Importance of the Moon for Earth life

The giant-impact theory of the moon can account for the origin of the moon and many of Earth's unusual features, such as the right rotation rate, axial tilt, atmosphere, iron core, magnetic field, plate tectonics, and tidal history, all of which have created a unique environment for the existence of complex life-forms.

Origin of the Moon

The giant-impact theory of how the moon was formed not only explains the origin and unusual features of the moon but also accounts for many unique properties of the earth that make it the only habitable planet in the solar system. Several historical theories for the origin of Earth's moon have been proposed, but none has been completely successful.

Immanuel Kant's co-accretion theory suggested that Earth and the moon formed from the same dust cloud. British astronomer and mathematician George Darwin's fission theory assumed that the moon split from Earth. American astronomer Thomas See's capture theory tried to explain how Earth's gravity could pull a planetesimal into its orbit. However, none of these theories could account for all of the moon's unusual features, including its large size relative to Earth, its orbital properties, its low density, its small iron core, and evidence from the Apollo space missions that the moon lacks volatiles and once had a magma ocean.

The origin of the moon and its unusual features were finally explained by the giant-impact theory, which succeeded by combining various aspects of earlier theories. The giant-impact theory was first proposed by Canadian geologist Reginald Daly at Harvard University in 1946. Daly suggested that a glancing collision of a large planetesimal with the Earth early in its history would blast vaporized debris into orbit, which would eventually coalesce to form the moon. This theory was ignored until after the Apollo missions, when American planetary scientists William K. Hartmann and Donald R. Davis began to apply computer programs to the problem of planetesimal formation in the early solar system and revived Daly's giant-impact idea in 1974.

About the same time, astrophysicist Alastair Cameron (another Canadian, also based at Harvard) and his student William Ward began to develop computer simulations of a glancing collision, finding that formation of the moon required an impactor about ten times larger than the moon itself. According to this model, one-half of the debris that had blasted into space would remain in orbit, and within a few weeks some of it would coalesce to form the moon. These simulations showed that the collision would increase Earth's mass by about 10 percent, increase its rate of rotation to about five hours, and produce a moon deficient in iron and volatiles.

The giant-impact theory made little progress until 1984, when a post-Apollo conference was held in Kona, Hawaii, about the origin of the moon. Several conferees presented papers on the giant-impact theory, leading to a growing consensus in favor of the theory. More improved simulations followed the Kona consensus, especially by Cameron, who had retired to Arizona, and by American astrophysicist Robin M. Canup at the Southwest Research Institute in Boulder, Colorado. The two scientists began using a method developed for modeling bomb explosions called the smooth particle hydrodynamics (SPH) method, in which their simulations differentiated between rock and iron particles (several thousand of each). These new simulations showed the melted iron core of the impactor blasting into space and then falling back to Earth and sinking into its core.

Planetesimal accretion models suggest that the giant impact occurred about forty million years after the formation of the solar system as determined from the oldest meteorites. An interesting extension to the theory was proposed in 2011 based on a computer simulation, suggesting that the ejected material from the giant impact formed two bodies, which eventually collided to form the moon. This proposal explains why the far side of the moon has a thicker crust and more highlands, resulting from the smaller body striking the larger on what is now the far side. NASA scientists conducted additional simulations to learn more about the giant impact theory. High-resolutions conducted in 2022 revealed that the moon may have formed immediately or in hours after the collision.

It has long been recognized that Earth's moon provides several benefits for life on Earth. These benefits include keeping time based on the phases of the moon, illumination of the night sky, and lunar tides for cleansing and oxygenating the oceans. The giant-impact theory suggests a number of additional benefits that are critical to providing the conditions that are needed for life on Earth. Ten such essential factors appear to be related to the formation of a large moon, assuming that complex life requires liquid water. The first five of these beneficial results relate to the giant impact itself, and the last five result from the moon's subsequent influence on Earth. The absence of any of these benefits might have prevented the development of life on Earth.

