Where Does The Moon Come From?
In some ways, the debate on the moon’s origin resembles the debate on the origin of the Universe, in the sense that we can see how it might have happened, without being able to explain the mechanisms involved. For all that though, it seems way past strange that science can explain the origin of celestial objects that are billions of light years away, but not the origin of the Moon, which is barely a quarter of a million miles away.
Nonetheless, over the years, many theories and hypotheses on how the moon came to be have come and gone, but one theory, that the moon was gouged out of a young earth during a collision has prevailed, although exactly how the collision happened has not been resolved satisfactorily. In this article then, we will take a closer look at some of the various issues surrounding the origin of the moon, as well as a few obsolete theories.
The Giant Impact Theory.
Most planetary scientists agree that it is feasible that the moon could have formed as the result of a collision between a young earth and a massive object, but there is as yet no consensus on the size and mass of the impactor. Some investigators maintain that to satisfy the observed orbit and angular momentum of the earth-moon system, the impactor had to have had at least 90% of the mass of Earth, while others are of the opinion that a collision withan object of the size of Mars would have been sufficient to eject enough material during the collision to have formed the moon.
Whatever the mass of the impactor, the real issue revolves around the composition of the impactor. It is well known that no two bodies in the solar system have the exact same chemical composition as a result of all bodies having formed in different parts of the accretion disc that formed the solar system. This fact forms the crux of the problem, since the impact theory predicts that the moon should consist of, or at least contain, as much as 70-, to 90% of the material that comprised the impactor, with the rest being material from Earth.
However, this is not the case. Admittedly, the moon contains almost no volatile material, which would have boiled off during and soon after the collision, as the impact theory predicts, but moreover, during initial analyses of moon rocks, it turned out that there was no appreciable differences between earth and moon rocks. In fact, in some cases, the rocks from the moon and those found in Earth’s crust and mantle were practically identical.
This presented a serious problem since no trace of the impactor was found, and here it must be remembered that analysis of theisotropic composition of rocks in the form of meteorites are so accurate and reliable that the origin of space rocks can be pinpointed to individual, and specific asteroids and other bodies. Thus, not finding any trace of the impactor through the presence of unique isotopes (varieties of the same molecule) forced investigators to claim that the impactor must have been chemically almost identical to the young earth, which is stretching the imagination, to say the least.
However, other possibilities, such as that the mixing of the ejected material with that of the impactor was almost total, thus accounting for the similarities between moon and earth rocks, or that the earth could have collided with a much smaller, and less massive object. The latter possibility suggests that most of the impactor would have been vaporized upon impact, which accounts for the preponderance of Earth material in the moon. Nonetheless, neither of these possibilities can be made to explain the size of the moon relative to the earth, or the observed orbit and angular momentum of the earth-moon system.
A possible solution?
“Solution” is perhaps too strong a word, but renewed studies led by Daniel Herwartz of the University of Cologne in Germany, involved a re-evaluation of the oxygen content of moon rocks obtained from NASA (courtesy of Apollo missions 11, 12, and 16), to preclude the skewing of results by the space weathering of meteorites that had been on earth for millions of years.
The experiments, usingan upgraded mass spectrometer, involved extracting all the oxygen from the moon rocks, and it revealed that the ratio between two oxygen isotopes, O17, and O16 , was different from that of earth rocks by 12 parts per million. However, this is the only significant difference between moon and earth-derived rocks found to date, and while the finding seems to support the impact theory, the investigators readily acknowledges that the difference can be ascribed to other causes as well.
One possibility suggested by the Cologne team is that Earth could have been subjected to a bombardment of material containing a lower oxygen isotope count, but the mechanism whereby the moon had escaped the same bombardment remains to be explained.
Although none of the notions on how the moon could have formed never graduated into the “theory” class they are nevertheless interesting, if only for the fact that they served to demonstrate the total lack of empirical knowledge of the processes that formed the moon. Below are three such notions, and they are mentioned only because they enjoyed a certain amount of consideration from professional scientists.
The moon was captured by Earth.
Popular until the early 1980’s, this idea states that the moon was captured by Earth as it passed through the solar system, and the moon’s size, orbit, and the fact that it is tidally locked to Earth, counts somewhat in its favor.
However, close encounters of this magnitude usually involves altered orbits, or collisions, so for this notion to work, there had to have been a very large (and dense) atmosphere around Earth to slow the passing object down sufficiently to either prevent a collision, or to allow Earth to gravitationally capture it before it zipped right out of the solar system again, or worse, fell into the Sun. Moreover, the notion of a captured moon cannot explain the similar, if not the nearly identical ratios of various oxygen isotopes found in earth and moon rocks.
The moon broke off Earth.
In the 1800’s George Darwin, the son of the famous Charles Darwin, proposed an idea that could explain the formation of the moon by saying that at some time in the distant past, Earth rotated so rapidly that centrifugal forces caused a piece of the planet to break off.
This piece of Earth was supposedly flung into space, where it stabilized in its present position and orbit. Darwin speculated that the piece of Earth that broke off is where the Pacific Ocean is now; however, it is known that the Pacific Ocean basin is only around 200 million years old, which precludes the possibility of this area providing the material for the moon.
Nonetheless, even though the moon is known not to contain any ocean crust, it does not disprove the break-away theory, since the material that makes up the moon could conceivably have originated from another piece of Earth’s crust. The biggest difficulty the break-away theory has is the fact that it cannot explain the observed angular momentum of the earth-moon system.
This notion stated that the earth and moon formed together from the same material in the accretion disk that formed the rest of the solar system. While this may account for the chemical similarities between earth and the moon, it can explain neither the observed angular momentum of the earth-moon system, nor the pronounced difference in the ratio between the diameters of the two bodies’ iron cores.
Earth has an iron core that accounts for 50% of its radius, while the moon’s core amounts to only 25% of its radius, which could not have happened had the two bodies formed at the same time- and so close together to boot.
So while we know almost nothing of how the moon formed, we do know everything about its orbit and its path around both Earth and the Sun, which causes spectacular lunar eclipses, and which can be predicted with great accuracy.
The Cologne investigators do not claim their finding as any sort of vindication of the Giant Impact Theory of how the moon came to be, merely saying that it “...supports the view that the Moon formed by a giant collision of the proto-Earth with [an impactor.]”
Commenting on the finding, planetary scientist David Stevenson of the California Institute of Technology in Pasadena said that “... It is a relief that a [disparity in ratios] has been found, since the total absence of difference between Earth and moon would be hard to explain.” Adding to his comments, Stevenson also said that “...now that a difference has been found, many will work to confirm or deny it and do battle over what it means,” which is another way of saying that science is no nearer now to explaining the origin of the moon than it ever was in the past.
The Cologne investigators are of the opinion that the moon is comprised of a roughly 50-50 mixture of Earth and impactor material, and that the relatively high oxygen content of the moon samples points to the possibility that the impactor was composed of “enstatite chondrite”, an exceedingly rare material that occurs in only 2% of meteorites that make it to Earth’s surface, which promptedplanetary scientist David Stevenson to say that “...the possible significance of enstatite chondrites is interesting, but at present we are stuck with speculating about the bodies that went into making Earth, since they are no longer around.”
Which statement confirms the view that at present...