Asteroids bombarding Earth early in its history. The Earth is thought to have formed from the accretion of smaller particles of rock around the Sun some 4.5 billion years ago. Around four billion years ago, long after the main phase of accretion, there was a second period of frequent impact events, known as the Late Heavy Bombardment, in which repeated impacts melted much of the crust. It is this period that formed most of the craters and basins visible on the surface of the Moon and Mercury. The surface of Earth is highly geologically active, however, and visble signs of the bombardment have all been eroded, covered or otherwise destroyed.