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A Look at the Case for Moon Life
Science in the News Fervently optimistic about life's existence elsewhere in the cosmos, some scientists have abandoned the old notion that life requires an Earth-like home. A satellite (moon) orbiting a giant planet that in turn orbits a star resembling our sun at a distance nearly identical to Earth’s distance from the sun could be, they say, a life site. 1-3
First, while planets outside our solar system do exist, and while they may (though it is unlikely) orbit their stars at appropriate-for-life distances,4-5 their orbits would likely be too unstable to accommodate a moon-life scenario. Giant planets do not (in fact, cannot) form close to their stars. They either drift there or are bounced there under the influence of gravity and other physical forces. The likelihood that such a planet would end up a habitable distance from its star and with a stable, nearly circular orbit is miniscule.6-11 The extra-solar (outside our solar system) planets we do see anywhere close to life-habitable distances from their stars have such highly elongated orbits as to make life on any of their moons impossible.12 Their seasonal temperature differences would be far too extreme. An important and yet unanswered question is whether or not giant planets can even hold onto their satellites as they (the planets) migrate. Second, a moon orbiting far enough away from its planet to avoid "tidal locking" (sticking by gravity with the same face always toward the planet) will at one point in its orbit be too close to its star (thus too hot) and at the opposite point too far from its star (too cold) for life to survive. Ameliorating the temperature extreme through a very dense atmosphere will not work as the accompanying article by Guillermo Gonzalez demonstrates.13 On the other hand, a moon in close orbit about a planet like Jupiter would be bombarded by speeding charged particles (accelerated by the planet’s magnetosphere). These energetic particles would sputter away the satellite’s atmosphere, making the place uninhabitable. Wind from the parent star also would contribute significantly to the sputtering of the satellite's atmosphere. Escape from this catastrophe would be possible only if the moon were to possess a strong magnetic field. However, just a few solar system bodies have strong magnetic fields: the sun, Jupiter, and Earth. Ganymede, the largest solar system moon and the only one with an undisputed magnetic field, has a very weak one, less than one percent as strong as Earth’s.14, 15 It is too weak to prevent sputtering in Ganymede’s polar regions, and this sputtering would inhibit life-essential cloud formation.16
Third, a life-supporting moon would need enough mass to hang onto (by
gravity) an appropriate atmosphere. On that basis alone, such a moon would
have to possess at least 12% the mass of Earth.17 Other factors
beyond mass and gravity, however, affect a body's capacity to sustain an
adequate atmosphere. 18 The long list sounds like "The House
that Jack Built," but it ends with the carbonate-silicate cycle, which
requires a just-right balance of dry land, plants on the dry land, the
right ratio between dry land and ocean surfaces to permit sufficient
weathering, and plate tectonic activity.19, 20 These factors add
up to the need for a moon that's nearly one-fourth as massive as Earth. In
that case, it would be eight times more massive than Ganymede, which orbits
Jupiter, and more than twice as massive as Mars, which is the seventh
largest planet in our solar system—not a satellite we're likely to find
anywhere in the cosmos. Incidentally, this need for dry land and plate
tectonics eliminates the totally ice-water environments such as Jupiter's
moon Europa as long-term life sites. More reasons than these could be cited
for skepticism about moons as life sites.21, 22 No doubt even
more reasons will be discovered in the next few years. While I cannot yet
say that research has sealed the case, evidence strongly suggests that a
long-term home for life requires divine engineering.
References:
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How feasible is such a suggestion, and, if feasible, how would it impact
our conclusions about God’s design of the solar system? The following three
considerations, among others, shape my response: 
