The origin of the Moon: Two impossible sister planets
Fifty years ago, Neil Armstrong, Buzz Aldrin and Mike Collins came home from the moon, bringing with them the first ever samples of lunar rocks to see and study for all. Heads have rolled since then, and new hypotheses on the birth of the moon popped up like mushrooms.
Origin of the Moon: from the world's largest catastrophe
to invaluable companion.
Our planet has continuously changed throughout its long history. But most changes were gradual and step-wise, for example the oxygenation of the atmosphere or the erosion of mountain ranges. Rarely the earth was shaken by something sudden or dramatic at a global scale, except maybe for the asteroid that silenced the dinosaurs. But one particular event eclipsed all others. The formation of the moon, as elegantly described in Prof. Dr. Robert Hazen's book The Story of Earth, shook our planet on its foundations. Without this stellar drama, maybe life would not have been so successful as to perdure almost 4 billion years till now.
Also the research on the origin of the moon caused quite a stir.
Much has been written about the moon, in many fields of literature. Since early humanity, the moon has exerted an irresistible attraction on us. The Greeks associated their oldest gods, the Titans, with the celestial bodies.
Theia, one of the titans, was the goddess of the heavens. She gave birth to Selene, the goddess of the Moon.
The moon appears as an inoffensive milky-white disc, ever changing shape and place. Terrestrial life is rocking to the rhythm of the moon. Before the use of instruments, humans kept track of time by the phases of the moon.
At full moon, life is restless, more active. During new moon, the night is shrouded in darkness, but the stars shine all the brighter. Peoples from all over the world read upcoming disasters from a blood moon or a super moon. Humans attribute lunar phenomena to the whims of the gods and of fate.
Twice a day, the world's oceans are drawn to the moon, causing the tides. Less evident are the tidal effects of the earth and the moon themselves, which deform under the mutual forces of gravity. In the long run this will reduce the rotation speed of the earth. The moon, which is only one of many moons in our solar system, is yet so important for life.
The moon keeps the movements of the earth in check. The earth not only rotates around its axis like a spinning top, but the axis itself also wobbles, as reflected in more or less regular climate fluctuations over millennia. Without the moon, the earth would dangerously wobble out of control, throwing the climate from one extreme into another, pushing any surviving life to its limits.
Has the moon, our invaluable companion, always been there? The answer is yes and no. The solar system and the planets were created almost 4.6 billion years ago, but earth was still moonless. However, the moon did not take long to make her appearance, some 50 million years after the formation of the solar system, according to recent studies [i].
Before the moon, the solar system originated from a rapidly rotating protoplanetary disk of cosmic matter, with the young sun in the middle. Obeying the laws of gravity, smaller objects, boulders and planetesimals were attracted towards the larger ones, in a process called planetary accretion. Eventually a dozen larger, growing protoplanets remained.
These larger protoplanets ploughed through the solar system like gigantic vacuum cleaners, consuming any remaining debris, meteors and miniplanets that crossed their paths. The formation of the planets happened fairly quickly, geologically speaking, in less than a million years [ii]. At this early stage, the young, incandescent, moonless earth was incessantly bombarded by meteors. The earth's surface was a raging wasteland, covered with magma seas and towering lava fountains at the locations where the meteors had hit.
With each impact, the surface of the earth became a liquid surface again, to solidify quickly, in the chilled temperatures of the surrounding universe. But these meteor rains and initial collisions did not produce a moon.
To understand better where our ideas on moon formation come from, because they indeed came a long way, we turn back to the present, not to today but to the last century.
Let's see what the older generations of scientist, i.e. the ones from before the Apollo program missions, came up with. Their ideas were based on one striking observation.
The Case of the Giant Moon
The moon is gigantic compared to the earth [ii]. Some even refer to the earth-moon system as a double planet. The moons of other planets of the solar system are very small compared to their host planet. Mars is orbited by tiny Phobos and Deimos, whereas Ganymede and Callisto are enormous moons, but they pale into insignificance next to Jupiter.
The diameter of our moon, on the other hand, is more than a quarter than that of the earth. Researchers suffered major headaches explaining this Case of the Massive Moon. Unlike the other moons, which are 'hijacked' asteroids, the origin of our giant moon required a more complex explanation.
Asteroid or ejected hunk?
Before 1969, there were three working hypotheses. George H. Darwin, son of another famous Darwin, speculated that a chunk of matter spun off the earth, which then rotated much faster around its axis. In this fission theory, the ejected material would form the moon. The second theory is that of capture, in which the orbits of the earth and an asteroid crossed and the asteroid was eventually captured in the gravity field of the earth. In the co-accretion theory, the moon formed at the same distance from the sun as earth, and materialized from remaining debris from the initial solar nebula.
First moon samples
The 1969 moon landings of Apollo 11 and subsequent missions sent a tsunami through the scientific community. For the first time, pieces of the moon could be touched, scientists could see what the moon was made of. They discovered that the minerals making up the moon, and their relative abundances, were very similar to those of the Earth. But there are also important differences.
