If we condense the time of Earth’s existence into 24 hours, the moon would form just 10 minutes after the Earth was born. The Earth formed 4.56 billion years ago, and the Moon formed about 30 million years later. An impactor about the size of Mars struck the Earth at an oblique angle, and removed some of the magmatic mantle which was put in orbit around the Earth, together with some of the debris from the impactor itself, and this material eventually formed the Moon.
The tidal effect of a body increases as a cube of the distance, so the effect of the Moon’s tidal forcing on the Earth was extremely high at this time and provided some additional energy to the heating from radioactive elements present. As Earth started to cool, the Moon still was a source of heating that may have had some geological effect, keeping the Earth’s magma hot and perhaps forcing additional convection in the Earth’s mantle. With further cooling, the first crust started to float on top of the magma. During this period the Earth was subjected to increased meteor bombardment. Many of the large basins on the Moon are evidence of this late heavy bombardment. In this way, the Moon is a history book for the inner solar system and the Earth.
The usefulness of the moon for learning about early Earth:
The Earth was hit more often than the Moon because Earth is larger and has more gravity. When some of these impactors hit the Earth, the explosion caused some rocks and dirt from Earth to shoot up and land on the Moon. There is potentially a LOT of material from early Earth buried beneath the surface of the moon--up to a few hundred kilograms of Earth material per square kilometer of the Moon’s surface. It would be very interesting to dig this material up and sample these rocks from early Earth because almost nothing from this time period has survived on the Earth tiself. This is because of tectonic recycling of the crust plates and atmospheric weathering. We would try to detect some organics within those rocks, and that could tell us about the history of organic chemistry on Earth. Some of these rocks could even have preserved fossils of life. Such rocks could help us look further back into the fossil record, which now stops at 3.5 billion years ago. This way, we could possibly learn about the emergence of life on Earth.
By exploring the Moon, we also can get clues on how the Earth has evolved. We can study processes on the Moon that have also shaped the Earth, like volcanism and tectonics. Because the Moon is smaller than the Earth, the Moon’s radiogenic heating dissipated much faster. Because the Moon offers different conditions than the Earth, we can better understand how physical processes work generally by studying a larger range of parameters than just the Earth’s.
Lunar effects other than tides:
There are people who propose that the tidal effect of the Moon may have helped trigger the convection on the Earth that led to the multi-plate tectonics. The other planets don’t have the same tectonic cycle. For most of them, the crust is like a lid that doesn’t move much horizontally, and the magma and heat are blocked by this lid on the surface. The Earth instead has rolling convective motion that drags the crust, and then the crust plunges back down into the mantle and gets recycled.
There are some very subtle effects of the Moon in the climate and the oceans. One pattern that has been found recently is related to the El Niño phenomenon. You have a cold undersea current coming from the Antarctic sea, and that creates the Humboldt stream which keeps the sea around the South American coast near Peru and Chile quite cold. Because of this, there are fewer clouds and less precipitation there. Sometimes this current drifts away from the coast, and then you have much more cloud formation and a period of very bad weather over South America which we call El Nino. People have connected some newly discovered streams with how the Moon’s tidal effect influences the mixing of the deep ocean. If you took away the Moon suddenly, it would change the global altitude of the ocean. Right now there is a distortion which is elongated around the equator, so if we didn’t have this effect, suddenly a lot of water would be redistributed toward the polar regions.
In addition, the Moon has been a stabilizing factor for the axis of rotation of the Earth and thus has contributed to the maintenance of a stable climate. Mars, in contrast to Earth, has wobbled quite dramatically on its axis over time due to the gravitational influence of all the other planets in the solar system. Because of this obliquity change, the ice that is now at the poles on Mars would sometimes drift to the equator. But the moon has helped stabilize our planet so that its axis of rotation stays in the same direction and consequently, we had much less climatic change than if we had no moon. And this has changed the way life evolved on Earth, allowing for the emergence of more complex multi-cellular organisms compared to a planet where drastic climatic change would allow only small, robust organisms to survive.
We know that the Moon has influenced biology because of tides, but it has had other influences on biology, as well. For instance, the eyesight of many mammals is sensitive to moonlight and the level of adaptation of night vision would be very different without the Moon. Thus the moon has influenced evolution in this respect.
Human vision is so sensitive that we are almost able to work by the light of the Milky Way. The full Moon has more light than we need to see at night. For most of our history, we were hunting and fishing or doing agriculture, and we organized our lives by using the Moon. It determined the time for hunting, or the time where we could harvest. That’s why most of our calendars are based on the Moon and there are many examples of human behavior that may have been based on lunar phases.
Studying the Moon helped us determine distances in the solar system and the size of celestial objects. By studying lunar phases, for example, people were able to determine how far the Moon is from the Earth, the size of the Earth, and our distance from the sun. The Moon has inspired humankind to learn how to travel to space, and to bring life beyond Earth’s cradle.
Understanding the influences of the moon on life on Earth is important for recognizing the importance of moons to life in general. If moons are deemed necessary for life, then the search for life can be further narrowed to only planets with moons. Is the moon essential for life on Earth? I am not sure. But there are many helpful influences of the moon on Earth. The one that seems particularly interesting (besides tides which i discussed in an earlier blog) is that of the moon's role in the stabilization of Earth's rotation around its axis. I had never thought about how this related to the stability of Earth's climate, but this may be an important consideration for habitability. This also depends on how many other planets are in the solar system, however, and how the other planets influence the rotation of the exoplanet of interest. Nonetheless, this is an intersting thought. Learning more about our moon both helps us learn more about Earth itself, but also about the importance of moons in general. Now the question is: And are the presence of moons any indication of increased habitability on exoplanets?