The Heavy Bombardment and Life on Earth
It is estimated that Earth formed around 4.5 billion years ago, yet life-even the simplest sort of microscopic life that must have started things off_did not begin right away. Until about 3.9 billion years ago, Earth was subject to an intense barrage of large objects from space, a time called the “heavy bombardment.” Radiometric dating of Moon rocks shows that most of the Moon’s visible impact craters, or holes formed on its surface from impacts, must have formed during this early period of the solar system’s history. This is not surprising: According to our modern theory of solar system formation, the planets were built as larger and larger chunks of rock (sometimes mixed with metal or ice). or planetesimals, collided with one another. When a colliding planetesimal stuck to a growing planet, the planet got larger, increasing its gravity and allowing it to draw in even more planetesimals. Even after the planets had reached essentially their current sizes, there must still have been many planetesimals floating around; some of them still remain today, as the objects we call asteroids and comets. Those planetesimals that had orbits intersecting the orbits of the planets were doomed to eventual collisions. and most of those collisions must have occurred early in the solar system’s history, when the number of planetesimals was still large. In other words, the heavy bombardment was the period of time during which impacts were most common, and the evidence from the Moon tells us that this period ended by about 3.9 billion years ago.
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