An artist's rendering of what the environment around Pleiades star HD 23514 might look like as two planets collide. Small, rocky planets that could resemble the Earth or Mars may be forming around a star in the Pleiades star cluster, astronomers reported on Wednesday. (Gemini Observatory/Lynette R. Cook/Reuters)
The moon was one of the larger bodies orbiting Tiamat and was captured by Marduk after the collision, but only briefly, as it joined with Earth to share the new orbit of the remains of Tiamat. The center of balance (I forget what it's called) is a mile or two beneath the Earth's surface.