Immediate Benefits of the Giant Impact

In the giant-impact theory, the glancing collision itself produced a minimum of five effects on the Earth that helped prepare it to support life. The first of these was an increase in Earth's spin to an initial five-hour rotation rate, much faster than any other rate in the solar system. This initial rate was fast enough so that over the time for life on Earth to develop, the rate could be slowed to the current twenty-four-hour day by the moon's tidal action on Earth's oceans. Earth's current rotation rate makes photosynthesis possible and moderates temperatures between the freezing and boiling points of water over most of the planet. By comparison, Mercury's rotation rate of fifty-nine days produces long 100 kelvin (-173 degrees Celsius [C]) nights and 700 kelvin (427 degrees C) days.

A second benefit of the giant impact was to provide Earth with a favorable axial tilt, leading to the current inclination of Earth's equatorial plane of about 23 degrees relative to its orbital plane (ecliptic). This axial tilt gives the earth its relatively mild seasonal variations; the tilt is large enough, however, to stimulate evolutionary processes. By contrast, Mercury's axis has no tilt and thus no seasonal variations. The tilt of Earth appears to have remained fairly steady through time, as evidenced by the growth pattern of an 850-million-year-old stromatolite. Analysis based on the assumption of growth toward the noontime sun suggests a 26.5 degree tilt at that time.

A third life-supporting benefit of the giant impact was the apparent removal of greenhouse gases. Several investigators have suggested that a glancing collision would have stripped Earth of its primordial atmosphere. By comparison, the atmospheric pressure of Venus without a glancing collision is ninety times that of Earth, producing greenhouse temperatures of about 700 kelvins, which has boiled away all surface water. On Earth, surface water absorbs much of the excess carbon dioxide.

After the giant impact and the resulting magma ocean on Earth, a new atmosphere formed from outgassing and comet collisions. Eventually, a new crust formed and water condensed and formed oceans to absorb carbon dioxide. The reformulated atmosphere was then thin enough to prevent a runaway greenhouse effect and to eventually allow photosynthesis to occur with its associated production of oxygen.

There are two further benefits from the giant impact: first, the action of the molten iron core of the impactor falling back to Earth and sinking into its core; and second, the transfer of the impactor's mass to Earth. Both of these benefits have been revealed by modern computer simulations.

The dynamo theory of Earth's magnetism shows that the enlargement of the liquid iron core, together with a much faster rotation rate, increased Earth's magnetic field to about one hundred times that of any other rocky planet. A strong magnetic field deflects charged particles in the solar wind. Without the magnetic field, this wind would have stripped away much of Earth's atmosphere and threatened its emerging life.

Computer simulations show that most of the mass of the Mars-size impactor transferred to Earth, increasing its mass by about 10 percent. This increase in mass was critical for life because it provided sufficient gravity to keep Earth's water vapor from escaping to space before it could condense to form the oceans.

Later Benefits for Life from a Large Moon

Five further benefits for life on Earth emerged after the giant impact, but resulted from the impact and the large moon it produced. The first of these benefits was to contribute to the conditions required for plate tectonics, which provides strong support for life and occurs on no other known planet. The glancing collision removed up to 70 percent of Earth's crust, added significantly to its core and mantle heat, and it increased radioactive isotopes to sustain this heat.

When a thinner crust re-formed on Earth after the collision, Earth was more susceptible to cracking and the driving forces of heat convection, enhanced by increased internal heat. Continuing plate tectonics built the mountains and continents of Earth, without which it would be mostly covered with water and have little chance for developing land-based life. Tectonic activity also recycles the crust, bringing minerals to the surface and controlling long-term climate by the carbon-rock weathering cycle, which helps to balance atmospheric carbon dioxide and prevent temperature extremes.

A second benefit that followed the giant impact came from huge tides early in Earth's history, which eroded the land and enriched the oceans with the minerals needed for life. Calculations show that the moon formed about fifteen times closer than it is today, when Earth had its initial five-hour day. In the late nineteenth century, George Darwin showed how tidal action slows Earth's rotation and increases the orbital distance of the moon. When the moon was about ten times closer than it is now, and when the day had slowed to perhaps ten hours, the tidal forces would have been one thousand times larger and hundreds of times higher than today's tides. Huge tides from the early moon would have eroded minerals from far inland about every five hours, enriching the oceans with life-sustaining minerals.

