Fig. 2: THE "BIG SPLAT" THEORY The Moon formed about 4.52 billion years ago when Earth was a still accreting, mostly differentiated, and molten planet with a metallic core. Another Mars size body tangentially impacted Earth with such cataclysmic force it blew off a large amount of Earth's less dense upper mantle as splattered liquid blobs orbiting the Earth. These numerous liquid splatters quickly coalesced into a molten proto-Moon orbiting the Earth. Much of the impactor's mantle material plus its metallic core were absorbed into the still molten Earth.