The composition of the moon and the peculiarities in its relationship to the Earth have led scientists to the conclusion that the moon was almost certainly formed when a small planet the size of Mars smashed into the Earth early in its history. Traveling at 25,000 miles per hour, this massive projectile would have met a violent end, generating the explosive power of a billion, trillion tons of TNT, and ejecting five billion cubic miles of the Earth’s crust and mantle into orbit. This shattered material formed an enormous ring of debris around the planet. The intense heat generated by the impact also ejected some of the Earth’s atmosphere, most of it carbon dioxide at the time, into space. Much of the planet’s recently-solidified crust and plastic mantle melted back to magma, allowing the heavier elements, such as iron and nickel, to migrate to the Earth’s core. The fragments of rock orbiting the Earth had a gravitational attraction to one another, and over time they coalesced into one solid object, becoming the moon as we know it today. The massive devastation caused by the impact ultimately created conditions on our planet that made it far more suitable for the evolution of the great diversity of life that now exists.
The most obvious effect of the moon on the Earth is its gravitational attraction, which causes the ocean tides. Water, not being tied down, is attracted to the moon when it is overhead, causing it to increase its rate of movement towards the moon’s strong gravitational pull. There is a surprising reverse effect as well. The bulge of tidal water also pulls on the moon, causing it to move, and over time to increase its speed. Tidal water loses its momentum when the moon moves on, but the moon has been increasing its orbital speed over the years—and this is causing it to move further from our gravitational field. It is now thought that the moon was originally about 20,000 miles from the Earth, and has moved to its position 240,000 miles out over the past four billion years.
The mass or size ratio of the Earth-moon pair is more than ten times that of any other known planet-satellite pair in the solar system. That is, while several other satellites are heavier than the moon, no other planet possesses a satellite having a mass which is such a large fraction of the planet’s mass.
Curiously, the size of the sun and moon as seen from Earth are the same. The moon is 400 times smaller than the sun, but it is also 400 times closer to the Earth. Because of this situation the moon is able to occasionally eclipse the sun exactly. Computer studies furthermore show that this perfect eclipse condition is unique among all the known moons of the solar system. Because the moon is slowly moving away from the Earth (at a speed close to that of fingernail growth, about an inch a year), the precise fit of moon over sun is a temporary arrangement.
Energy within a given system is conserved, so if the moon is gaining speed and energy, the Earth must be losing some. And indeed it is. The eastward rotation of the Earth is retarded by the westward motion of the tides, slowing down the rotation (the Earth rotates to the east, while the moon orbits to the west around the Earth). This slowing of the rotational speed of the Earth means that the day is getting longer by about 0.002 seconds per century. It doesn’t sound like much, but over billions of years it adds up. By counting the growth rings in 400-million-year-old coral fossils and in 3-billion-year-old stromatolites, geologists calculate that Earth was rotating four times faster when it formed than it is today. The tidal effects of the moon and, to a much lesser degree, the sun have lengthened the day at the rate of one second per hundred thousand years, from six hours to twenty-four hours over four billion years. This is good news for those who complain there’s never enough time in the day.
When the fragments of space debris orbiting the sun first accreted into and formed the Earth, the Earth’s axis of spin would have been at 90° to the sun, in a straight upright position. But the Earth’s axis is now tilted at an angle of 23.5° to the sun. The planetessimal that smashed into our planet and initiated the formation of the moon also knocked the Earth’s axis over to its present position.
What if this collision had never occurred; what if there were no moon? If the Earth had not been hit by a small planet in the early years of its development, there would be no 23.5° tilt relative to the sun, and thus there would be no seasons on Earth. Summer and winter have alternated for four billion years because first the northern hemisphere and then the southern hemisphere tilts toward the sun, receiving alternately increasing and decreasing amounts of solar radiation each year. Without this tilt, life on Earth would be far less diverse because ecosystems would vary much less over the course of a year. There would be little or no bird and mammal migration, as this movement of life is tied to the changing seasons and to following the energy of the sun. Heating in the tropics and cooling in the high latitudes would both be greater, making high and low latitudes more inimical to life.
