Symbolic
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. 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 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].
Evidence of small basaltic eruption on Moon (irregular mare patch).
Credit: NASA/GSFC/Arizona State University
Mission to the Moon
As commented previously, the various missions to the Moon brought not just the astronauts who went there back with them, but rocks too. Some of which have only recently been opened for scientific analysis, these rocks were sent back to Earth and analysed with X-rays for a cross-section of the rock, and mass spectrometry, allowing for molecules of rock to be identified, so as to find out, for instance, how similar the composition of lunar rocks are to those on our Earth [5].
The first lunar sample to be delivered back to Earth was in Apollo 11, with 22kg of rock from the Sea of Tranquility (Mare Tranquillitatis), leading to the discovery of lunar basalt of 3.6 billion years in age, revealing the Moon’s volcanic past. The lighter anorthosite rock (rich in calcium) was also found, being originally from the lunar highlands, backing up the Giant Impact Hypothesis since the early stage of the Moon explains this anomaly (as the various rocks coalesced into one body).
Moreover, this particular sample included rocks known as “breccias”, which are fragments of rock which melted together because of the intense heat caused by meteorite impacts, showing that the lunar surface was heavily bombarded at some point in its history. Indeed, some say Apollo 11’s samples alone accounted for ~80% of our understanding of the Moon today [6]. In total, 382kg of samples have been brought to Earth by the Apollo missions [7], and many more was brought back by other missions such as the Soviet “Luna” missions.
5.5kg of lunar rock from Apollo 11 in Houston. Credit: NASA
One small step for man… into an alien puddle?
Well, not quite! On Monday, NASA announced the most recent bit of science found out about the Moon: it has water. Not as much as on Earth though. In fact, the Sahara desert contains 100x that which was discovered on the Moon.
Using NASA’s/DLR’s (German Aerospace Centre) SOFIA (or Stratospheric Observatory of Infrared Astronomy), being “the world’s largest flying observatory” [8] evidence for water has been found on the Moon, in the Clavius Crater on the southern hemisphere’s Earth facing side. It had long been though that the Moon had no water because it would all evaporate off into space because of the Moon’s lack of a sufficient atmosphere in order to keep this water.
The data from both SOFIA and the Apollo landings’ lunar samples has been used to reveal the possibility that micrometeorites delivered small amounts of this water to the surface, or that the solar winds from the Sun delivered hydrogen to minerals containing oxygen on the lunar surface, creating hydroxyl, before micrometeorites delivered radiation to transform this into water. This water could then either be trapped in beadlike structures within the lunar soil from the temperature of the micrometeorite impact, or in the grains of lunar soil, to shelter from the Sun (of which the Clavius Crater has in plentiful supply), which would evaporate it, then explaining why it's still here.
This is exciting because of how NASA plans to head its Artemis program in the near future, bringing people to live on the Moon, needing to find resources to sustain life there, with one of the most important ones being water, needed not only for drinking, but for farming, for example, meaning human presence on the Moon can not only happen but can be sustainable.
This is the first time SOFIA has ever observed the Moon [9], and the first time all these pieces of the “jigsaw” have been pieced together to make such a dramatic discovery. Hopefully, we will find even more deposits as years go by, and maybe even find enough to sustain astronauts on the lunar surface without needing water from Earth.
Clavius Crater (south up). Credit: NASA
By George Abraham, ADAS member
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References
"The Moon: One of the earliest human symbols". BBC Culture. Archived from the original on 31st October 2020.
"How was the Moon Formed?" Space.com. Archived from the original on 31st October 2020.
"Volcanism on the Moon: Sinuous Rilles". Oregon State University. Archived from the original on 31st October 2020.
"Volcanism on the Moon". Oregon State University. Archived from the original on 31st October 2020.
"Long-Sealed Moon Rocks Collected on the Apollo Mission Just Opened for the First Time". Live Science. Archived from the original on 31st October 2020.
"What have the Apollo 11 Lunar Samples taught us about the Moon?". The Sky at Night Magazine. Archived from the original on 31st October 2020.
"Lunar Rocks and Soils from Apollo Missions". NASA. Archived from the original on 31st October 2020.
"SOFIA Discovers Water on a Sunlit Surface of the Moon". Youtube. Archived from the original on 31st October 2020.
"NASA's SOFIA Discovers Water on Sunlit Surface of Moon". NASA. Archived from the original on 31st October 2020.
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