Earth without the Moon

 A scientific speculation, if there were no Moon

The Moon, der Mond, la lune. Its name and even its gender vary from language to language but there is no question that it is key to our image of Earth. Can you imagine Earth without a moon? No beautiful, bright object traversing the night sky, hovering on the horizon, peeking through the trees on a cold winter’s night? No romantic moonlight, no Blue Moon and no lunar landings. Not only would we miss it, without the Moon, we might not even exist. Soaring temperatures, a flooded landscape, violent winds…. What would our planet be like without the Moon? Without the Moon, Earth’s geology, biology, and climate, as well as human philosophy, would be different in many significant ways. Earth did not always have a moon, so where did it come from? The leading scientific theory is that an object about the size of Mars, called Theia, collided with Earth about 4.5 billion years ago. Striking at an oblique angle, it raised a cloud of debris that then coalesced to form the Moon. This had profound effects on Earth. If Earth had no Moon, the postulated origin of the Moon through a collision between the proto-Earth and a Mars-sized object we now call Theia would never have occurred. The Earth/Theia collision shattered both worlds and the larger Earth attracted much of Theia’s iron and heavier elements. The resulting larger iron core within Earth created a stronger magnetic field which protected the planet from the radiation effects of energetic solar particles, allowing the earliest life-forms to evolve. Without the shield of a strong magnetic field, the evolution of life on Earth would have taken a different path.

The Moon is thought to have formed in a high-speed impact, when a body the size of Mars slammed into the young Earth about 4.5 billion years ago. The resulting molten rock, vapour and shattered debris mixed with debris from Earth to form a ring around our planet. Over time, this debris coalesced to make the Moon. Earth and its newly formed moon exerted a gravitational force on each other, slowing the rotation of Earth and lengthening the Earth day from 5 hours to 24 (Touma & Wisdom, 1998). In fact, to this day, the Moon continues to slow down the rotation of Earth, although only by 0.002 seconds per century. After its creation, the Moon was about 10 times closer to Earth than it is today. When Earth’s oceans formed over 3 billion years ago, the greater gravitational pull from the Moon’s proximity created titanic ocean tides hundreds of feet high which washed far inland. The violent churning of the oceans helped mix and distribute the waterborne chemistry which sparked the evolution of life on Earth. But with no lunar gravity driving the tides and assisting currents, those oceans would have become largely stagnant. Earth rotates faster than the Moon orbits Earth, causing friction as the land rotates under the tidal bulge. The friction between land and the tidal bulge pulls the tidal bulge forward so that it is ahead of the line of attraction between Earth and the Moon. The friction force between Earth and the ocean acts as a brake. This force is called tidal braking and it ‘pulls’ Earth backwards in its orbit, effectively slowing the rotation of Earth. Tidal braking also affects the Moon through force, which ‘pulls’ the Moon forward in its orbit, effectively speeding up the rotation of the Moon. This is what causes the orbit of the Moon to slowly increase, causing it to slowly move further from Earth. 

Even at its current distance, lunar gravity pulling on the tidal bulge creates friction between the water and the ocean basins, slowing Earth’s rotation. Without the Moon, our days would be much shorter. And without the stabilizing effect of the Moon’s gravity, our rotational axis would wobble, altering or periodically eliminating the seasons. Thus, without the Moon stabilizing the seasons, plant life could have evolved, but animal life may not have. Many plant and tree species are far more radiation- and harsh-climate-resistant than animal life, so a moonless Earth might be lush and green, but without the chirp of birds or the howl of a wolf. The gravitational attraction between Earth and the Moon also stabilised the tilt of Earth’s axis, and it is today’s constant tilt of 23.5° which gives Earth its predictable, fairly constant climate and its seasons. Without the Moon, however, the axis would have continued to wobble. The gravitational attraction between Earth and the Moon stabilises the tilt of Earth’s axis, giving Earth its predictable, fairly constant climate and its seasons. Because the Moon orbits Earth and is closer to it than any of the planets, its gravitational pull is both stronger than theirs and almost constant. Without the Moon, Earth would be subjected to the pull of the other planets as they orbited the Sun: when Jupiter was close, it would pull Earth in one direction, when Mars was close, it would pull in another direction. Earth would therefore be pulled by various forces over time and its axis would wobble. 

