The two worlds were battered by debris about 3.85 billion years ago, during the so - called Late Heavy Bombardment (LHB), but it is unclear whether rocky asteroids or icy comets were the key culprits. (newscientist.com)
Evidence shows that some areas are geologically ancient, dating back to the end of Late Heavy Bombardment during when the Kuiper belt and the outer solar system rearranged about 4 billion years ago. (astrobiology.nz)
Mars, on the other hand, is thought to have been shaped mostly during the period of primordial accretion and the so - called Late Heavy Bombardment, when asteroids carved out massive impact basins and pushed up huge volcanoes. (sciencedaily.com)