The Sumerians believed that a planet called Tiamat lay between Mars and Jupiter. However, there is some debate about where this planet is today. In his book, Dark Matter, Missing Planets and New Comets, Tom van Flandern claimed that the planet was destroyed 65 million years ago and became the asteroid belt.
Zecharia Sitchin disputed this in his books The Twelfth Planet and The Cosmic Code. Instead, he declared that Tiamat had changed orbit and is now Earth. Sitchin said that Tiamat changed orbit after colliding with a hypothetical planet called Marduk and its three moons.
Sitchin claimed that the collision formed a new planet that broke in half. Two chunks of it moved closer to the Sun to become the Earth and the Moon while leftover debris became the asteroid belt. Sitchin added that Tiamat’s former moons were also flung into new orbits. He believes that one of the moons crashed into Mars and created the great rift.