Another feature of our planet is its oceans: more than 70% of Earth’s surface is covered by salt water, rising and falling on a 12.5 h tidal cycle. The forces which create tides are complex, involving not only the centrifugal forces of Earth’s rotation but also the gravitational pull of both the Moon and the Sun. The effect of the Moon, however, is twice that of the Sun; this is because the gravitational force that one object exerts on another depends on both its mass and its distance. The gravitational attraction of the Moon causes the oceans to bulge towards the Moon. Another bulge occurs on the opposite side, since Earth is also being pulled toward the Moon (and away from the water on the far side). Because Earth spins, these bulges (high tides) occur twice daily at any one spot. The tides also show a pattern linked to the lunar cycle. When the Moon and the Sun are aligned, their combined gravitation pull is strongest and the tides are highest. When the Moon is in its first quarter or third quarter, the tides are lowest. We do not know how close the Moon was to Earth when it first formed, but we do know that it was farther than 12 000 km and closer than it is today (about 384 400 km). This means that it initially caused much larger tides than we experience today, tides which are thought to have been important in mixing the oceans and in the early evolution of life, some 3.8 billion years ago. Interestingly, the tides and the rotation of Earth have an effect on the Moon. Together, they pull on the Moon, making it spin just a little faster, and as it spins faster and faster, it moves further away from Earth, albeit at a rate of only 3.82 cm/year.

Even if humanity had evolved on a moonless Earth, only the distant, pinpoint planets and starry realm of the Milky Way would be our nightly companions. Archaeological evidence shows that 20,000 years ago, there was already interest in the Moon’s cycles. Without the nearby Moon sparking our curiosity about other worlds, our space exploration program would be vastly different. Humanity might not even be a spacefaring species. Marine turtles tend to lay their eggs at spring tides, when the highest high tides occur. These tides allow the female turtles to swim up the beach to lay their eggs above the high-water mark (where they hatch best). Jellyfish (Cnidaria) and many other groups of marine and freshwater zooplankton move up and down the water column according to a daily rhythm. If the Moon were to disappear, Earth days would become shorter and the animals would need to adapt to the shorter daily rhythm. Supposing the Moon just vanished tomorrow? We and all other organisms on Earth would be in serious trouble: we have evolved to live under a particular set of conditions and would then be faced with an entirely different environment. These changes would happen over the course of thousands to millions of years, which may sound like a long time, but the changes would be dramatic.

Without the Moon, the stability of Earth’s axis would be lost again, and with it, our predictable temperatures. Life on Earth, and humanity itself, owes its existence and part of its legacy to the silver orb that circles our green Earth. Many organisms, such as deer, mate at specific times of year. What effect might the loss of the Moon, and of our seasons, have on these organisms? Moving might be one option, but not for all organisms. Coral reefs, for example, are sensitive and complex ecosystems which might not be able to adapt fast enough to the changing water temperature and would probably die. Furthermore, as the temperatures changed, Earth would lose its reliably cold regions: the poles, which contain huge amounts of ice. This ice would melt and the oceans would rise, changing the coastlines all around the world. With the lack of stability in Earth’s tilt, we would also lose our regular seasons, with far-reaching consequences. And drastic changes in temperature would affect the growing season and climate for plants, making food production for the billions of people on Earth more complex. If we lost our Moon, and thus our regular seasons, how would this affect deciduous trees, which provide beautiful autumn colour at different places. 

What would have happened on Earth if, about 4.5 billion years ago, Theia had passed peacefully on its way without striking Earth and forming a moon? Well, life of some sort would probably exist on Earth, but humans almost certainly wouldn’t. Think of the very long course of evolution, the small changes, the minute adaptations that organisms make to their environment. It would only have taken small changes to Earth’s environment to have dramatically altered the course of evolution. And if the Moon had never formed, Earth would be a very, very different place. An Earth day would be only 8-10 h long, with no moon to slow it down. The faster rotation would cause winds of 160-200 km to sweep Earth’s surface. The tilt axis of Earth would wobble, resulting in dramatic changes in temperature over thousands to millions of years. And although our seas would still be tidal, the tides would be much smaller, caused only by the Sun. You might not expect to find so much overt speculation in a science article. However, encouraging people to imagine a world without a moon is a fun exercise to illustrate all the interesting ways that the Moon makes Earth the wonderful planet we know. Such an exercise not only introduces some complex physics in a simple context, but also gives all an opportunity to think about the course of evolution, and the way in which every aspect of our lives is affected by our environment in the present day universe.