托福阅读official74 Why does the author mention "variations in crater density"? 答案解析【小站备考】

Much is still unknown about the Moon's origin.Until recently, the most widely held hypothesis argued that the formation of the Moon paralleled that of Earth and the other planets. That is, the Moon formed from minute rock fragments and gases that composed a disk-shaped structure (the solar nebula) that orbited the early Sun. Debris from this disk collided and accumulated into larger masses that, in turn, accreted into planetary-sized bodies.

A new hypothesis, which has recently gained support from many scientists, suggests that a giant body collided with Earth to produce the Moon. The explosion caused by the impact of a Mars-sized body with a semi-molten Earth is thought to have ejected huge quantities of mantle rock from the primordial Earth. A portion of this ejected material remained in orbit around Earth, while the remainder either escaped or impacted upon Earth's surface. In a manner similar to that proposed in the earlier hypothesis, the material orbiting Earth then began to accumulate, eventually producing the Moon. Though the giant impact hypothesis provides a plausible mechanism for the Moon's formation, many questions must be answered before this proposal can be considered viable.

Despite the fact that the origin of the Moon is still debated, planetary geologists have been able to work out some of the basic details of the Moon's history, using among other things variations in crater density (quantity per unit area). Simply stated, the higher the crater density, the longer the topographic feature has existed. During its early history, the Moon was continually impacted as it swept up debris from the solar nebula. This continuous bombardment and perhaps radioactive decay generated enough heat to melt the Moon's outer shell and quite possibly the rest of the Moon as well.

When a large percentage of the debris had been gathered, the outer layer of the Moon began to cool and form a crystalline crust. From samples obtained by Apollo astronauts, the rocks of the primitive lunar crust are thought to be composed of a high percentage of a calcium-rich feldspar (anorthosite). This feldspar mineral crystallized early and, because it was less dense than the remaining melt, floated to the top and formed a surface scum. While this process was taking place, iron and other heavy metals probably sank to form a small central core. Even after the crust had solidified, its surface was continually bombarded. Remnants of the original crust occupy the densely cratered highlands, which have been estimated to be as much as 4.5 billion years old.

The last period of heavy bombardment recorded in the lunar highlands occurred almost 500 million years after the crust had formed. It is not known with certainty whether this final episode of bombardment was simply a clean-up phase where the remaining large particles in the Earth-Moon orbit were swept up or whether it was an influx of bodies from farther out in the solar system.

The next major event in the Moon's evolution was the formation of maria basins, which are large craters that filled with lava flowing up through cracks in the Moon's surface. The meteoroids that produced these huge pits ejected mountainous quantities of lunar rock into piles rising 5 kilometers or more. The Apennine mountain range, which typifies such an accumulation, was produced in conjunction with the formation of the Imbrium Basin, the site explored by the Apollo 15 astronauts. The crater density of the ejected material is greater than that of the surface of the associated basin, confirming that an appreciable time elapsed between the formation and filling of these basins. Radiometric dating of the maria basalts (a type of rock) puts their age between 3.2 and 3.8 billion years, some what younger than the initial crust. In places, the lava flows overlap the highlands, another testimonial to the lesser age of the maria deposits.

The last prominent features to form on the lunar surface were the rayed craters as exemplified by the crater Copernicus. Rays of material ejected from these young depressions are clearly seen blanketing the surface of the maria and many older rayless craters. By contrast, the older craters have rounded rims,and their rays have been erased by the impact of small debris. However, even a relatively young crater like Copernicus must be millions of years old. Had it formed on Earth, erosional forces would have long since obliterated it.

OFFICIAL74 Why does the author mention "variations in crater density"?

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