A Lunar Lagoon


The Moon is something that has always been very symbolic to humans, being Earth’s only natural satellite and usually the brightest object in the night sky across the Earth. It has been engrained into human culture since the beginning, seen in Ancient Egypt generally on top of the god Khonsu (“traveller” or “pathfinder” in English) being a defender against demons, and by Bronze Age Celts as something to help souls navigate after the person has died.

It has also been seen in numerous paintings, like Vincent Van Gogh’s “Stary Night” (1889) and Caspar David Fridrich’s “Man and Woman Contemplating the Moon” (1818), all showing the moon as something key to life, whether it be to meditate over or as “King of the Skies”. [1]

Map of Moon overlaid with Knowth markings

Map of Moon overlaid with Knowth markings. Source: BBC News. Accessed 31/10/20

Ancient Evolution

There are many theories for the formation of the Moon, but there are three that are the most accepted

The first is the “capture” theory, whereby the Earth pulled in a passing body of rock, backed up by how Mars’ moons, Phobos and Deimos, were asteroids that were taken into Mars’ orbit; along with how the lunar rock collected by the Apollo missions, has shown that its composition is different to Earth’s. However, the Moon, unlike Phobos and Deimos, is spherical and orbits in the ecliptic (a plane in the solar system where most astronomical bodies in it can be found). This seems a bit strange to then assume that this is whole truth, so another theory exists that explains why the Moon is not like Phobos and Deimos, but more “planet” shaped and oriented.

It is the “Co-formation” theory. This is where the Earth and Moon were formed at the same time, with the particles that make up the Moon and that of the Earth got gravitationally bound together at the same time. This can happen, and could be the case since the Moon has a similar composition to Earth, and it’s orbit is explained by this theory. However, it seems that this may also not be the whole story, since, if this were true, then the Moon would have a similar density to Earth, since its core would have the same heavy elements within it, but it doesn’t; it has a lower density than Earth. There is, however, one more widely accepted theory (bar the lunar cheese theory of course!) which could be the closest to the real answer.

It is that of the “Giant Impact Hypothesis”. This has by-far the most exciting name, but an even more exciting explanation. It all starts with a Mars-sized body called “Theia”, which impacted Earth when it was only young, leading to Earth ejecting some of itself. Then, through gravitational attraction, the ejected parts of Earth and Theia came together into what we now call the Moon. This event was believed by NASA to be 100 million times larger than the asteroid impact that brought an end to the dinosaurs. However, the Apollo missions’ rock samples suggest that, again, this theory isn’t correct. This is because models show that 60% of the Moon’s rock should be made of Theia’s material, but this doesn’t seem to be the case. [2]

What ever the answer, each of these theories are likely to play some part, but we may never know what actually happened, since we can only use what is in front of us right now.

Artist's impression of collision of two object around the star HD172555, like the Large Impact Hypothesis.

Artist concept of two objects colliding in HD172555 system, like Large Impact Hypothesis. Credit: NASA/JPL-Caltech

Explosive Past

As well as in the possible events that happened in the Giant Impact Hypothesis, the Moon has had a few more violent episodes in its past, not least of which is its surprising volcanism.

It may not have large volcanoes like Earth has, such as in Hawaii, but what it does have is basaltic (the type of runny lava found in shield volcanoes such as those in Iceland) lava fields, being visible from Earth, each with the name of a “Mare” or “Sea” from the Latin (since it was thought that they were great oceans of water). They are vast planes, 19 in all, and with the addition of formations such as rilles (looking like rivers or valleys, like Hadley Rille[3], on the centre of the visible side of the Moon, for example), shows that the Moon is indeed volcanic.

However, they are slightly different to ones found on Earth. Lunar volcanoes are old compared to Earth’s; 3-4 billion years old in fact (being the typical age of a sample from a mare), compared to most of Earth’s positively spritely age of a few 100,000 years old. Also, it’s not got any recent volcanic evidence, unlike Earth, with volcanic activity happening all the time, and it has no plate tectonics (large fragments of crust that move across the molten rock mantle below) like Earth. Instead, it has near circular basins or mares, appearing where the crust is thinnest, being on the Moon’s near side (only less than 2% appear on the opposite thicker far side). Also, the lunar gravity of a 6th of Earth’s leads to runnier lava which spreads over a wider area (not in the cone shape which is seen on Earth, leading to the "mare" shape) [4].