The science of the origin of the Moon seems to be taking on a different direction today compared to 10 or 15 years ago. I have written about a number of issues in planet formation but have not really addressed our Moon too much, except for my 2008 paper for the International Conference on Creationism, called “Our Solar System: Balancing Biblical and Scientific Considerations.” I have often observed that one of the best ways to refute evolutionary science is to just wait and let the evolutionists refute each other. The trouble is often that when older ideas are refuted textbooks don’t catch up and the false impression promoted to the public from early ideas may not be corrected. You used to read that lunar rocks have practically no water or other “volatiles” in them, for example. “Volatiles” refers to substances easily boiled away. But now scientists know that there is actually water in lunar basalts. The Moon is considered to be low in iron, when you consider the Moon as a whole, but again basalts on the Moon’s surface have significant amounts of iron. In recent years lunar scientists have been realizing the Earth and the Moon are more similar in composition than they thought.
The accepted theory for the origin of the Moon has been what’s referred to as the “Giant Impact Model.” This general idea is that a small planet struck the Earth before Earth formation was complete. This impact is believed to have happened sometime between about 50 million to 200 million years after Earth began to form. The ejecta from the impact melted a significant part of the Earth’s mantle and ejected part of Earth’s mantle and crust into space to mix with material from the impactor planet. This theory was originally put forward in the mid 1970’s following the Apollo program. At that time the primary “fact” to explain was how could the Moon be of different composition than Earth, especially in the amount of iron it possesses. The Moon was thought of as being of a composition that was similar to but different from the Earth. So a large impact provides a mechanism for mixing Earth material and material from the small planet.
By the time of the Apollo missions in the early 1970’s there was already a realization that none of the old theories on the Moon’s origin were plausible. These theories were 1) the fission theory, 2) the capture theory, and 3) the condensation theory. The fission theory never had any good physics behind it. It said that the early Earth spun so fast that the Moon was literally “spun off” of the Earth. But if the Earth spun this fast, the Earth itself would probably fly apart or be left smaller than it is today. The capture theory said that Earth captured the Moon in the early years of the solar system. The Moon would be presumed to have come close to Earth enough to be caught by Earth’s gravity. But this idea was put forward before the dynamics of such an event was really understood well. Such an event would mean the Moon would initially have to be in a highly elliptical orbit and then you have to explain how the Moon could be slowed down, as well as how the Moon’s orbit could be rounded off and come to be as it is today. The third old idea is the condensation theory. This is actually the idea that is assumed to be the “usual” process for the formation of moons in our solar system in general. Planet moons are normally assumed to have condensed and accreted out of dust and gas along with the planet they orbit. There are some moons believed to be exceptions to this rule though, where it is believed they could have been captured by planets such as Jupiter or Neptune for example. So the condensation theory would propose that our Moon formed with the Earth. The chief problem with this idea in minds of scientists has always been that if the Moon condensed from dust with the Earth, why does it have so much less iron, and in general why is it’s composition different?
The Giant Impact theory for the Moon’s origin has been worked on in many computer simulations by scientists for a long time now. There are varying views of how this event happened and when it happened. Some scientists argue it happened around 50 or 55 million years after Earth began forming and some have argued it was later. The difference in the time of when the giant impact happened matters because if it was earlier then Earth is thought to have been less solid, hotter, and it would not be it’s full present size. If the impact happened later Earth would be larger, colder, and harder. Scientists believe life did not evolve on Earth-and survive-until after this large impact event. The angle the impacting planet made when it struck the Earth has also been varied in computer simulations. Generally scientists describe it as a “glancing blow” to the Earth. But some have argued it was at a steeper angle (or that the impactor was larger) so as to eject more of Earth’s mantle into space. There are many variables and it is very difficult to do a physical simulation of such a large impact. Such an event cannot be checked against any experiment, not even against large craters in the solar system. This giant impact is a planet-changing event for Earth and formation of the Moon would have been just a lucky outcome of it for us. The Moon benefits life on Earth by stabilizing Earth rotation axis and there are other benefits from the tides.
