Answer:
For decades, researchers have suspected that the gargantuan basin was created by a head-on collision with a very large, very fast meteor. Such an impact would have ripped the moon's crust apart and scattered chunks of lunar mantle across the crater's surface, providing a rare glimpse at what the moon is really made of.
Explanation:
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A glance at a world map shows that Europe is in fact a small peninsula jutting from the enormous landmass we call "Asia." It was the Greeks who first divided the world into Europe and Asia, with the waters of the Bosporus as the conventional dividing line. Yet the language they spoke originated, like ours, in the vast steppe areas beyond the Caspian. Men of neolithic times, who moved freely from the borders of China to the Atlantic coasts of Europe, would have found the division meaningless.
At the beginning of recorded history, some time in the third millenium BC, one of the Indo-European or Indo-Aryan speaking peoples of these steppelands succeeded in domesticating the horse, revolutionizing warfare and transforming themselves almost overnight into a formidable fighting force. Wave after wave of horse nomads swept across Europe and western Asia, meeting resistance only from the sedentary civilizations of Mesopotamia and Egypt, which were able to withstand the assault only by adopting chariot warfare - if not mounted cavalry - themselves.
These nomads, speaking closely related languages and sharing a common social organization, were the ancestors of, among others, the Greeks, Romans, Persians, the Indo-Aryan speaking conquerors of India, and of many other lesser-known peoples who were later to play an important role in the history of the various segments of the Silk Roads.
Time and distance obscured the common geographical and linguistic origin of these widely scattered peoples, and it was not until the 19th century that the relationships among all their languages was fully worked out and their homeland in the Asian steppes identified. When Alexander fought Darius at Gaugamela, he had no notion that the Persians, at least linguistically, were cousins of the Greeks. The Greek and Roman historians who later chronicled his campaigns derived a great deal of dramatic play from the contrast between stern Macedonian virtue and the decadent luxury of the East, between Greek freedom and Persian slavery, between Europe and Asia. These attitudes penetrated deep into the European consciousness - they surface occasionally today - and erected a mental barrier at times almost as impassable as the Pamir Mountains that protected the farthest outposts of China from those the Chinese called "the western barbarians."
For the Chinese, like the Greeks - but perhaps with more reason - divided the world into civilized and barbarian. They, like their counterparts in India, Mesopotamia and Egypt, had had to face the fierce mounted bowmen of the steppes, and to survive had had to adopt their enemies' methods of warfare.
The pattern established in the second millennium BC - the settled, agriculturally-based urban civilizations of China, India and the Middle East regularly exposed to attack by mounted horsemen from Central Asia - did not end with the settling of the Indo-European speaking nomads. As they were transformed, as a result of the success of their own conquests, into urban civilized peoples themselves - Greeks, Romans, Persians and Indians - they in their turn had to defend themselves against new attacks by mounted horsemen from the Eurasian steppes - Parthians, Huns, Turks and finally Mongols. The last great wave of invasion out of Central Asia occurred in the early 15th century of our era, when Tamerlane and his Turkic- and Mongolian-speaking hordes devastated the Middle East.
It is no wonder that Ibn Khaldun, the 14th-century Arab philosopher of history, saw the history of the Middle East in terms of urban peoples periodically assaulted by mounted nomads, who then adopted the civilized ways of the peoples they conquered, became thereby decadent and in their turn submitted to a new wave of nomadic invaders. Had Chinese historians been able to read Ibn Khaldun, they would have found his paradigm borne out by their own experience.
No fully satisfactory explanation has ever been offered for the periodic explosion of nomadic peoples from - or through - Central Asia, but the pattern is clear: The region has historically been a sort of dynamo generating population movements that have affected Europe, Asia and America since the beginning of human occupation of the Eurasian landmass.
The Chinese fear of the peoples to the west was therefore not without foundation. In the third century BC the short-lived but powerful Qin Dynasty linked up a series of earlier bulwarks and formed the Great Wall, effectively separating the settled and cultivated lands of China from the nomadic herdsmen without. The Great Wall stretches from Gansu to Manchuria, a distance of 2,400 kilometers (1,500 miles). It was an effective defence against nomads who lacked both siege
