The moon, 1/4 of the diameter of the earth, non-rotating and only 1/80th of
the density of the Earth, circles the earth in less than 24 hours and, along with the
gravitational force of the sun, influences tides and winds worldwide.
But where did our moon come from, and why is it less dense and absent an iron core like that of Earth? There are several theories that have been proposed… but most have no explanation for the lack of iron nor for the lack of water … both components that would be expected on a planet developed in conjunction with ours. Here are some ideas proposed:
- The molten earth, continuing in its formative rotation, developed massive waves,
not merely hundreds of feet high, but potentially thousands of feet high causing a literal separation of billions of tons of surface matter, spun off from the surface that excluded the already densely formed iron core…too heavy to break free. Both bodies continued to form for billions of years…the moon, absent a rotating core and nearly no atmosphere, cooled and formed, while the earth continued to rotate and evolve (Rachel Carson, The Sea Around Us).
- Others once believed the Moon came from another part of the Universe with little iron, strayed near the Earth’s gravitational field, and was drawn into orbit. This, too, failed for three main reasons. This is called the Capture Hypothesis. One was due to the fact that the isotope composition of Moon rocks is very similar to that of rocks from our planet. Second is the
necessity for a very small (thus, highly unlikely) encounter velocity. And third, is the absence of a very thick atmosphere that should have been present to provide a so-called gas drag that would have helped in the capture process.
the density of the Earth, circles the earth in less than 24 hours and, along with the
gravitational force of the sun, influences tides and winds worldwide.
But where did our moon come from, and why is it less dense and absent an iron core like that of Earth? There are several theories that have been proposed… but most have no explanation for the lack of iron nor for the lack of water … both components that would be expected on a planet developed in conjunction with ours. Here are some ideas proposed:
- The molten earth, continuing in its formative rotation, developed massive waves,
not merely hundreds of feet high, but potentially thousands of feet high causing a literal separation of billions of tons of surface matter, spun off from the surface that excluded the already densely formed iron core…too heavy to break free. Both bodies continued to form for billions of years…the moon, absent a rotating core and nearly no atmosphere, cooled and formed, while the earth continued to rotate and evolve (Rachel Carson, The Sea Around Us).
- Others once believed the Moon came from another part of the Universe with little iron, strayed near the Earth’s gravitational field, and was drawn into orbit. This, too, failed for three main reasons. This is called the Capture Hypothesis. One was due to the fact that the isotope composition of Moon rocks is very similar to that of rocks from our planet. Second is the
necessity for a very small (thus, highly unlikely) encounter velocity. And third, is the absence of a very thick atmosphere that should have been present to provide a so-called gas drag that would have helped in the capture process.
- Co-Formation Hypothesis proposes that both the Moon and the Earth were formed at practically the same time and in the same region in the solar nebula, drawing materials to each other from the dust around them. While this hypothesis is consistent with the proximity of the two bodies, it is difficult to explain why they the Moon doesn’t have as much iron as the Earth.
- Of all of these hypotheses that strive to explain the Moon’s origin, it is the Giant Impact that has received the widest acceptance. In this hypothesis, the Moon is believed to have been originally a part of the Earth’s crust, whacked out by a collision between the Earth and another body bearing
the size of Mars. As we consider the Giant Impact Hypothesis note that since the whacked-out
piece may have certainly come from the outer layer of the Earth (the crust), then this explains the lack of iron in the Moon. Furthermore, computer simulations also show how this theory is also consistent with angular momentum measurements.
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