  With approximately one ninth of Earth's mass, Mars is widely considered to be
a stranded planetary embryo that never became a fully-grown planet. A currently
popular planet formation theory predicts that Mars formed near Earth and Venus
and was subsequently scattered outwards to its present location. In such a
scenario, the compositions of the three planets are expected to be similar to
each other. However, bulk elemental and isotopic data for martian meteorites
demonstrate that key aspects of Mars' composition are markedly different from
that of Earth. This suggests that Mars formed outside of the terrestrial
feeding zone during primary accretion. It is therefore probable that Mars
always remained significantly farther from the Sun than Earth; its growth was
stunted early and its mass remained relatively low. Here we identify a
potential dynamical pathway that forms Mars in the asteroid belt and keeps it
outside of Earth's accretion zone while at the same time accounting for strict
age and compositional constraints, as well as mass differences. Our uncommon
pathway (approximately 2% probability) is based on the Grand Tack scenario of
terrestrial planet formation, in which the radial migration by Jupiter
gravitationally sculpts the planetesimal disc at Mars' current location. We
conclude that Mars' formation requires a specific dynamical pathway, while this
is less valid for Earth and Venus. We further predict that} Mars' volatile
budget is most likely different from Earth's and that Venus formed close enough
to our planet that it is expected to have a nearly identical composition from
common building blocks.
