Length of Day

Another potential problem is the length of a moon's day. Computer models show that any large moon orbiting a giant planet or brown dwarf becomes locked into synchronous rotation — one side of the moon always facing the planet — within a few hundred million years. This, of course, has happened with Earth's own Moon and many other large moons in the solar system.

Assuming that large moons typically have orbital periods of 2 to 16 days, any potentially habitable moon would have a "day" several times longer than Earth's. Simple calculations by Stephen Dole of the Rand Corporation in the 1960s showed that a body with an Earth-like atmosphere would become uninhabitable when the period of rotation exceeds 4 days, due to large swings in surface temperature.

In reality the situation for slowly rotating moons is probably not so bleak. Monoj Joshi and Robert Haberle (NASA/Ames Research Center) and their colleagues have investigated the effects of synchronous rotation on the livability of planets closely orbiting red-dwarf stars. Such a planet would become tidally locked so that one hemisphere always faces its sun while the other experiences perpetual night. Joshi and Haberle's computer models have shown that an atmosphere with a carbon-dioxide pressure of only 1 to 1.5 bars (a bar is the atmospheric pressure on Earth) not only maintains habitable conditions on a synchronously rotating planet but even allows liquid water on the planet's perpetually dark side. A thick carbon-dioxide atmosphere (an infrared-trapping "greenhouse") retains heat better than a thin Earth-like (nitrogen-oxygen) atmosphere and also transfers this heat to the night side via global circulation.

The situation with a slowly rotating moon should be less extreme than for a synchronously rotating planet. While simulations with such a moon have yet to be performed, even modest additions of carbon dioxide to a moon's atmosphere (probably a natural consequence of the carbonate-silicate cycle) could keep it clement despite having a day as long as several weeks. Clouds and large bodies of water, which were not taken into account in the models, should further moderate temperature extremes.

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