The Formation of the Moon, or How Did That Get There?
Many mysteries of the Moon continue to astound us. Where did the water come from? Is it geologically active? Does it have an atmosphere? But these all might be dwarfed by the origination question: how did the Moon form? If you want to escape now before we dive into this mess, do so now. This is where many disciplines of science converge and the mess that ensues is what we call the Moon.
Putting aside religious and pseudoscience explanations, some of the first work in determining the current theory of the origin of the Moon was done in the second half of the 19th century. In 1879 George H. Darwin was able to use mathematics and observations to show that the Moon was receding away from us and that if you went backward it would have eventually been a part of us. But scientists were puzzled as to how a chunk of the Earth could have escaped from us and where the missing material would be. After all, the Moon is a big rock and we don’t have a divot in the surface large enough to explain that missing mass. Scientists began to think of the Earth as a mix of solids, liquids, and gases in an attempt to figure this out (Pickering 274).
They knew that the interior of the Earth is warmer than the surface and that the planet is continuously cooling off. So thinking backward, the planet had to be warmer in the past, possibly enough for the surface to be molten to a degree. And working the rotation rate of the Earth backwards shows that our planet used to complete a day in 4-5 hours. According to William Pickering and other scientists like George Darwin at the time, the spin rate was sufficient for centrifugal forces to work on the gases trapped inside our planet, causing them to be released and thus the volume, mass, and density were all in flux. But by the conservation of angular momentum, the smaller radius increased our spin rate. Scientists wondered if the rate was sufficient along with the weakened surface integrity to cause pieces of Earth to fly off. If the crust was solid then some remains should still be visible but if it was molten then the evidence wouldn’t be visible (Pickering 274-6, Stewart 41-2).
Now, anyone who looks at a map notices the Pacific Ocean seems to be circular and is a large feature of the Earth. So some started to wonder if it was possible the site of a break off with Earth. After all, it being void seems to point to the Earth’s center of gravity not matching with the center of the ellipsoid itself. Pickering ran some numbers and found that if the Moon did some off the Earth in the past then it took with it ¾ of the crust, with the remaining fragments forming the plate tectonics (Pickering 280-1, Stewart 42).
Theia or the Giant Impact Theory
Scientists continued with this line of reasoning and eventually developed the Theia hypothesis from these initial inquiries. They figured out that something had to hit us in order for the material to escape the Earth rather than its initial rotation rate. However, it was also likely for the Earth to have captured a satellite. Moon samples however, pointed the smoking gun to the Theia Hypothesis, otherwise known as the Giant Impact Theory. In this scenario, about 4.5 billion years ago during the birth of our solar system the cooling Earth was impacted by a planetesimal, or a planet-developing object, the mass of Mars. The impact tore off a section of the Earth and made the surface molten again while the magma chunk that broke off from Earth and the remnants of the planetesimal cooled and formed the Moon as we know it today. Of course, all theories have challenges and this one is no exception. But it addresses the spin rate of the system, the low iron core of the moon, and the lack of volatiles seen.
Problems, Solutions, and General Confusion
Much of the evidence for this theory came about through the Apollo missions of the 1960s and 1970s. They brought Moon rocks such as troctolite 76536 which told a chemical tale of complexity. One such sample, dubbed the Genesis Rock, was from the period of solar system formation and revealed the Moon to have had a magma ocean on its surface at nearly the same time frame, but with about 60 million years separating the events. This correlation meant the lunar capture theory as well as the co-formation idea were busted, and it was through this that Theia gained ground. But other chemical clues offer issues. One of these has to do with levels of oxygen isotopes between the Moon and us. Moon rocks are 90% oxygen by volume and 50% of their weight. By comparing oxygen-17 and 18 isotopes (which make up 0.01% of the oxygen on Earth) with the Earth and the moon we can get a grasp on the relationship between them. Ironically, they are almost identical which sounds like a plus for the Theia theory (for it implies a common origin) but according to models those levels should actually be different because a majority of the material from Theia went into the Moon. Those isotope levels should only happen if Theia it us head on rather than at a 45 degree angle. But scientists at the Southwest Research Institute (SwRI) created a simulation that not only accounts for this but accurately predicts the mass of both objects upon completion. Some of the details that went into this model included having a Theia and Earth of nearly identical masses (4-5 current Mars-sized) but with a final rotation rate nearly 2 times the current one. However, early gravitational interactions between the Earth, Moon, and Sun in a process called eviction resonance may have stolen enough angular momentum so that the model does indeed match expectations (SwRI, University of California, Stewart 43-5, Lock 70, Canup 46-7).
