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The origin of Earth’s Moon
During the last two centuries, astronomers developed three different hypotheses for the origin of Earth’s Moon, but these traditional ideas have failed to survive comparison with the evidence. A relatively new theory proposed in the 1970s may hold the answer. You can begin by testing the three unsuccessful theories against the evidence to see why they failed.
The first of the three traditional theories, the fission hypothesis, supposes that the moon formed by the fission of Earth. If the young Earth spun fast enough, tides raised by the sun might break into two parts. If this separation occurred after Earth differentiated, the moon would have formed from crust material, which would explain the moon’s low density.
But the fission theory has problem. No one knows why the young Earth should have spun so fast, nearly ten times faster than today, nor where all that angular momentum went after the fission. In addition, the moon’s orbit is not in the plane of Earth’s equator, as it would be if it had formed by fission. （反对fission theory的理由： 1-地球应该转得快， 2- angular momentum去了哪？什么是angular momentum？3-月球的轨道应该在地球的赤道平面 ）
The second traditional theory is the condensation (or double-planet) hypothesis. It supposes that Earth and the moon condensed as a double planet from the same cloud of material. However, if they formed from the same material, they should have the same chemical composition and density, which they don’t. The moon is very poor in certain heavy elements like iron and titanium, and in volatiles such as water vapor and sodium. Yet the moon contains almost exactly the same ratios of oxygen isotopes as does Earth’s mantle. The condensation theory cannot explain these compositional differences. （反对condensation theory的理由：1-构成物质的化学成分和密度应该一样。 但月球含iron and titanium, 以及易挥发的物质如water vapor and sodium很少）
The third theory is the capture hypothesis. It supposes that the moon formed somewhere else and was later captured by Earth. If the moon formed inside the orbit of Mercury, the heat would have prevented the condensation of solid metallic grain, and only high-melting-point metal oxides could have solidified. According to the theory, a later encounter with Mercury could have “kicked” the moon out to Earth.
The capture theory was never popular because it requires highly unlikely events involving interactions with Mercury and Earth to move the moon from place to place. Scientists are always suspicious of explanations that require a chain of unlikely coincidences. Also, on encountering Earth, the moon would have been moving so rapidly that Earth’s gravity would have been unable to capture it without ripping the moon to fragments through tidal forces. （反对capture theory的理由： 1-地球应该转得快，）
Until recently, astronomers were left with no acceptable theory to explain the origin of the moon, and they occasionally joked that the moon could not exist. But during the 1970s, planetary astronomers developed a new theory that combines the best aspects of the fission hypothesis and the capture hypothesis.
The large-impact theory supposes that the moon formed from debris ejected into a disk around Earth by the impact of the large body. The impacting body may have been twice as large as Mars. In fact, instead of saying that Earth was hit by a large body, it may be more nearly correct to say that Earth, and the ejected debris formed the moon. Such an impact would have melted the proto-Earth, and the material falling together to form the moon would have been heated hot enough to melt. This theory fits well with the evidence from moon rocks that shows the moon formed as a sea of magma.
This theory would explain other things. The collision must have occurred at the steep angle to eject enough matter to make the moon. The objects could not have collided head-on. A glancing collision would have spun the material rapidly enough to explain the observed angular momentum in the earth-moon system. And if the two colliding planetesimals had already differentiated, the ejected material would be mostly iron-poor mantle and crust. Calculation show that the iron core of the impacting body could have fallen into the larger body that became Earth. This would explain why the moon is so poor in iron and why the abundances of other elements are so similar to those in Earth’s mantle. Finally, the material that eventually became the moon would have remained in a disk long enough for volatile elements, which the moon lacks, to be lost to space.
The moon may be the result of a giant impact. Until recently, astronomers have been reluctant to consider such catastrophic events, but a number of lines of evidence suggest that some planets may have been affected by giant impacts. |
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