江苏 迟 琳

Since the dawn of human history,the moon has influenced religion and poetry.But as the moon appears in the sky,we wonder,“What is the origin of the moon?”

Before the invention of the lunar probes (月球探测器),scientists had proposed three theories.The first declared the moon was a fragment(碎片)produced from an early earth that rapidly turned.The second origin theory assumed that the moon and the earth formed side by side from the cooling down of original gases and particles.The third theory said the earth and the moon originated at widely separate points in the solar system,but as they got close,the moon was pulled into orbit around the earth.

But in 1975,after studying moon rocks and close-up photographs of the lunar surface,scientists proposed another,more probable theory—“planetesimal impact”.According to this theory,a very huge object known as a planetesimal struck the earth more than 4 billion years ago.At first,scientists estimated the object was about the size of the planet Mars.But in 1997,a US computer simulation showed it must have been at least two-anda-half to three times Martian size.The theory is that the destructive impact forced parts of the earth and this giant object into space,where the fragments eventually came together to create the moon.

By the 1990s,this theory had become the most widely accepted explanation of the moon's origin.But it still poses a major problem: Where on the earth's surface is the crater(坑)created by the planetesimal's impact?

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