So, all good right? Not a chance. For while those oxygen levels in the rocks were easy to explain, what isn’t is the water found. Models show how the hydrogen component of water should have been released and sent into space when Theia impacted us and heated the material. Yet hydroxyl (a water based material) is found in Moon rocks based on infrared spectrometer reading and cannot be a recent addition based on how deep it was found inside the rocks. Solar wind can help transport hydrogen to the surface of the Moon but only so far. Ironically, this finding only happened in 2008 when renewed interest in the lunar soil was brought up because of lunar probes. Clementine, the Lunar Prospector, and LCROSS all found signs that water was present, so scientists wondered why no evidence had been found in the lunar rocks. Turns out the instruments of the age were not refined enough to see it. While it is not enough to overturn the theory, it does point to some missing components (Howell).
But could one of those missing components be another moon? Yes, some models do point to a second object having formed at the time of the Moon’s formation. According to a 2011 article by Dr. Erik Asphaug in Nature, models show a second smaller object escaping Earth’s surface but eventually collided with our Moon courtesy of gravity forces compelling it to fall in. It impacted one side and caused the Moon to become asymmetrical with regards to its crust, something that has long been a mystery. Eventually, that side now faces us and it much smoother and flatter than the far side with its mountains and craters. Sadly, evidence from the GRAIL mission probes Ebb and Flow, charged with mapping the gravity of the Moon, was inconclusive for finding evidence of this but did prove that the thickness of the moon was smaller than expected, a plus for the Theia theory as it caused the density of the moon to line up better with Earth's. Some simulations even show that a dwarf planet the size of Ceres could have impacted instead and resulted not only in a weaker near side and a built-up far side (courtesy of the material falling down from the other side of the impact zone) but also bring new elements to cause Earth-Moon values to fluctuate as seen, but this is all according to simulations (Cooper-White, NASA "NASA's GRAIL," Haynes "Our").
Well shucks. Could evidence of how the molten state of the Moon be a different clue? It would help to first know how the Moon cooled. Models point to a rapidly cooling object after its formation but some show that it took longer to cool off than anticipated. If theory is right, then as the Moon cooled it formed crystals of olivine and pyroxene which were heavy and sank towards the core. Anorthites also formed and are less dense and therefore floated to the surface rapidly as the Moon cooled, where their white color is visible to this day. The only dark patches are from volcanic activity which occurred 1.5 billion years after the Moon formed. And magma pushed to the surface by carbon combining with oxygen to form carbon monoxide gases, leaving traces of carbon that also match with Earth levels. But once again, Moon rocks were a clue that all may not be right with our theory on this. They show that the anorthites floated to the top almost 200 million years after the Moon formed, which should only have been possible if the Moon was still molten. But then the volcanic activity seen should have been affected by the increased activity yet it is not. What gives? (Moskvitch, Gorton)
The best idea to fix this presents multiple molten stages for the Moon. Initially, the mantle was more of a semi-liquid which allowed for volcanic activity early in the Moon’s history. Then evidence for that was erased with the activity which occurred later on in the Moon’s history. It is either than or that the timetable for the formation of the Moon is wrong, which goes against much evidence collected, so we go with the lesser of the consequences. Occam’s razor applies (Ibid).