Ocean tides would be approximately one-third as high as they are today, responding only to the gravitational pull of the sun. This loss of tidal energy would decrease the agitation of nutrients pouring onto the continental shelf from rivers, diminishing the vitality of the coastal ecosystems. Tides would have been vastly more powerful back when the moon was much closer to the Earth, and this powerful interface of the chemistry of land and sea as huge tides advanced and receded may have been critical in the early formation of life on the planet.
The collision of the smaller planet with the Earth resulted in the ejection of the majority of the Earth’s primordial atmosphere into space, much of it composed of carbon dioxide. If this collision had not occurred, we would likely have had an atmosphere similar to that of Venus, which is 80 times as dense that of the Earth (equivalent to being one mile beneath the ocean). Such a thick atmosphere on Venus resulted in a runaway greenhouse effect, creating a bone-dry planet with a surface temperature of 800°F. The Earth would have suffered a similar fate if the majority of its primordial atmosphere had not been ejected into outer space. In fact, the Earth is 20% more massive than Venus and further away from the Sun, both factors of which should have lead to a terrestrial atmosphere much thicker than that of Venus. For some strange reason, we have a very thin atmosphere, just the right density to maintain the presence of liquid, solid and gaseous water all at the same time, a condition necessary to life.
The drag on the Earth created by the gravity of the nearby moon has slowed the Earth’s rotation so much that day length has increased from six hours to twenty-four hours. If the Earth’s rotation had not slowed, the faster speed of revolution would result in much higher winds than exist on Earth today; they would reach 100 mph regularly and in storms would reach at least 300 mph. Life on land under these circumstances might be impossible, and certainly there would be no upright organisms such as trees or large terrestrial animals on the planet.
Another fortuitous result of the collision of the Mars-sized planet with the Earth is the presence of the Earth’s large and heavy metallic core, which increased its iron and nickel content at the time of the impact. In fact, the Earth has the highest density of any of the planets in our Solar System. Our large nickel-iron core is responsible for the powerful magnetic field that encircles the planet. This magnetic shield protects the Earth from radiation bombardment. If this shield were not present, life as it exists today would not be possible on Earth. The only other rocky planet to have any magnetic field is Mercury, but its field strength is 100 times less than the Earth’s. Even Venus, our sister planet, has no magnetic field.
The “dark side” of the moon (it is not actually dark, just not visible from Earth) is more peppered with craters from meteor impacts than the side that faces the Earth. Many of these meteors would have been on a trajectory towards the Earth, so it can be said, at least in a poetic sense, that the moon is watching over the Earth, protecting it from further extraterrestrial impacts.
That early collision with a planetessimal is very likely a critical factor in the initiation of the constant movement of the Earth’s crust that we call plate tectonics. The Earth’s crust is much thinner (two miles thick) than that of Venus (twenty miles thick). Tectonic processes cannot occur with such thick plates as exist on Venus, as heat from the planet’s core cannot penetrate it. If much of the crust of the Earth had not been blown away during the formation of the moon, it would not be possible for the convection currents rising from the Earth’s semi-molten core to crack and move the plates making up the Earth’s crust. Without plate tectonics all land would be washed into the Earth’s great oceans in short order (geologically speaking; e.g., millions of years). We would live on a water planet. Nutrients critical to life would be washed into the Earth’s sedimentary rock and be lost forever to the biosphere. Life, if it was ever initiated, would almost surely be extinguished.
We are left with the unsettling insight that an unlikely and exceedingly violent collision with another planetary body apparently increased the viability of life on Earth, greatly enhancing the conditions that lead to diverse and abundant life.
References: What If the Moon Didn’t Exist? Neil Comins, 1993, and Where Did the Moon Come From? 3 min, BBC. Dana Visalli is a naturalist living in Twisp, Washington. He produces a quarterly magazine, The Methow Naturalist, which is where this essay originally appeared. Read more of his writings here.