Why Are There Craters on the Surface of the Moon?

The Moon is the only natural satellite of our planet. Although it is the most important to Earth's inhabitants, it is only the 5th largest satellite in our Solar System. It has a diameter of 3,474.8 km and is composed mainly of oxygen, silicon, magnesium, iron, calcium, aluminum, chromium, titanium and manganese.

As with the Earth, the internal structure of the Moon is made up of a core, mantle and crust. It is believed the lunar core is composed of iron, sulfur and another unknown element. Its mantle is a rocky layer made up of iron and magnesium. A much greater variety of elements can be found in the crust of the Moon.

This planetary satellite is synchronized with the Earth's orbit. It is in a rotational movement with respect to our planet, going through an oval-shaped orbit trajectory and taking 27.3 days to make a complete turn. It is this movement that gives rise to the phases of the Moon:

• New Moon: when the Moon is between the Earth and the Sun, it is eclipsed by the light of the sun. This means it is not visible from earth with the naked eye.
  
  
• Full Moon: when the Earth is the placed between the Moon and the Sun, the Moon is illuminated by which we can see it completely. Since we can see all of it, we know it as the full Moon.
  
  
• Waxing Moon: when the Moon is in an intermediate position we can only observe a quarter of its total. When the Moon is waxing, it means more sunlight is progressively shining onto it and it will eventually lead to a full Moon.
  
  
• Waning Moon: also only a quarter of the Moon is visible, but in this case the illumination is becoming less and it is heading toward a new Moon phase.

The gravitational influence of the Moon on Earth produces alterations in the tides. Together with its prominence in the sky and its regular cycle means this satellite is an object of great cultural influence both in the calendar, as well as in language, art and mythology.

We cannot see all of the Moon with the naked eye, but we can get a closer look with telescopes. Up until the advent of space flight, this was the best way to observe the Moon. Since the Apollo program from NASA (National Aeronautics and Space Administration) allowed landing on the Moon, we have been able to learn much more about the Moon's geography and other characteristics. One of the most intriguing are the Moon's carious craters.

Before NASA's first Moon landing and its subsequent Apollo missions, various theories had existed to explain the craters on the Moon's surface. One of the most common was the belief that the Moon had various volcanoes which had become inactive over time. This belief stated that eruptions similar to volcanoes on earth had created the craters. This is because Moon craters were mountainous and of a similar shape to Earth volcanoes.

One of the reasons this theory was eventually rejected was due to the location of the craters in relation to the surface of the Moon. While the lunar crater is at ground level, the vents of Earth volcanoes are located on top of the mountain. This difference meant that it was necessary for investigations and hypotheses to continue to develop.

Another theory developed in the early 20th century was known as the Welteislehre theory, created by a an enginerr known as Hanns Hörbiger. This theory claimed that ice was the basic substance for all of the various processes in the cosmos. It was this ice that created the craters of the moon. He did not come to his conclusion through scientific enquiry, but believed he was given it in a vision. It has never been accepted widely by astronomers.

An American astronomer put forward the currently accepted theory. He affirmed that the lunar craters are the result of the collision of asteroids at very high speeds. The impact of the asteroids left various imprints on the Moon's surface which resulted in the craters. The size of the asteroids and the velocity of its impact resulted in the diameter and depth of the crater. This means the Moon has not always had the same appearance.

This theory of Moon crater origins asserts the Moon was constantly growing initially as its gravitational force attracted rocks, dust and asteroids that floated around it. These either became part of its structure or they collided with it, leaving the craters we can see from the Earth.

When this process slowed down, the radioactive elements of some rocks influenced the Moon's topography. The radioactivity attracted dust which caused the formation of lava flows. In turn, these eliminated some craters, giving rise to vast dark areas. These were again deformed more recently by the continuous collision of asteroids that occurs throughout the universe.

A fact that supports this theory is due to the satellites and space probes humans have sent into space. They also present these notches or footprints. Although space scientists and those who study selenography believe this to be the most likely reason for the craters on the Moon, it is always possible this theory can change and develop. However, since the Moon landings, there have been recordings of craters being created by asteroid impact.

Although from Earth we observe the Moon with a characteristic brightness or glow, the Moon does not have its own light. What we observe is simply the reflection of sunlight. To get a better idea of how this works, we can compare it to the reflectivity of snow. Snow is capable of reflecting 100% of the light that falls on it, the Moon is only capable of reflecting only around 7%.

The Moon is not spherical. It has an oval shape due to its rotational movement and its acceleration. This same acceleration of its orbit is causing it to gradually distance itself from Earth by about 3.8 cm per year.

The Moon's atmosphere is very thin. The result means there is no balance in its temperature which can range from 134 º/C273.2 ºF in the sunniest areas and -153 ºC/-243.4 ºF in the darkest. There is also hardly any wind or precipitation, another reason there is little erosion of the craters and lunar relief.

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