Recent research on the giant impact formation of the Moon finds that it is very difficult to make the concept work. Not that scientists have given up on the theory, but as some issues are clarified, it is becoming more challenging to make it work. Between the motion of the Earth and moon, the tidal forces, and the amounts of various elements including iron, oxygen, titanium, and other elements, the simulations never explain everything. The research on the impact origin of the Moon has perhaps clarified some questions about very large impacts. After the impact with Earth, the model says that the Moon would actually form mostly from the impactor, the small planet. Scientists build into their simulation the assumption that the small planet had an iron or iron-nickel core. Most of the impactor would turn into a debri disk around the Earth, essentially a temporary ring. But, the theory says that the metal core of the impactor would more or less stay intact and sink into the Earth, combining with Earth’s core. Some of the material ejected from the impact into space would fall back to the Earth. But any material ejected far enough away from the Earth is thought to be able to pull together by gravity into the Moon. The Moon has to form at a distance where the tidal forces from Earth would not pull apart the Moon as it forms. Because different elements vaporize and condense at varying temperatures, a large impact would have a tendency to separate materials by melting point, boiling point, and density. This should make the Moon’s overall composition somewhat different than Earth.
In 2013 some scientific papers were published raising serious questions about the giant impact theory of the Moon’s origin. Consider the following from Nature Geoscience from 2013 (by Linda Elkins-Tanton of the Carnegie Institute, Nature Geoscience Vol. 6, Dec. 2013, pp 996-998.)
“But recent work may in fact have undone previous progress on the giant impact hypothesis. A crisis in the field has been created by the growing realization that the Moon and Earth are exceptionally similar in composition — so similar, in fact, that the emerging constraints are difficult for the giant impact hypothesis to meet.”
The paper goes on to say
“The Earth and Moon seem to share identical isotopic signatures in oxygen, iron, hydrogen, silica, magnesium, titanium, potassium, tungsten and chromium. . . . That all these isotopic compositions are the same on the Earth and Moon, to high precision, places stringent constraints on physical scenarios for making the satellite. . . . The simulations of a Moon-forming impact have yet to produce a moon that fits all the puzzle pieces, geochemical and otherwise.”
Another well known planetary scientist, Robin Canup from the Southwest Research Institute in Boulder, CO has made statements comparing the composition of the Earth and Moon similar to the above. In Nature, Dec 5, 2013, Canup said “Lunar-origin studies are in flux. No current impact model stands out as more compelling than the rest.”
Canup goes on to say that there are new concerns that for the giant impact concept to work requires a sequence of events that could be considered too improbable. I’ve observed over the years that as origins models have more and more problems, ideas that complicate them are often added and eventually it raises the question, “Would this be likely to happen in the real world?” Canup makes an interesting comment about this, “As the number of steps increases, the likelihood of a particular sequence decreases. Current impact models are more complex and seem less probable than the original giant-impact concept.”
So my own conclusion is that when you rely only on known natural processes and rule out the possibility of superatural creation by God, this actually handicaps you in being able to explain it. I cannot prove that a giant impact did not form the Moon, but as the details are better and better understood it does not make the impact idea look more likely. It makes it look less likely, perhaps even impossible.
On the other hand, for Earth and the Moon to be of very similar composition may be consistent with some ideas young-age creationists have been proposing. First, it may suggest that the Moon and the Earth are similar by intelligent design. Some creationists working on astronomy and planetary science questions are thinking in terms of God creating matter on the first day that is not formed into objects until the fourth day. So because everything is created in a very short time frame, objects in space are created supernaturally and rapidly from material nearby. This makes it reasonable that the Moon and the Earth would have similar compositions, but it doesn’t rule out differences either. (I think there is a lot to say for the concept of created uniqueness in our solar system planets and moons.) I’ve also come to consider the possibility that impacts could have been part of the rapid formation of the Moon on the fourth day of creation. These impacts on the fourth day would not have affected Earth but could have affected objects across the solar system on the fourth day. Then another impact bombardment event of a smaller scale would have happened during Noah’s Flood. Then when the creation week ended the supernatural creation processes stopped and conservative natural processes became the rule of normal operation.
Psalm 89:37 speaks of the Moon as “the faithful witness in the sky” (NIV) that confirms that God’s promises are something we can count on. Scientific models always have their limits, especially in matters of origins. Scripture doesn’t give us scientific models but it does tell us some information about our origins and the origin of our special planet. Isaiah 40:8 says “The grass withers and the flowers fall, but the word of our God endures forever (NIV).” Linda Elkins-Tanton quoted above made the comment that “The Earth and Moon should not be the special exception or a statistical improbability.” Why not? What if our planet Earth and its Moon are special exceptions, that is consistent with the old term “special creation” which I think should be brought back into modern usage. Creation requires supernatural processes because natural processes are not adequate. What is created for humans on Earth is special because it is intelligently designed for our benefit. So the Moon should be a reminder to us God’s word still stands.