The Lunar Influence

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Even if the Moon is rare, so what? If Earth were Moonless, then poets through the ages would have lost a source of inspiration. Perhaps mankind's scientific development would have been affected, since historically the Moon has played a large role in advancing our understanding of astronomy. But would life itself really have been any different?212

There are several ways in which the Moon exerted (or continues to exert) an influence on Earth. For example, the Moon raises ocean tides. Soon after the Moon formed it was much closer to Earth than it is now, so the tides of 4 billion years ago would have been huge — a surfer's paradise. It has been suggested that tides were a factor in getting life started, perhaps by acting as a giant mixer of the primordial soup and causing nutrient-rich pools where life may have started. This suggestion is not totally convincing, because even without the Moon we would still have ocean tides: the Sun raises tides about half as large as the present lunar tides. We would, however, miss the spring and neap tides, which depend upon the relative positions of Sun and Moon. The suggestion, therefore, cannot be ruled out.

figure 60 A montage of Earth and Moon (the Moon is shown comparatively larger here than in reality).

A more subtle lunar tidal effect is its influence upon the Earth's crust. The effect of the Moon's gravity may have amplified volcanic activity on Earth and increased continental drift. So it is possible (though not certain) that a Moonless Earth would have been less geologically active; Earth's atmosphere, which formed by volcanic outgassing, may have taken much longer to reach the stage where life could arise. We discussed the importance of plate tectonics in the previous section.

The most important effect to consider, though, is how the Moon influences the Earth's obliquity. The planets all orbit the Sun in or near one plane in space; the obliquity — or axial tilt — of a planet is the angle of inclination of its equator to this orbital plane. Earth's obliquity of 23.5° gives rise to the pleasant seasons we enjoy. Other planets are not so lucky. Mercury has an obliquity of 0°, so its equatorial regions resemble Hell. Life as we know it could not survive. (Interestingly, an observer at either of Mercury's poles would see the Sun always on the horizon. Relatively little solar energy can be absorbed at the poles, and indeed the polar regions of Mercury are ice-covered.) Uranus, which has an obliquity of 98°, is almost lying on its side. One pole receives sunlight for half of the Uranian year, while the other pole is in darkness. Again, these are less than ideal conditions for life. Earth — from our biased point of view — seems to be "just right."

December Jure

(Winter in Northern Hemisphere) (Summer in Northern Hemisphere)

December Jure

(Winter in Northern Hemisphere) (Summer in Northern Hemisphere)

figure 61 It is Earth's obliquity — its tilt relative to the plane of its orbit around the Sun (the ecliptic plane) — that produces the seasons. For planets with a "moderate" obliquity like Earth, most of the solar energy falls in the equatorial regions, where the midday Sun is always high in the sky. The polar regions are in constant illumination for 6 months, but in constant darkness for 6 months too; even when the Sun is in the sky, it is never higher in the sky than the obliquity allows — 23.5° in the case of the Earth — so the ground is never heated really strongly by sunlight. Thus, the polar regions are cold and the equatorial regions are hot. (The figure is not to scale.)

The impact event that formed the Moon would have caused Earth's axis of rotation to shift from its initial position. More importantly, as computer simulations have shown, the Moon plays a large role in stabilizing Earth's axial tilt over a period of many millions of years. This is important because even small changes in obliquity can cause dramatic changes in planetary climate. For example, Earth's obliquity oscillates by about ±1.5° with a period of oscillation of 41,000 years. This is only a small variation, yet it seems to be linked to the succession of ice ages that Earth has experienced over the past few million years. Mars has no stabilizing influence on its obliquity (Phobos and Deimos being merely boulders, with insufficient mass to have any influence). The obliquity of Mars is currently 25°, but this value ranges between 15° and 35°, with a period of 100,000 years. Calculations indicate that, over longer timescales, the obliquity of Mars changes chaotically: over the last 10 million years it may have ranged from 0° to 60°. With no Moon to act as a stabilizing influence, Earth's obliquity would also wander chaotically, to values as large as 90°. Even a relatively large satellite — up to half the mass of our Moon — would be unable to stabilize the obliquity; Earth requires a large satellite to prevent its obliquity from wandering and its climate shifting from one extreme to another.

Life on Earth has adapted well to climate change in the past, but had the Martian pattern of obliquity shifts been repeated here, it is difficult to see how advanced land animals could have prospered. Perhaps life on Earth would not have evolved into the forms we see today.

There are many "ifs", "buts" and "maybes" in the above discussion. We do not know whether a large satellite is necessary if a planet is to provide a suitable home for complex life-forms. Our own view is necessarily biased. We believe the Moon has been beneficial for the development of life here, but we do not know whether the Moon was necessary for life. Perhaps if we lived on a Moonless world we would be grateful we did not have one of those huge chunks of rock hanging so close to us in the sky.

And yet that nagging suspicion remains. Perhaps double planets like our Earth-Moon system are necessary for life, and yet they seem to form in rare, chance events. Perhaps the uniqueness of our satellite explains why we are alone. Perhaps that is the tragedy of the Moon.

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