Few celestial objects are understood better than our very own Moon. Indeed, its appearance in the night sky is taken for granted. Yet one question above all has baffled experts for years: why is the geological makeup of its famed dark side so different from the part that we can see? Well, it appears that we now have an answer.
“Everyone is a moon and has a dark side which he never shows to anybody,” Mark Twain once wrote. Though whether those words hold a semblance of truth or not is another matter. For what Twain’s remark certainly reveals is the ubiquitous nature of the so-called dark side of the Moon in human consciousness. It is the mythical section of a body otherwise well known.
Though the Moon is far from just a decorative presence in our skies; it plays a pivotal role in the Earth’s processes in more ways than one. Indeed, it controls the tides and provides light in the otherwise dark night sky.
However, experts believe that the presence of our Moon is possibly more important than most of us could ever imagine. That is because, without it, the Earth may never have evolved as it has. According to Space.com, it is possible that the gravitational pull of the Moon helped moderate fluctuations in the tilt of the Earth’s axis – therefore stabilizing our climate. And the result of this may have been life on our planet.
Yet for much of human history, relatively little was known about the Moon other than what could be observed from Earth. To many cultures, it was actually viewed as a god-like object. The Egyptians called the astronomical body Thoth, the Greeks knew it as Artemis, while Hindus called the Moon Chandra.
Naturally, our knowledge about the Moon increased over time. Italian astronomer and physicist Galileo Galilei was the first to challenge the long-held belief that the Moon was perfectly smooth. He found that instead, it was rocky and rough with brighter highlands and low-lying regions.
More groundbreaking discoveries were then made over the following centuries. These were facilitated by the fact that – in celestial terms at least – the Moon is close to Earth. It is only 380,000 kilometers away, which is very close in relation to other objects in our solar system. In comparison, Venus – the planet that orbits nearest us – comes within a mere 41 million kilometers from Earth.
But how did the Moon even come to be? Well, the widely held belief among scientists is that our planet and the satellite were formed by a huge collision between a body roughly the size of Mars and an early proto-Earth. This former object has been named Theia, after the mother of Selene, who is the Moon goddess in Greek mythology.
The Earth and Moon are, therefore, inextricably linked. Our home, of course, is the larger of the two bodies, and after the collision involving Theia it was able to retain enough heat to possess the type of tectonic activity that the Moon is almost entirely lacking. The astronomical body, it was believed, simply cooled down and practically froze from a geological perspective. However, recent discoveries have now challenged this long-held belief.
Many theories regarding the Moon’s formation and geology are disputed, though other theories have become fact. The satellite’s orbit time, for example, has long been established as 27.3 days. Meanwhile, it takes 29.5 days it to fully orbit the Earth – marking the beginning of another lunar cycle or new Moon.
Yet it was only in the 1960s that technological advancements allowed humans to finally begin physically exploring the Moon. The last year of that decade saw a lunar module pilot called Buzz Aldrin and commander Neil Armstrong become the first people to land on the satellite. The latter famously said at the time, “That’s one small step for man, one giant leap for mankind.”
Of course, setting foot on the Moon was an incredible human achievement. Yet even before then, the astronomical body was being explored at close quarters using unmanned spacecraft. Satellites were launched into space to take photographs of our Moon, and scientists were able to increase their knowledge due to the favorable vantage points afforded.
The advent of space travel naturally heralded a new era when it came to discovering the Moon’s secrets. Furthermore, NASA’s manned missions brought nearly 400 kilograms of rocks from the satellite, which helped scientists gain a better understanding of our closest celestial neighbor. According to Space.com, University of Tennessee, Knoxville, planetary geologist Larry Taylor noted the impact of the NASA missions, saying, “We answered so many fundamental questions.”
But there’s one particular riddle which has baffled scientists for years. Why is the geological makeup of the Moon’s far side – which cannot be seen from Earth – so geologically different from the part that we can see with the naked eye? Indeed, the answer to this has proven to be stubbornly elusive.
But what exactly is the dark side of the Moon? Well, due to how it rotates in conjunction with our planet, the result is that there is a part of the satellite that perpetually faces us. At the same time, therefore, there is a huge section of it that we can never see from Earth. This is the famed dark side of the Moon as immortalized in British rock group Pink Floyd’s eponymous 1973 album.
And how much of the Moon can even be considered “dark” in any case? Due to changes in the astronomical body’s viewing angle as it rounds Earth – a phenomenon known as libration – considerably more of the Moon’s surface can be seen from right here. Indeed, the website EarthSky notes that a casual observer can see around 59 percent of it from our planet.
For its part, the Soviet Union was particularly instrumental in the early breakthroughs in our understanding of the Moon’s dark side. The U.S.S.R.’s unmanned spacecraft Luna 3 allowed mankind to view it for the very first time, and the results were very surprising.
But why exactly were scientists so shocked at the images that Lunar 3 and other subsequent spacecraft sent back? Well, it turns out that the Moon’s surface geology is far from asymmetrical. Simply looking at the satellite with the naked eye will confirm that the surface we can see is full of light and dark areas. And for many years, observers believed the dark patches to be seas – similar to those that we have here on Earth.
The notion that the Moon’s surface contained seas was dispelled over a century ago, however. For what can actually be seen are large dark plains which are known scientifically as maria. This is in fact the Latin name for seas, so a nod to those earlier theories remains. But what images from Lunar 3 confirmed for the first time was that the dark side of the Moon had far fewer of these maria.
