When I consider your heavens, the work of your fingers, the moon and stars, which you have set in place, what is man that you are mindful of him, the son of man that you care for him?
(Psalm 8:3-4, RSV)
Did God create the moon just to provide “a light for the night,” or a celestial calendar for “times and seasons?” Research since the Apollo missions to the moon suggests an answer. Although the moon was found to be barren of any possibility of life, it appears to be essential for life on earth, revealing details of how God cares for humans, in particular, how he prepared a place for them to flourish.
Although the moon was found to be barren of any possibility of life, it appears to be essential for life on earth.
Our moon is unique in the solar system in composition and large size, making the earth and moon together a virtual double planet. Over the last 200 years, scientists have proposed several theories for the moon’s origin, but none could account for its unusual properties. Finally in the 1980s, the giant-impact theory emerged from Apollo discoveries, and computer simulations could successfully account for the moon’s origin and properties. These simulations required a glancing collision with the earth early in its history by a Mars-size impactor, a collision that would blast vaporized debris into orbit to form the moon.
The giant-impact theory suggests a number of benefits critical to providing conditions needed for life on earth. At least ten essential results appear to be related to such a glancing collision. Five of these are produced by the giant impact itself, and five follow from the moon’s subsequent influence on the earth. Comparisons with neighboring planets suggest that the absence of any of these results might have prevented the development of life on earth.
The Impact of the Giant Impact
1. The glancing collision increased the earth’s spin to an initial rotation rate of 5 hours, faster than any other planet in the solar system. This 5-hour day was first calculated by George Darwin, son of Charles Darwin, from the known rate of slowing of the earth’s rotation by lunar tides over several billion years to reach the current 24-hour day. This current rate moderates temperatures between water’s freezing and boiling points over most of the earth’s surface. Compare that to Mercury’s rotation rate of 59 days, which produces long nights far below freezing and long days far above the boiling point of water.
2. The earth’s axis shifted to its axial tilt of 23.5 degrees (relative to the axis of its orbit around the sun). This gives earth its relatively mild seasonal variations, associated life cycles, and rich biological diversity. By contrast, Mercury’s axis has no tilt and thus no seasonal variations.
3. Greenhouse gases were apparently removed. A glancing collision would strip the earth of its earliest atmosphere, which was probably similar to the opaque carbon-dioxide atmosphere of Venus. Venus’s atmosphere is 90 times heavier than on earth, and its greenhouse temperatures have boiled away all water. After the giant impact and the resulting magma ocean on earth, a new atmosphere formed as trapped gasses were released and comets collided with earth. Eventually a new crust formed and water vapor condensed to form oceans that absorb much of the excess carbon dioxide. This new atmosphere was thin enough to prevent a runaway greenhouse effect, moderate earth temperatures, and later allow photosynthesis, which is key to producing oxygen.
4. Computer simulations reveal that the molten core of the impactor fell back to earth and sank into its core. With a larger liquid-iron core and a faster rotation rate, the earth’s magnetism increased to about 100 times that of any other rocky planet. Thus charged particles from the solar wind—particles that can kill emerging life and ultimately strip away much of earth’s atmosphere—were deflected.
5. The earth’s mass increased by about 10 percent. This increase is critical for life since it provides sufficient gravity to keep the early earth’s water vapor from escaping into space before it could condense to form the oceans.
The Difference the Moon Makes
Five further benefits for life from the giant impact followed after the collision and the formation of a large moon.
But it now appears that the most important feature for life on earth might be the right size moon and the right kind of glancing collision to produce it
1. It provided the conditions for plate tectonics, which occur on no other known planet. The glancing collision removed more than half of the earth’s original crust, added to its core and mantle heat, and increased its internal radioactivity to sustain this heat. A thinner crust was more susceptible to cracking and the driving forces of heat convection. Plate tectonics built the mountains and continents of earth that keep it from being completely covered with water. It also recycles the crust by bringing essential minerals to the surface.
2. The moon created huge tides early in earth’s history. The moon’s initial orbit was about 15 times closer to the earth than it is now. As tidal action slowed the earth’s rotation, it increased the orbital distance of the moon. When the earth’s rotation had slowed to about 10 hours (and when the moon was about 10 times closer than it is today), the tidal forces would be about a thousand times larger and tides would be hundreds of times higher than today. These huge tides eroded minerals from far inland every few hours, enriching the oceans with life-sustaining minerals.
3. The moon had a slowing effect on the rotation of the earth. Its early rapid rotation produced super-hurricane winds with severe threats to most life forms. A slower rotation rate optimizes wind circulation and surface temperatures for life as we know it.
4. These early tides helped form intertidal pools, which concentrated nutrients by evaporation. These rapid and large tides produced more and larger intertidal pools. Longer cycles between the highest spring tides and the lowest neap tides allowed for several days of evaporation, providing ideal conditions for the development of early life forms.
5. A large moon stabilized the tilt of the earth’s axis. The gravitational forces from our large moon keep the earth’s axis tilted in a narrow range between 22 and 25 degrees, producing regular seasons that stabilize annual climate in a favorable range for living organisms. Thus our large moon prevents the large and chaotic changes in tilt that have been shown to occur on Mars, which has no large moon.
All of these results, suggested by computer simulations of the giant-impact formation of a large moon, appear to be necessary for life on earth. Earth has many other features that make it a “goldilocks” planet with just the right conditions for life, including a favorable location in the galaxy, the right size sun, the right distance from the sun, the right amount of water, the ozone layer around the earth to protect it from ultraviolet radiation, and many others.
But it now appears that the most important feature for life on earth might be the right size moon and the right kind of glancing collision to produce it. Such an event requires at least five independent parameters: the right size impactor, the right time for the impact to occur, the right direction for an effective glancing collision, the right point of impact on the proto-earth, and the right speed to place enough debris in orbit for a large moon. If we assume a probability of less than a tenth for each of these factors, their product gives about one chance in a million for such an event—and for creating a planet we could thrive on.
Joseph Spradley is emeritus professor of physics at Wheaton College, Illinois.