The moon contains more titanium, and a different amount of chromium. Furthermore, the moon has much less iron, and its core is significantly smaller than that of Earth. The moon lacks the volatile elements (gases) that are so abundant on earth and that make life possible, including nitrogen, carbon and sulphur. The isotope ratio of the element oxygen (isotopes are atoms of the same element, with a varying number of neutrons) is the same. The moon and earth cannot be distinguished from each other, based on isotopes. And there are no traces of water-bearing minerals on the moon. The moon thus looks very much like the earth, but why are there so many differences?
The advocates of the older hypotheses were in deep trouble with this wealth of new data. One by one, the old hypotheses were proven wrong. The co-accretion theory was the first to fall short. If true, the composition of the moon must be identical to that of the earth, which is not the case. The capture theory also met unsurmountable obstacles, because the earth could only capture an object if it formed at the same distance from the sun as the earth, in which case the composition must also be fairly similar.
And even if a wandering moon or object from a faraway place in the solar system settled in an earth-crossing orbit, then the speed of this moon would be very high. Computer model show that at such speeds, no object could be captured by earth's gravity field.
Only the fission theory could explain the compositional difference. The ejected chunk had no or little material from the earth's core, which was a good argument for the lack of iron in the moon. But this hypothesis suffered a final blow by the laws of physics. Computer models calculated that the earth never rotated fast enough around its axis to throw such a chunk in orbit.
During the 1970s and 1980s, scientists worked frantically on new ideas. At a revolutionary conference on the origin of the moon[iii], held in Kailua-Kona, Hawaii, in October 1984, the 'Giant Impact' (GI) hypothesis rose from the ashes of the older hypotheses.
As the ambitious name implies, the moon was born after a huge impact. This theory gave the best solution for the apparently irreconcilable properties of the moon. But how should we envisage the Great Impact? Which celestial bodies clashed upon each other?
Fifty million years after the formation of our solar system, proto-Earth almost had its current size, but was still occasionally bombarded by smaller debris and particles. This had little effect on proto-Earth, which was already very large, and glided inexorably along its orbit. Until Theia, a Mars-sized proto-planet, started to close in on Earth.
For a period of time, Earth and Theia, the sister planets, chased like two racehorses in similar orbits around the sun, a situation that could not last long. The laws of astrophysics are adamant: Two planets of similar size cannot share the same orbit for a long time. The sister planets whizzed by each other at a dangerously close range. A collision was inevitable.
Theia smashed into Earth at a grazing angle. The possible collision scenarios have been modelled in increasing detail. Thousands of simulations were run to see what boundary conditions of mass, size, velocity and impact angle yielded a Moon-Earth system. Many did not, but some were successful.
One particular simulation [ii], as described by Dr. Robert Hazen, shows how the drama unfolds apparently in slow motion. Both planets approach each other, until they make contact, gently nudging each other. They just hang there. Minutes later Theia starts to deform, like a dough ball. The Earth is still round. As Theia is deformed beyond recognition, the Earth is also thrown out of balance, losing its round shape.
Half an hour later Theia is reduced to vapor, the Earth severely dented. Vaporized rock material jets out of the impact zone and envelopes the collided worlds in a thick atmosphere of vaporized rock.
In another version, Theia bounces off Earth like an elongated blob, after an initial impact, before being drawn back into Earth to deliver the final blow. In most simulations, Theia disappears completely. Theia disintegrated into a glowing cloud of rock debris and vapor, which orbits Earth. Earth does not escape unharmed. Part of the earth's mantle and core are shattered as well, and get mixed with material of Theia. A small part of the dust escapes to space, but most of it retained in the firm grip of Earth's gravity field.
The Earth is reduced to a seething magma world. The heavy metals quickly sink back to Earth's core, forming a new, larger core than before. For weeks it rains hot material that feeds the red-hot magma-ocean. The material that orbits Earth at a certain distance, clumps together to form an increasingly larger, celestial body: the Moon. After a few years, the moon has reached its current size. Both Moon and Earth are made up of material of the proto-Earth and Theia.
Can we now breathe with relief that we finally know how the Moon-Earth system formed and that the Case of the Giant Moon is solved?
Not yet! Nothing is further from the truth. The Great Impact Hypothesis is back under fire and scientists frantically seek solutions. The Earth, the Moon and Theia do not give away their secrets so easily. We'll come back to this topic!
In the video below, go to 5:40 to see a simulation of the collision that formed the Moon:
[i] Thiemens M M, Sprung P, Fonseca R O C. et al., 2019, Early Moon formation inferred from hafnium-tungsten systematics. Nature Geoscience. 12, p. 696-700
[ii] Hazen, R. The Story of Earth: The First 4.5 Billion Years, from Stardust to Living Planet.Penguin Books. (2013), 320 p.
[iii] Hartmann W K, Phillips R J, Taylor G J, 1986, Origin of the Moon. Papers presented at the Conference on the Origin of the Moon, held in Kona, Hawaii, October 1984. The Lunar and Planetary institute, Houston