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Black holes of various masses are thought to have sprung into existence within 1 second of the big bang, as elementary particles clumped together at extreme energies. But Einstein's theory of general relativity predicts the smallest of these "primordial" black holes should have already evaporated, through a quantum process called Hawking radiation.
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The maria were formed after large impacts from meteors carved out basins in the lunar crust. When the Moon was volcanic, magma seeped to the surface, filled the basins and eventually hardened, resulting in the relatively smooth flat areas seen today. Scientists can tell this happened recently, in geological terms, because the maria have fewer impact craters than the highland areas.