video: In this animation, Earth and the smaller planet that hit Earth, named Theia, are represented by many particles with a fixed mass, shown as small balls. The color of the ball indicates the material: iron cores and rocky mantles. Before the impact, Earth's shape is an oblate spheroid because the day is only 2.3 hours long. In this potential Moon-forming impact, Theia has half the mass of Mars and strikes at 20 km/s. Theia penetrates all the way to the core of the Earth and throws material out, temporarily forming a deep hole in the planet. Theia and part of the Earth are vaporized and expand around the planet. Some material is ejected quickly enough to escape the Earth. The final disk is massive enough to make the Moon and composed primarily of material from Earth (green balls). The disk has almost no iron as Theia's iron core merges with Earth's core. This impact scenario agrees with the observed the masses of the Earth and Moon, the low iron content of the Moon, and the similar isotopic composition of the Moon and Earth's mantle. After the impact, the Earth has a day of 2.7 hours. This video relates to a paper that appeared in the October 17, 2012, issue of Science Express, published by AAAS. The paper, by Matija at Harvard University in Cambridge, Mass., and colleagues was titled, "Making the Moon from a Fast-Spinning Earth: A Giant Impact Followed by Resonant Despinning." view more
Credit: [Video courtesy of Sarah T. Stewart, Harvard University / Matija Cuk, SETI Institute]