Weird Science | How the Moon has fascinated humanity
Vikram's landing on the Moon's south pole is just one of many firsts, as mysteries about the Moon's formation remain
Vikram the lander’s successful touchdown on the Moon’s south pole, part of ISRO’s Chandrayaan-3 mission, will likely be just one of many firsts, for the Moon offers many more mysteries for humans to solve. Beyond its unexplored territories, there remains the question about how the Moon formed in the first place, even though one among various theories is widely accepted today. Beyond its origin, again, the Moon continues to fascinate modern humans as much as it did our ancestors, partly for science, partly with myths.
The Moon was not just worshipped as a deity but also held responsible (it still is) for diverse events. Even today, the myth persists that a full moon influences people into making irrational decisions, an idea with no scientific basis. Wolves were thought to howl at the moon, giving rise to the myth about werewolves, humans who supposedly turn into wolves when the moon is full.
That said, the Moon does influence life on Earth. It is responsible for the tides in the oceans, dictates the migration of birds, and influences the reproduction cycles of some animals. And Earth’s natural satellite has not only revealed some of its mysteries but also inspired science.
What Moon has told us
Ancient humans used the Moon as a calendar system, secure in the knowledge that it follows a regular orbit around our planet. As knowledge of geometry evolved, scientists calculated the Moon’s distance from Earth based on the shadow cast during a lunar eclipse.
In the 17th century, Galileo’s telescopes revealed, for the first time, mountains and craters on the lunar surface. Since the 1950s, manned and unmanned lunar missions have revealed more. Among these discoveries were views of the far side of the Moon.
Although the Moon is rotating on its axis, it always looks the same from the Earth, presenting a face with features that we identify as “the man in the Moon”. The Moon and Earth are gravitationally tied in a way that the Moon takes the same amount of time to rotate about its axis as it takes to make one orbit around Earth. This rotation causes the same side of the Moon to always face Earth.
As a result, the far side of the Moon was a mystery until the Soviet Luna-3 probe clicked the first photographs in October 1959. Over the next few years, the USSR released clearer and clearer images. In December 1968, astronauts on NASA’s Apollo 8 mission, became the first humans to see and photograph the far side, although they did not land on the Moon. Most recently, China’s robotic spacecraft Chang’e-4 in 2019 became the first mission to land on the far side.
All these missions have enabled scientists to identify and name several craters and other features on the far side over the years. But one mystery endures: how was the Moon formed?
What it hasn’t told us
The most widely accepted theory about the Moon’s origin is known as the giant impactor theory. It proposes that an object, which is often described as “Theia” and is estimated to have been about the size of Mars, struck Earth just after the formation of the Solar System. This would have ejected large volumes of material from both objects, and this material eventually combined to form the Moon.
While many scientists contest the giant impact theory, it is still seen as more plausible than other theories that have been proposed and discounted over the years.
>The fission theory suggests that the Moon was once a piece of Earth that broke loose from its outer layers, with the separation giving rise to the Pacific Ocean. The British astronomer George Howard Darwin (Charles Darwin’s son) championed this idea. What he proposed was thought to be possible during his time, but scientists today discount it. Among the grey areas, the theory does not explain how materials on the Moon were baked more than those on Earth.
>The capture theory, on the other hand, proposes the Moon was formed independently of Earth, and was later captured into Earth’s orbit. For such a capture, there would need to have been some event that slowed the Moon down by just the right amount at just the right time. Such “fine-tuning” is believed improbable.
>The condensation theory proposes that the same dust cloud that led to the formation of Earth also created the Moon. The Moon has receded ever since, but was never part of Earth. While the Moon indeed continues to recede from Earth, ever so slowly, the condensation theory would require that the Moon and Earth should have nearly the same composition, which they do not.
To come back to the giant impactor theory, first proposed in the 1970s, it can explain the composition of the Moon to a far greater extent than the other theories. Those who are not convinced, however, raise various questions. If there was indeed a giant impact, a magma ocean would have formed on Earth’s surface, but there is no evidence that Earth ever had one. Scientists note that the ratios between various elements on the Moon are not explained by the giant impactor theory. Also, if Earth were impacted, why wasn’t Venus struck by a similar object, which should have resulted in Venus having its own Moon?
Hunt for an answer
The book Asimov’s New Guide to Science discusses the fission, capture and condensation theories and notes how certain aspects remain unresolved. Moon rocks brought back to Earth should have resolved the mystery, but Isaac Asimov dwells on how their composition raises questions about each of the theories.
“… Scientists have been heard to mutter that if the evidence for the Moon’s origin is carefully considered, then the only possible conclusion is that the Moon is not really out there — a conclusion, however, that just means they must continue the search for additional evidence,” Asimov writes.
“There is an answer, and it will be found.”
Kabir Firaque is the puzzles editor of Hindustan Times. His column, Weird Science, tackles a range of subjects from the history of inventions and discoveries to science that sounds fictional, but isn't it.