When it comes to space rocks slamming into Earth, two stand out. There's the one that killed the dinosaurs 65 million years ago (goodbye T-rex, hello mammals!) and the one that formed Earth's Moon.
When the Moon-forming impactor smashed into Earth, much of the world became a sea of melted rock called a magma ocean ( if it wasn't already melted). After this point, Earth had no more major additions of mass, said Simone Marchi, a planetary scientist at the Southwest Research Institute who creates computer models of the early Solar System and its planetary bodies, including Earth.
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