Problems for ‘giant impact’ origin of moon
Evolutionary astronomers have great trouble accounting for the origin of the moon. There have generally been three competing hypotheses, but they all have serious physical problems:
Fission theory, invented by the astronomer George Darwin (son of Charles). He proposed that the earth spun so fast that a chunk broke off, with the Pacific Ocean as the probable scar (or a modification of the theory that had the earth molten at the time). But this theory is universally discarded today. First, the moon is too chemically different from the earth; second, the earth could never have spun fast enough to throw a moon into orbit; and third, the escaping moon would have been shattered while within the Roche Limit.
Capture theory — the moon was wandering through the solar system, and was captured by Earth’s gravity. But for one approaching body to enter into orbit around another, it would need to lose a lot of energy, which is why spacecraft sent to orbit other planets are designed with retro-rockets. Otherwise the approaching body would have been ‘slingshotted’ rather than captured, a phenomenon the Voyager probes exploited. Finally, even a successful capture would have resulted in an elongated comet-like orbit.
Condensation (or co-creation) theory — earth and moon formed at about the same time from the same portion of the swarm of planetesimals which supposedly orbited the sun in the early phases of the evolution of the solar system. However, it’s unlikely that the gravitational attraction could have been strong enough, and it doesn’t account for the moon’s low iron content.
The evolutionary astronomer Lissauer affirms that these three theories have insoluble problems.1 He even cited an only half-joking statement in a university astronomy class about 20 years ago by Irwin Shapiro: since there were no good (naturalistic) explanations, the best explanation is that the moon is an illusion! This counts as strong evidence for the moon’s special creation.2,3
Lissauer’s article was actually commenting on a paper4 supporting what evolutionary scientists consider a fourth promising hypothesis for the origin of the moon, developed during the past decade. It is called the Giant Impact Hypothesis. This hypothesis suggests that the proto-Earth and a Mars-sized protoplanet had a glancing collision 4.5 billion years ago. The moon subsequently formed from the ejecta. A variant of the hypothesis, the Impact-triggered Fission Hypothesis, propounds that, instead of one giant impactor, the moon formed from the debris of multiple impacts of smaller planetesimals. However, recent dynamical and geochemical analyses call the Giant Impact Hypothesis into question.
Computer models have been constructed to simulate such a giant impact. Although such computer models are simplified and depend too much on initial conditions, the results have strained the hypothesis to the breaking point. One of the new dynamical results is that the debris from the collision would rain back down onto Earth instead of remaining in orbit and forming the moon.5 To hurl the debris far enough from the earth, the impactor would need to be three times the size of Mars. The results of such a collision are hard to understand, much less model. And if the moon did form after such a collision, the orbit would likely be unstable with a distance of only 14,000 miles above the earth and circling it every two hours. Lissauer also noted the unsolved problem of losing the excess angular momentum.1
Planetary scientists are trying to work through all the dynamical problems to patch up this hypothesis by employing multiple computer simulations.6 Of course, multiple computer attempts with different initial conditions and physics are bound to come up with something plausible. But, some researchers are sceptical that such computer models are realistic:
‘However, Jay Melosh (University of Arizona) argued that we do not know the equations of state well enough to calculate the energy of such an impact and that we may have grossly underestimated them, to the point that specific dynamic models are currently unjustified.’ 7
In spite of a growing consensus in favor of the Giant Impact Hypothesis, some workers remain sceptical of the hypothesis on both dynamical and geochemical grounds.7
Ruzicka, Snyder and Taylor reviewed the geochemical data,8 especially the diagnostic elements of Ni, Co, Cr, V, and Mn. These elements have been used to argue in favor of the Giant Impact Hypothesis, but these researchers, after reviewing observed data from the moon and meteorites, conclude ‘... that there is no strong geochemical support for either the Giant Impact or Impact-triggered Fission hypotheses.’ 9 Much of the geochemical support for the hypothesis was based on genitive models, which of course are simplified with too few variables. It is the observed data that call these hypotheses into question. The researchers also add that the reason the Giant Impact Hypothesis has become popular lately is because other hypotheses don’t work:
‘This [hypothesis] has arisen not so much because of the merits of [its] theory as because of the apparent dynamical or geochemical short-comings of other theories ...’ 9
Planetary scientists won’t give up. They must have a naturalistic hypothesis for all origins, including the moon’s, so will believe almost any hypothesis to fill the void. In regard to the moon and despite a long history of theorizing, ‘The origin of the Moon is still unresolved.’ 9 The idea that the moon was specially created ex nihilo at its present distance and in its present orbit some 6,000 years ago is still the most reasonable explanation for its origin.
- Lissauer, J.J., It’s not easy to make the moon, Nature 389(6649):353–357, 1997. Return to text.
- Whitcomb, J.C. and DeYoung, D.B., The Moon—Its Creation, Form and Significance, Baker Book House, Grand Rapids, Michigan, 1978. Return to text.
- Sarfati, J.D., The Moon: The light that rules the night , Creation 20(4):36–39, 1998. Return to text.
- Shigeru Ida et al., Lunar accretion from an impact generated disk, Nature 389(6649):353–357, 1997. Return to text.
- Anonymous, Recipe for a moon, Discover 18(11):25–26, 1997. Return to text.
- Halliday, A.N. and Drake, M.J., Colliding theories, Science 283:1861–1863, 1999. Return to text.
- Halliday and Drake, Ref. 3, p. 1862. Return to text.
- Ruzicka, A., Snyder, G.A. and Taylor, L.A., Giant Impact and Fission Hypotheses for the origin of the moon: a critical review of some geochemical evidence, International Geology Review 40:851–864, 1998. Return to text.
- Ruzicka et al., Ref. 5, p. 851. Return to text.