A third benefit after the giant impact resulted from the slowing effect on Earth's rotation caused by the tidal action from a large moon, also shown by Darwin. Early rapid rotation produced super-hurricane winds, similar to those produced on Jupiter by its rapid ten-hour rate of rotation, which would have posed severe threats to most life forms. Geological evidence for the slowing of Earth's rotation involves alternating layers of silt and sand offshore from tidal estuaries, showing that the earth's rotation had slowed to about an eighteen-hour day by 900 million years ago. A slower rotation rate optimized wind circulation and surface temperatures for the development of life.

Another benefit from early tides was their role in forming intertidal pools, which have long been recognized as ideal locations for concentrating nutrients by evaporation for emerging life forms. Rapid tidal cycling occurred when the day was shorter and the moon was closer, so that the tides would have been larger and tidal pools would have covered larger areas. Longer cycles between spring and neap tides might have given several days for drying. Some investigators have suggested that cycles of tides and evaporation along the shorelines of the early oceans might have provided the kind of environment in which protonucleic acid fragments could begin to associate and assemble molecular strands, leading to the origin of life.

A final beneficial result of a large moon is its stabilizing effect on the tilt of Earth's axis. In the early 1990s, scientists showed that gravitational forces from Earth's large moon keeps Earth's axis tilted in a narrow range between 22 and 25 degrees, stabilizing annual climate variations in a favorable range for living organisms and producing the regular seasons that occur on Earth. Thus, Earth's large moon prevents the kind of large and chaotic changes in tilt that have been shown to occur on Mars, which has two small moons.

Implications for Planetary Life

All the results described in computer simulations of the giant-impact theory and of the large moon it produced are apparent contributions to making life on Earth possible. It appears also that the lack of any one of these results might have prevented the development of complex life forms, including human life. Not only is it remarkable that Earth has all these life-sustaining features but these features also appear to be the result of just the right kind of glancing collision to form a large enough moon. Beyond these lunar features, Earth has many other properties that are needed for life, such as a favorable location in the galaxy, the right size sun, the right distance from the sun, a sufficient amount of water, and an ozone layer to protect from ultraviolet radiation. These conditions greatly restrict the possibilities of life elsewhere in the galaxy, even without taking into account the probable requirement of a large moon. Optimistic estimates, however, claim that more than 10,000 planets with intelligent life should exist among the more than 100 billion stars in the Milky Way galaxy alone.

The unlikely possibility of a giant-impact formation of a large moon is consistent with an infrared survey by the National Aeronautic and Space Administration's Spitzer telescope of more than four hundred young stars just a few million years past their planet-forming age. The survey revealed only one dust cloud signature large enough to be a possible moon-forming collision. Computer studies have shown that any accreting planet has some chance of being hit by a planetesimal object about one-tenth its size (as in the apparent formation of Earth's moon).

However, it is also evident that the right kind of glancing collision has a very low probability. Any estimate of this probability should take into account a minimum of five independent parameters, each with its own estimated probability of less than one-tenth: 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. The product of these factors gives an estimated probability of about one-millionth for such an event, which combined with other requirements, makes it surprising that intelligent life exists on even one planet.

Principal Terms

accretion: the accumulation of gas and dust in the solar disc, forming celestial objects such as asteroids, planetesimals, planets, and moons

dynamo theory: an attempt to explain the magnetic field of some celestial objects in terms of rotation, heat convection, and electrical conduction in a fluid and metallic inner core

ecliptic plane: the plane of Earth's orbit around the sun

escape velocity: the minimum speed required for an object to escape from the gravity of the earth or other celestial object

giant-impact theory: a theory that describes a glancing collision with Earth of a Mars-size planetesimal that blasted enough debris into orbit to form the moon

greenhouse gases: atmospheric gases such as water vapor, carbon dioxide, and methane that trap heat by absorption of solar radiation and re-emission of longer wavelengths that cannot escape from the atmosphere

magma ocean: a deep layer of molten rocks, volatiles, and solids that may have covered large portions of Earth and the moon in their early stages of formation

planetesimals: celestial objects that form by the accretion of dust and gravitational attraction that eventually combine to form planets

plate tectonics: a geological theory describing the large-scale motions of the earth's lithosphere that account for continental drift and mountain building

solar wind: the stream of high-energy charged particles flowing from the upper atmosphere of the sun, consisting mostly of protons and electrons

volatiles: chemical elements and compounds, such as oxygen, hydrogen, nitrogen, carbon dioxide, and methane, that are found in Earth's crust and atmosphere and which have low boiling points

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