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A fixture in the night sky for billions of years, the moon has captivated humanity through the ages with its magical charm. Its mysterious, romantic qualities have inspired prose, song, and reflection. But in recent years, as scientific findings concerning its origin have been advanced, the moon has also generated philosophical disquiet.
Our moon is like no other. The ratio of its mass compared to the mass of its host planet (the earth) is about fifty times greater than the next-closest known ratio of moon mass to host-planet mass. In addition, our moon orbits the earth more closely than any other known large moon orbits its host planet.
Dynamical models dating back to my graduate school days all predicted that the moon's existence was impossible. The moon was too large, it was too close to the earth, and the earth was too close to the sun for the moon to have formed out of the sun's protoplanetary disk. All collision scenarios either resulted in the destruction of the earth, the failure to form the moon, or the formation of an earth-moon system with orbital features radically different from what astronomers actually observe. At the time, some astronomers commented that the moon's presence had to be some kind of miracle.
Today, astronomers understand that the existence of the moon does not violate any physical laws. Nevertheless, the earth had to have sustained a just-right impactor at the just-right time under the just-right conditions and circumstances for the moon to have formed as it did. Astronomers have assigned the name Theia to this impactor.
Developing the Moon's Origin Scenario
Over the past fifteen years, astronomer Robin Canup has developed progressively more sophisticated and detailed models demonstrating that the moon formed as the result of a collision between a planet with about twice the mass of Mars (Mars's mass = 0.107 of the earth's mass) and the newly formed earth. She also shows that the angle of impact had to be about 45 degrees, and the velocity at impact very low—less than 12 kilometers per second.1
In order for an impact to generate a debris disk from which a lunar-sized satellite can form, vapor gases cannot dominate the debris disk.2 Such gases will generate spiral shocks that would lead to the destruction of the circumterrestrial disk (the debris disk surrounding the earth) within just a few days. Fortunately, the volatile-poor nature of the sun's newly born planets (volatiles include nitrogen, water, carbon dioxide, ammonia, hydrogen, methane, and sulfur dioxide) helped limit the quantity of available vapor gases. However, even without significant quantities of volatiles, a gas-dominated circumterrestrial disk could still arise if the impact energy were high. High impact energy would vaporize the rocky material in the earth, the impactor, or both.
It is challenging, to say the least, to develop a solar-system scenario that produces such a low—impact-energy collision by such a massive collider that simultaneously produces the moon, rids the earth of so much of its primordial atmosphere and liquid water, and enriches the earth's core with all the iron, uranium, and thorium needed to both sustain a long history of aggressive plate tectonics and establish a strong, stable magnetic field. (Advanced life requires both the plate tectonics and the magnetic field). Consequently, some researchers have suggested that the collider actually shared the earth's orbit around the sun. Newtonian mechanics allows for this possibility if the collider was small enough and if it was situated 60 degrees backward or forward along the earth's orbit.
This stability condition assumes, however, that only three massive bodies were involved: the sun, Theia, and the earth. The presence of other planets in the solar system, particularly the presence of Jupiter and/or nearby planetesimals (bodies as large or larger than the biggest asteroids), means that—given sufficient time—the smaller planet sharing the earth's orbit would have been jiggled away slightly from its 60 degrees backward or forward position. When this happened, there was a substantial possibility that the smaller planet would have crept toward the earth and eventually collided with it at a low velocity and a low impact angle.
Hidenori Genda and Yutaka Abe, two Japanese astronomers, have demonstrated that the existence of a deep liquid-water ocean on the surface of the primordial earth was crucial for ensuring that the moon-forming impact event would blast away enough of the earth's initial atmosphere and ocean.3 Deep liquid water at the impact site would have lowered the shock impedance compared to bare ground (think of dropping a bowling ball 100 feet into a water-filled swimming pool versus an empty pool). A low shock impedance and plentiful liquid water means that the impact would have generated a huge amount of superheated steam. It is this steam that ejected almost all of the earth's primordial water and atmosphere into interplanetary space. To guarantee that neither too much nor too little of the earth's primordial atmosphere and ocean was removed, the earth's primordial ocean depth must have been highly fine-tuned.
Such fine-tuning was not lost on Canup, who remarked in a recent Nature review article, "Current theories on the formation of the Moon owe too much to cosmic coincidences."4 Indeed, the required "coincidences" continue to pile up. New research has revealed that the moon has a similar chemical composition to the outer portions of the earth, a result that Canup's models cannot explain, unless: (1) the total mass of the collider and the primordial earth was four percent larger than the mass of the present-day earth; (2) the ratio of the collider's mass to the total mass lay between 0.40 and 0.45; and (3) a fine-tuned orbital resonance with the sun removed the just-right amount of angular momentum from the resultant earth-moon system.5
Cosmic Coincidences Pile Up
Astronomers Matija Ćuk and Sarah Stewart found another way to explain the similar composition. In their model, an impactor about the mass of Mars collided with a fast-spinning (rotation rate = 2.3–2.7 hours) primordial earth.6 The fast spin of the primordial earth generated a moon-forming disk of debris made up primarily of its own mantle material, thus explaining the similar chemical composition of the present-day moon and the earth's present-day outer layers. But as with Canup's most recent model, a fine-tuned orbital resonance between the moon and the sun is needed.
In an article published in the same issue of Nature as Canup's review, Stewart concludes, "In the new giant-impact models, lunar material is derived either from a range of depths in the proto-Earth's mantle or equally from the entire mantles of two colliding half-Earths."7 But either way, while "each stage of lunar evolution is possible," she can't help but wonder, "with the nested levels of dependency in a multi-stage model, is the probability of the required sequence of events vanishingly small?"8
Canup suggests that perhaps a small (Mars-sized) collider model that didn't need so much of the added fine-tuning of the Ćuk-Stewart model could be retained if the collider's initial chemical composition were more earth-like than Mars-like. However, extra fine-tuning may be needed to explain this required initial composition.
In yet another article in the same issue as Canup's review, earth scientist Tim Elliott observes that the degree and kinds of complexity and fine-tuning required by lunar origin models appear to be increasing at an exponential rate. Among lunar origin researchers, he notes, "the sequence of conditions that currently seems necessary in these revised versions of lunar formation have led to philosophical disquiet."9
Implications beyond Science
What is this philosophical disquiet? The moon-forming impact event presents astronomers and all humanity with one of the most dramatic sets of evidence for supernatural, super-intelligent design for the specific benefit of humanity. Thanks to the exquisitely fine-tuned nature of this impact event, the collision:
Such ingeniously fine-tuned features may beget philosophical uneasiness for some, but further—and honest—philosophical inquiry may lead to rewarding discoveries of supernatural activity for the benefit of all humanity. •
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