This is a series of posts about our celestial neighbors, highlighting facts you may not have known about them. In this first installment we will learn about how we owe our very existence to the closest neighbor on the block: The Moon.
Just the Facts:
The moon is a bit over a quarter million miles away from the Earth, or about 60 times the radius of the Earth. The moon has a radius roughly one quarter of the radius of the Earth and the mass is about 1/80 that of Earth. Putting those facts together means that the gravity you would feel on the moon is about 1/6 of that on Earth. The moon has little to no atmosphere which means no wind to erode craters or astronaut footprints.
Where did it come from?:
There have been many different origin stories for the moon throughout human history ranging from the supernatural to the scientific. However, the currently accepted theory, first seriously proposed in 1976, is called the giant impact hypothesis. To fully understand the power of this theory, we need to travel back in time to the early days of the solar system.
As most people are aware, planets form slowly over time as particles and atoms are gravitationally attracted to one another and coalesce into a bigger body. As the object becomes bigger, it gets hotter. Eventually it gets hot enough that all of the material in it melts into a giant ball of magma. However, just like how oil floats on top of water, the more dense elements such as uranium and iron (the metals in general) slowly sank down to the center of this ball of magma while the lighter elements (the ones that make up rock and the atmosphere) stayed closer to the surface. This separation process is called differentiation and as the Earth was well underway with this process is when something incredible happened.
Roughly around the time that the outermost layer of the Earth would have been cool enough to solidify, a planet roughly the size of Mars collided with Earth at an oblique angle. Such a collision would have been terrifically violent. Think about a meteor hitting earth today. There is a crater that is left behind from the rock and dirt that is ejected out from the impact. Now, scale that up to the size of another planet hitting Earth. There was so much material flung out into the far reaches of space that much of it is probably still flying through space somewhere, never hitting anything since. However, there was also a significant amount of material that was ejected with just the right amount of energy to fall into an orbit around the Earth. This stuff eventually coalesced into what we now call our Moon. There are many good reasons to think that the giant impact hypothesis is true (and a few to not think it is true or not entirely), but for the sake of brevity I will let you look at them yourself or do a separate post about it.
Why should I care about it?:
I will try to convince you that if it were not for the moon, human life, or even life itself, would not have evolved on Earth or had much of a hope of becoming a technologically advanced society.
Reason 1: The Tides and the Beginnings of Life
As everybody knows, the Moon is what has the biggest influence over the ocean tides. Also, many know that the Moon is slowly (38mm per year) moving farther away from Earth. However, this slow motion away from earth, over billions of years, is quite significant. Billions of years ago, when the first oceans were forming on Earth, the Moon was much closer to the Earth. That means that its gravitational influence on the tides would have been much greater. In fact, it would have been so great that as the Earth rotated there would have been two massive tidal waves that traveled around the Earth pointing to and opposite the Moon the whole time. It sounds too incredible to be true, but in this early Earth there was a massive wave that washed over the entire Earth twice a day!
You might be wondering how massive waves could help life develop, but you have to keep in mind the state of the Earth at that time. It was still quite volcanically active and hot. All of this thermal energy would have been able to initiate countless chemical reactions creating new, organic compounds left and right. However, this in itself would be unlikely to be sufficient for self-sustaining life. What needed to happen is the organic compounds created at one part of Earth to get to some other organic compounds created elsewhere. This is where the massive tidal waves play an integral part. These waves would have made sure that the oceans stayed well mixed so that every organic compound could interact with every other compound, eventually creating the right mix to form life!
Reason 2: Our Axis and the Seasons
All of us are familiar with the seasons that happen every year. We have summer, fall, winter, and then spring. These four seasons come one after another year after year. Most of you are also probably familiar with the fact that seasons happen because the Earth is tilted in relation to the sun. As the Earth revolves around the sun different sides of the Earth face the sun more directly at different times, creating the seasons. The regularity of the changing of the seasons have become so important for life on earth that all different kinds of life have adapted to them: trees, birds, fish, insects, mammals. Many different species have some behavioral trait, whether it be migration, hibernation, or shedding leaves that depends on the regular changing of the seasons.
What you may not realize is that we owe this steady progression to the moon. If we did not have such a large celestial satellite orbiting our planet, the axis of Earth’s tilt would be very different today than it is now. In fact, there is no way to really predict what it would be. The moon pulls on the Earth strongly enough to stabilize the tilt axis so that it virtually does not change over time. However, without the moon there, the axis would have been very unstable and would change quite unpredictably. This means that what is the north pole one year could lie on the equator only 1000 years later. This means that there would have been massive climate swings all over the planet; making the steady seasonal cycles we are accustomed to quite impossible.
Having stable seasons was a big requirement for the development of advanced life because it allowed plants, that cannot move to avoid arctic conditions, to take root in one location and flourish and evolve. More importantly for humans, regular seasons make things like farming and agriculture possible. It was our ability to grow our own food that allowed humanity to grow out of the hunter/gatherer phase of existence and create civilization.
Summing it all up:
The moon is a pretty cool neighbor to have. It is at once familiar because we see it every day and mysterious. For example, it was just recently proposed that the Earth had two moons at one time that merged into the one we have today. It may just be a big lump of rock for the most part, lacking the sex appeal of some other moons in the solar system; but it has inspired and helped the human race reach out and touch it. And now, as we strive to go further, let us not fall into the trap of selling the moon short in this wondrous thing called the universe, because without it, we would never have made it this far.