A barrage of asteroids hit the surfaces of Mercury, Venus, Earth, Mars, and the Moon 3.8 billion years ago. Scientists call that barrage the Late Heavy Bombardment (LHB). However, they cannot confirm where those asteroids came from.
Some scientists have suggested that the asteroids came from the remnants of Planet V, which lay between Mars and the asteroid belt that separates Jupiter from Mars today.
Scientists think that Planet V was smaller than Mars, which may explain why its orbit was heavily altered by the gravitational pull of Jupiter and other outer planets. Planet V soon became unstable and strayed into the asteroid belt, flinging asteroids toward Mars and the other inner planets. Planet V itself was later flung into the Sun or far into the solar system.
Alternatively, Planet V could have just steered clear of the asteroid belt and crashed into another planet. Some astronomers think it crashed into Mars and created the Borealis Basin, which covers 40 percent of Mars. If that happened, the asteroids that crashed into the inner planets were probably fragments flung into space during the collision.
Other astronomers say that the hypothetical Planet V never existed. They think that the LHB occurred after Jupiter and Saturn changed their orbits and flung asteroids from the asteroid belt toward the inner planets. Others say that the LHB was caused after the gravitational pull of Mars broke a large asteroid apart.