The side of the Moon that is perpetually facing our planet is comprised of a large proportion of these marias – a total of 31 percent, according to EurekAlert! The dark section, however, had very few examples of this geological feature. The website added that only 1 percent of its surface can be considered maria.
So, the question remained: why was the geological makeup of the Moon’s dark side – the part that perpetually faces away from Earth – so different from what we can see on our planet? Indeed, it appeared to make little sense.
Fortunately, a study released in June 2020 has provided scientists with a satisfactory answer. And it harks back to the very collision that originally formed the Earth and Moon. That huge impact resulted in a unique distribution of radioactive elements which led to the Moon’s strange asymmetrical composition.
The research was conducted by the Tokyo Institute of Technology’s Earth-Life Science Institute (ELSI), the Carnegie Institution for Science, NASA’s Johnson Space Center, the University of New Mexico and the University of Florida. And it involved examination of the samples of Moon rock that were brought back by NASA’s famous Apollo missions. Chemical analysis was subsequently performed on them in order to ascertain their geological composition.
Experts found that the chemical composition of the Moon rock was in fact due to volcanic activity. Not long ago, the scientific community had believed that the satellite was relatively passive in terms of volcanism – especially in comparison to Earth. But recent decades of research revealed that to be far from the case. Instead, the Moon’s volcanic activity had been more intense and recent than previously thought.
Previous research had resulted in the identification of a brand-new variety of rock unique to the Moon – dubbed “KREEP” due to the chemical composition of the minerals found in samples. These were potassium – with its chemical symbol K – rare-earth elements (REE) and potassium, with its chemical symbol of P.
It was the KREEP signature that gave certain patches of the Moon their dark appearance. These were the maria – those parts once thought to be seas. But while the near side of the Moon was full of this KREEP signature rock, the dark side was anything but, and this perplexed experts.
The new study called upon a combination of research practices to shed light on this phenomenon. Scientists relied on sampling and computer modeling in order to gain a better understanding of this lunar asymmetry. Crucially, the results were directly linked to a particular property found within the KREEP.
It is the presence of three unstable elements in particular that forms the basis of the new research results. Potassium, thorium and uranium are radioactive – meaning that they may be found in a number of differing atomic configurations. They also possess neutrons that are variable in number and are otherwise known as isotopes.
Some of these isotopes are, in fact, chemically unstable. The result is that they are constantly in a state of decay and emit radiation in the process. A byproduct of this is the yielding of further elements and the production of heat. Crucially, the latter is particularly important in this story.
The heat produced as a result of this radioactive decay can cause the rocks that contain these elements to melt. As well as a heating effect, the composition of the KREEP minerals lowered the melting-point of the rock which they were contained within. Hence the existence of the craters on the side of the Moon we can actually see. This also goes some way to explaining the co-localization of these elements.
The research team conducted a number of experiments, and each of these centered around the KREEP components of the Moon’s surface. In particular, the team focused on what impact these would have on volcanic activity on the Moon in terms of both when this activity occurred, and how much of it there was.
Not only did the research provide answers as to why there was asymmetry in the Moon’s geological makeup. An important element of the findings was that, finally, some context could now be provided in terms of timing. The order of certain processes which must have taken place on the Moon could now be established.
As a result of these research findings, we now understand better why the dark side of the Moon is so different to the side we can see. But that is not all, as ELSI co-author Matthieu Laneuville revealed more findings in a press statement.
“Because of the relative lack of erosion processes, the Moon’s surface records geological events from the solar system’s early history,” Laneuville said. “In particular, regions on the Moon’s near side have concentrations of radioactive elements like uranium and thorium unlike anywhere else on the Moon.”
“Understanding the origin of these local uranium and thorium enrichments can help explain the early stages of the Moon’s formation and, as a consequence, conditions on the early Earth,” Laneuville added. Essentially, a better grasp of the relative composition of different parts of the Moon – and the relationship between those – means that we learn more about our own planet.
The research project has also reached further conclusions. One is that the maria – the so-called seas which are enriched by these KREEP materials – have played an influential part in the evolution of the Moon’s geology since it was formed. Significantly, researchers believe that vital evidence of these processes which cause asymmetry are contained in other celestial moons within our solar system, and even on other rocky bodies found in the universe.
There should be little surprise that lunar revelations are still being unearthed. The Moon is a popular subject of scientific research, as understanding it better will help reveal more secrets about our solar system. To that end, NASA sent out another Moon probe called the Lunar Reconnaissance Orbiter (LRO) in June 2009. And this mission has led to a number of significant discoveries in the subsequent period.
Perhaps the LRO mission’s most significant finding has been the discovery of water on the Moon. Initially, the craft played a pivotal role in the observation of its sister probe LCROSS, which reached the satellite’s surface in October 2009. However, LRO and indeed subsequent craft have confirmed the existence of quantities of water in ice form found at the Moon’s north pole – among other places.
Of course, these aforementioned discoveries continue to pique our interest in our closest celestial neighbor. But at the same time, the fact that the Moon is resplendent in all its glory in our sky most nights will mean it continues to inspire and intrigue us, as it did our ancestors.
And one final point; it is actually erroneous to call the dark side of the Moon just that. It does, in fact, receive just as much sunlight as the part we can see. Therefore, a more accurate description would be “the far side of the Moon,” according to Space.com. And in terms of our location here on Earth, that’s exactly what it is.