What’s the origin of our mysterious moon? Was it split off from the Earth? Was it captured from space? Was it born together with the Earth? Or does it have another story entirely?
The grand old theories of lunar origin
Prior to the Apollo and Luna programs and subsequent research, science was dominated by three scenarios of the formation of the moon. Some believed it split off from the Earth; others that it was captured; and still others that it was born together with the Earth.
Fission: “The moon split off from the Earth”
The Fission Hypothesis was proposed in 1878 by George Darwin, son of Charles Darwin. He envisioned a young, molten Earth, rotating so quickly that it became distended, and eventually tore off a piece of itself. (One version of the theory even had the Earth spin itself into three pieces – of which the medium-sized piece became Mars.) The theory had a seemingly compelling piece of evidence: the “planet-wide scar” known as the Pacific Ocean. (Whitehouse, 238)
It also held a certain emotional appeal, casting the moon as a “daughter” born of “Mother Earth”. (Mackenzie, 80) But this model required the Earth to be spinning incredibly fast, and could not account for the current level of angular momentum. Surprisingly, although much declined in popularity, this was still the theory of choice for 3 of 20 lunar scientists surveyed in 1969. (Mackenzie, 86)
Capture: “The moon was captured”
The Capture Hypothesis, formulated by the controversial American astronomer Thomas Jefferson Jackson See early in the 20th century, proposed that the moon formed elsewhere in the solar system and was then “captured” into the Earth ’s orbit. (Mackenzie, p.94) The problem is that planetary bodies tend to either collide or deflect each other; a capture requires exceptionally precise timing, trajectory and momentum. (Whitehouse, p.239) An additional problem is how to get the moon close enough to be captured, but not close enough to be tidally disintegrated. Gerstenkorn and MacDonald attempted to resolve these problems in the 1960s, proposing an initially highly elliptical orbit that was later smoothed out by tidal forces into a near-circle. But even Gerstenkorn himself remained skeptical as to how the moon could have entered this orbit in the first place. (Mackenzie, p.101)
“The moon and the Earth were born together”
Lastly, the Co-Accretion Hypothesis proposed that Earth and moon formed alongside each other along with the rest of the planets. This appears on its face the most reasonable theory, but it cannot explain why, forming together, the Earth received almost all the iron while the moon got almost nothing (as evidenced by its much lower density). Nor can accretion alone account for the high angular momentum of the Earth-moon system. (Whitehouse, p.239)
“None of the above”
None of the three traditional theories was fully acceptable; all faced seemingly insurmountable problems. A poll of the Apollo scientists by Mitroff in 1969 revealed that they were for the most part indifferent to the lunar formation theories, seeing them all as highly speculative. (Mackenzie, p.115) The lunar samples returned by the Apollo and Luna missions of the late 60s and early 70s would at last render all three of these theories untenable, but it would take several more years for a new theory to emerge that could account for the observed data and lead to academic consensus.
“A planet collided with the Earth, ‘splashing’ the moon out into space”
The giant impact theory was first proposed by Reginald Daly in the 1940s, but the scientific community was apparently unready for any such “catastrophic” model of geological change, particularly anything to do with bombardment from space. After all, the consensus at the time was that the lunar craters were volcanic in origin. (Mackenzie, p.150) Essential groundwork was laid by Viktor Safronov in the 1960s, who advanced the now-accepted model of planet formation through the accretion of planetesimals. In this model, major impacts were not only possible but exceedingly common in the earliest days of the solar system. (Whitehouse, p.252)
In 1974, William Hartmann and Donald Davis presented a new incarnation of the giant impact theory. This theory calculated that an impactor approximately the size of Mars – now known as “Theia” – could account for the current angular momentum of the Earth-moon system. In a sense, our planet is carrying the momentum of that impact even today. The theory also accounted for the moon ’s compositional similarity to the Earth ’s mantle, since only the outer layers of the colliding planets would have formed the moon, with their iron cores sinking to the centre of the Earth. In addition, the moon ’s lack of volatiles could be explained by the extremely high temperatures of the collision boiling them off, allowing them to escape into space. Ten years after its proposal, most scientists were convinced that this was the best theory for the moon ’s formation. (Mackenzie, p.167)
Giant Impact
In the very young solar system of the giant impact, simulations have shown that dozens of Mars-sized “proto-planets” would have been roaming the inner solar system, often in irregular orbits. The pre-impact Earth, often referred to as “Earth Mark One”, would have been about half the size of our present planet, but already differentiated into an iron core surrounded by a silicate mantle, as it is today.
The impact is rightfully called “giant”: the kinetic energy of a moving planet is staggering, and the energy released by the collision must have been 100 million times greater than the impact that wiped out the dinosaurs. Great quantities of rock would be immediately vapourized, and a plume of rock vapour would have been fired out into space. Much of the impact energy would be converted into heat, superheating both planets. (Whitehouse, 246-248)
What happened to the moon?
The young moon would have been very hot indeed, possibly entirely molten, and certainly surrounded by a magma ocean (the existence of which was finally proven by the 1994 Clementine lunar mission). The surface is estimated to have solidified after about 50 million years, after which magma would continue to punch through in volcanic form for some time. Further accretion through not-quite-so-massive impacts contribute to bringing the moon as well as the Earth up to their present size. Yet after the excitement of the young solar system ’s intense-bombardment phase, the moon has been essentially unchanged for the past four billion years.
Sources
Mackenzie, Dana. The Big Splat, or How Our Moon Came to Be. United States: John Wiley & Sons, 2003.
Whitehouse, David. The Moon: A Biography. Great Britain: Headline Book Publishing, 2001.
What happened to the Earth?
The effects of the moon-forming impact on the Earth would have been immediate and catastrophic. They are parallel with the effects of any major impact, except that this impact was necessarily the greatest of the great. Clearly, if any biosphere existed at this early point, it would have been utterly obliterated, along with a sizable chunk of the mantle layer.
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