  Two fundamentally different processes of rocky planet formation exist, but it
is unclear which one built the terrestrial planets of the solar system. Either
they formed by collisions among planetary embryos from the inner solar system,
or by accreting sunward-drifting millimeter-sized 'pebbles' from the outer
solar system. We show that the isotopic compositions of Earth and Mars are
governed by two-component mixing among inner solar system materials, including
material from the innermost disk unsampled by meteorites, whereas the
contribution of outer solar system material is limited to a few percent by
mass. This refutes a pebble accretion origin of the terrestrial planets, but is
consistent with collisional growth from inner solar system embryos. The low
fraction of outer solar system material in Earth and Mars indicates the
presence of a persistent dust-drift barrier in the disk, highlighting the
specific pathway of rocky planet formation in the solar system.
