Meteorites reveal how Earth and Mars formed

Earth and Mars likely arose from collisions between giant moon-size rocks instead of the clumping together of tiny pebbles over time, a new study found.

Previous research suggested there may be two primary ways in which rocky planets such as Earth are built. The classic model proposes that moon-to-Mars-size rocks dubbed planetary embryos once regularly smashed together in the inner solar system, eventually assembling into full-size worlds. A more recent alternative concept envisions tiny pebbles from the outer solar system drifting inward toward the sun, gradually accumulating to form rocky planets, a process thought essential to the formation of the cores of giant planets such as Jupiter and Saturn.