There are good reasons to think that Earth and Mars originally formed a single protoplanet—Terramars—outside the orbit of Jupiter. Then, over 4 billion years ago, Terramars was pulled by Jupiter’s powerful gravitational field past the gas giant. As Terramars neared Jupiter, tidal forces heated it to the melting point, and Jupiter tore Mars away from Earth, leaving the Pacific Basin. Both planets, now turned into red-hot comets, sped off into the inner solar system.