But that approach does not work well when you find out the Moon is made mostly of Earth material. Simulations show that the Moon should be 70-90 percent Theia but when you look at the entire chemical profile of the rocks, they seem to show the Moon is essentially Earth material. No way for both to be true, so Daniel Herwartz and his team went hunting for any signs of foreign material. They looked for isotopes which may point to where Theia formed. This is because different regions around the Sun in the early solar system were undergoing unique chemical interactions. Ironically enough, those oxygen readings from earlier were a big tool here. Rocks were heated using fluorine gas, releasing the oxygen and thus able to be subjected to a mass spectrometer. Readings showed that certain isotopes were 12 parts per million higher on the Moon than on Earth. This could point to a 50/50 mix for the Moon, a better fit. It also shows that Theia formed elsewhere in the solar system before colliding with us, But a separate study in the March 23, 2012 issue of Science by Nicholas Dauphas (from the University of Chicago) and the rest of his team found that titanium isotopes levels, when taking external radiation into account, the Moon and the Earth matched. Other teams have found that tungsten, chromium, rubidium, and potassium isotopes also follow that trend. The tungsten is especially damning because it is correlated to the core of an object, with one isotope of it made via the radioactive decay of hafnium, which was abundant during the first 60 million years of the solar system. However, halfnium is not connected to the core of objects but their mantles. So the isotope of tungsten we have will tell us about the origin of the object, and based on levels seen it would have to imply that Their was not only in the same neighborhood as us but also co formed with us yet managed to avoid us for 60 million years prior to colliding with Earth.That hurts the mix theory. Folks, easy answers are not to be found here (Palus, Andrews, Boyle, Lock 70, Canup 48).
If so much evidence leads to contradictory results, then maybe a new theory is needed. One new entry into the theory pool that is gaining traction doesn't have us totally abandon our progress so far. Maybe the Theia impact completely mixed with the Earth in a higher energy collision, perhaps in a direct hit rather than a glancing blow, allowing materials to roughly be spread out evenly. Why? A higher impact would cause more material to be vaporized (and that and a sharing of material from the crust and mantle would be more easily achieved while leaving a relatively untouched core. But because of the spin of the Earth and the different densities of the materials at hand, faster moving objects would be able to go past the corotation limit (this is where the material on the equator of an object matches the orbital speed, hence the co-rotating) and congregate on the outside of our vapor cloud and slower ones on the inside, forming a torus-like shape made of rock vapor known as a synestia. This shape arises from the core contracting material in but the outer portions of the cloud being able to stay in orbit thanks to their high temperatures and fast orbital speed. Over a few decades, the Moon gradually forms from this as the vapor cools off and condenses onto Theia's core as molten rain, resulting in a magma ocean while the synestia continued to shrink. Eventually, the Moon would emerge from the perimeter of this while dust and vapor continued to coalesce onto the surface of the Moon. The beauty of this idea is the high levels of mixing we see but yet some differentiation, for the remaining vapor which fell to us and not the Moon would lead to different chemical levels we have seen such as the higher amounts of hydrogen, nitrogen, sodium, and potassium on Earth and yet roughly the same isotopic ratios. The volatiles we seem to lack on the Moon are also explained by this, for they would have had too much energy to have condensed while the Moon was within the synestia. It also matches simulations done by Simon J. Lock and Sarah T. Stewart, the two lead authors behind the synestia theory. They looked at the Earth spin rate and found if we backtrack from where it is today then the length of a day was only 5 hours. This was faster than had been thought prior to a new study that indicated a greater angular momentum exchange between the Earth and Sun than had been assumed in past years. The only way our planet could "start" with this value is if something gave it a direct hit rather than a glancing blow. Their simulations then showed the synestia formed and collapses with the features as outlined above (Boyle, Lock 71-2, Canup 48).
Maybe Theia wasn't that different from the Earth in terms of chemical make-up, explaining the similar chemical profiles. Simulations show that objects forming around the Sun were likely similar in composition based on the distance they formed at. Another major candidate as an alternate to the Theia theory is the moonlet theory, where a slow accumulation of tiny moons over a span of time after a major collision with Earth could have clumped together. However, most models indicate the moonlets would eject each other rather than merge with one another. More evidence will be needed and the theories worked out before anything definite can be concluded (Boyle, Howard, Canup 49).
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Palus, Shannon. “Body That Formed the Moon Came from a Different Neighborhood.” arstechnica.com. Conde Nast., 06 Jun. 2014. Web. 27 Oct. 2015.
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SwRI. “New Model Reconciles the Moon’s Earth-like Composition with the Giant Impact Theory of Formation.” Astronomy.com. Kalmbach Publishing Co., 18 Oct. 2012. Web. 26 Oct. 2015.
University of California. "Moon was Produced by Head-On Collision." Astronomy.com. Kalmbach Publishing Co., 29 Jan. 2016. Web. 05 Aug. 2016.
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© 2016 Leonard Kelley