of radioactive isotopes. This heat source decreases with time, however, and small bodies cool faster than large ones. As a result, large planets like Earth can support the carbonate-silicate cycle longer than small planets like Mars.
While the amount of internal heat required is still a subject of debate, it is estimated that a world's mass must be at least 25 percent that of Earth to maintain this cycle for 4.6 billion years, if radioactivity is the only heat source.
But large moons orbiting an extrasolar giant have an additional source of internal energy that our terrestrial planets lack: tidal heating. A spectacular example is Jupiter's moon Io. The constant flexing of Io's body as it is tugged between Jupiter and the planet's other moons generates enough heat to make it the most volcanically active body in the solar system.
Europa seems to experience a lesser degree of tidal heating, enough to maintain the ocean of liquid water that apparently exists beneath its icy shell.
Ganymede has a magnetic field and evidence of past geologic activity that both suggest this largest moon of Jupiter also underwent tidal heating as its orbit evolved through resonances with its siblings over the past few billion years.
The amount of tidal heating depends on a number of factors such as the masses of the moon and its planet, the moon's internal structure, the size and eccentricity of its orbit, and the orbits of its near neighbors. Large moons of extrasolar giants probably experience some tidal heating that could help maintain good living conditions far longer than much larger terrestrial planets maintain in isolation.
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