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Without the Moon there would be no moonbeams, no month, no lunacy, no Apollo program, less poetry, and a world where every night was dark and gloomy. Without the Moon it is also likely that no birds, redwoods, whales, trilobites, or other advanced life would ever have graced Earth.

Although there are dozens of moons in the solar system, the familiar ghostly white moon that illuminates our night sky is highly unusual, and its presence appears to have played a surprisingly important role in the evolution of life. The Moon is just a spherical rock 2000 miles in diameter and 250,000 miles away, but its presence has enabled Earth to become a long-term habitat for life. The Moon is a fascinating factor in the Rare Earth concept because the likelihood that an Earth-like planet should have such a large moon is small. The conditions suitable for moon formation were common for the outer planets but rare for the inner ones. Of the many moons in the solar system, nearly all orbit the giant planets of the outer solar system. The warm, Earth-like planets that are close to the sun and that fall within the habitable zone, are nearly devoid of moons. The only moons of the terrestrial planets are ours and Phobos and Diemos, the two tiny (10 kilometers in diameter) moons of Mars. Some of the solar system's moons are huge. Jupiter's Ganymede is nearly as large as Mars, and Saturn's Titan is nearly that large and has an atmosphere denser than our own, though much colder. Our Moon is somewhat of a freak because of its large size in comparison to its parent planet. The Moon is nearly a third the size of Earth, and in some ways it is more of a twin than a subordinate. The only other case in the solar system where a moon is comparable in size to its planet is Pluto and its moon, Charon.

Tilt!

The Moon plays three pivotal roles that affect the evolution and survival of life on Earth. It causes lunar tides, it stabilizes the tilt of Earth's spin axis, and it slows the Earth's rate of rotation. Of these, the most important is its effect on the angle of tilt of Earth's spin axis relative to the plane of its orbit, which is called "obliquity." Obliquity is the cause of seasonal changes. For most of Earth's recent history, its obliquity has not varied by more than a degree or two from its present value of 23 degrees. Although the direction of the tilt varies over periods of tens of thousands of years as the planet wobbles, much like the precession of a spinning top, the angle of the tilt relative to the orbit plane remains almost fixed. This angle is nearly constant for hundreds of millions of years because of gravitational effects of the Moon. Without the Moon, the tilt angle would wander in response to the gravitational pulls of the sun and Jupiter. The monthly motion of our large Moon damps any tendencies for the tilt axis to change. If the Moon were smaller or more distant, or if Jupiter were larger or closer, or if Earth were closer to or farther from the sun, the Moon's stabilizing influence would be less effective. Without a large moon, Earth's spin axis might vary by as much as 90 degrees. Mars, a planet with the same spin rate and axis tilt, but no large moon, is believed to have exhibited changes to its tilt axis of 45 degrees or more.

Because tilt of a planet's spin axis determines the relative amounts of sunlight that land on the polar and on the equatorial regions during the seasons, it strongly affects a planet's climate. On planets with moderate tilts, the majority of solar energy is absorbed in the equatorial regions, where the noon sun is always high in the sky. Each pole is in total darkness for half a year and has constant illumination for half a year. The highest altitude that the sun reaches in the sky at the pole is exactly equal to the number of degrees of the tilt of the spin axis. For moderate tilt angles, the sun is never high in the polar sky, and ground heating by sunlight is low even in the middle of the summer. The planet Mercury provides a spectacular example of what can happen on a planet whose spin axis is nearly perfectly perpendicular to the plane of its orbit. Mercury is the closest planet to the sun and most of its surface is hellishly hot, but radar imaging from Earth has shown that the poles of the planet are covered with ice. The planet is very close to the sun, but as viewed from the poles, the sun is always on the horizon. In contrast to Mercury's lack of tilt, the planet Uranus has a 90-degree tilt; and one pole is exposed to sunlight for half a year while the other experiences cryogenic darkness.

Although our viewpoint is certainly biased, our planet's tilt axis seems to be "just right." Constancy of the tilt angle is a factor that provides long-term stability of Earth's surface temperature. If the polar tilt axis had undergone wide deviations from its present value, Earth's climate would have been much less hospitable for the evolution of higher life forms. One of the worst possibilities is that excessive axis tilt could have led to the total freezing over of the oceans, a situation that might be very difficult to recover from. Extensive ice cover increases the reflectivity of the planet, and with less absorption of sunlight, the planet continues to cool. Astronomer Jacques Laskar, who made many of the calculations that led to the surprising discovery of the Moon's importance in maintaining Earth's stable obliquity, summarized the situation as follows:

These results show that the situation of the Earth is very peculiar. The common status for all the terrestrial planets is to have experienced very large scale chaotic behavior for their obliquity, which, in the case of the Earth and in the absence of the Moon, may have prevented the appearance of evoluted forms of life. . . . [W]e owe our present climate stability to an exceptional event: the presence of the Moon.

High obliquity has remarkable and seemingly counterintuitive effects on planets (see Figure 10.1). Consider a planet that is tipped 90 degrees. Averaged over the year, the poles would receive exactly as much solar energy as the equator would with no tilt angle. The north pole would become the Sahara! For the 90-degree tilt, however, the equatorial regions would receive much less energy averaged over the year and would become colder. If a planet is tilted more than 54 degrees, its polar regions actually receive more

Tilt ang|e Energy at pole

Tilt ang|e Energy at pole

Figure 10.1 The ratio of the annual amount of solar energy falling on a planet's pole to that falling on its equator varies with the angle of a planet's spin axis. With a tilt angle of 22.5 °, the Earth has very cold polar regions, but if the tilt exceeded 54°, the polar regions would actually receive more sunlight than the tropics. The lines parallel to the equator are the polar circles, where the Sun never sets in the midsummer and never rises in the midwinter.

energy input from sunlight than the equatorial regions. If the Earth were tilted more than this amount, the equatorial oceans might freeze and the polar regions would be warmer: a topsy-turvy world. Recently uncovered evidence has revealed that equatorial ice sheets did exist about 800 to 600 million years ago, and ice-rafted sediments of this age have been found in formerly equatorial regions. This has led to the "Snowball Earth" hypothesis that Earth may have actually have frozen over, as we saw in Chapter 6. It has been suggested that this may have been due to high tilt angle during a period of time when the Moon did not have full control. We do not know for sure how long the Moon has been successful in stabilizing Earth's obliquity.

In the distant future, the Moon will lose its ability to stabilize Earth's spin axis. The Moon is slowly moving outward from Earth (at a rate of about 4 centimeters a year), and within 2 billion years it will be too far away to have enough influence to stabilize Earth's obliquity. Earth's tilt angle will begin to change as a result, and the planet's climate will follow suit. Further complicating the future is the slow but unrelenting increase in the brightness of the sun. At the time when our planet's spin axis begins to wander, the sun will be hotter, and both effects will decrease the habitability of Earth.

There is currently much speculation about how rapid such changes of planetary obliquity might be in the absence of the Moon. Estimates for the time it would take Earth to "roll" on its side range from tens of millions of years to far shorter periods. Astronomer Tom Quinn of the University of Washington has suggested to us that the time of obliquity change could occur on scales as short as hundreds of thousands, rather than millions, of years. Such large-scale fluctuations would probably lead to very rapid and violent climate change. If the tropical regions became locked in a permanent ice cover in 100,000 years or less, there would certainly be a mass extinction of great severity.

Is the lack of a large moon sufficient to prevent microbial life from evolving into animal life? We have no information, but because deep-sea regions are insulated from climate change, it seems doubtful that rapid obliquity changes would deprive a planet of animal life. What it could do, however, is deprive a planet of complex life on land.

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