Hailstorm

Four billion years ago, the inner solar system turned into a cosmic shooting gallery. Earth and the other rocky worlds were pummeled by asteroids and comets — the leftover “building blocks” from the formation of the planets. But much of the details of this period are sketchy. Scientists aren’t sure how long it lasted, how intense it was, or what caused it in the first place.
This era is known as the Late Heavy Bombardment. The idea developed as scientists analyzed samples of the Moon picked up by Apollo astronauts. The samples indicated that three of the Moon’s large, dark impact basins formed at about the same time — roughly a half-billion years after the birth of the Sun and planets. Similar basins on the planets Mars and Mercury seem to be roughly the same age.
The leading idea is that about that time, the giant outer planets began rearranging their distances from the Sun. As they did so, their gravity kicked many bodies out of the asteroid belt. Many of those objects plunged into t