  Composition of terrestrial planets records planetary accretion, core-mantle
and crust-mantle differentiation, and surface processes. Here we compare the
compositional models of Earth and Mars to reveal their characteristics and
formation processes. Earth and Mars are equally enriched in refractory elements
(1.9 $\times$ CI), although Earth is more volatile-depleted and less oxidized
than Mars. Their chemical compositions were established by nebular
fractionation, with negligible contributions from post-accretionary losses of
moderately volatile elements. The degree of planetary volatile element
depletion might correlate with the abundances of chondrules in the accreted
materials, planetary size, and their accretion timescale, which provides
insights into composition and origin of Mercury, Venus, the Moon-forming giant
impactor, and the proto-Earth. During its formation before and after the
nebular disk's lifetime, the Earth likely accreted more chondrules and less
matrix-like materials than Mars and chondritic asteroids, establishing its
marked volatile depletion. A giant impact of an oxidized, differentiated
Mars-like (i.e., composition and mass) body into a volatile-depleted, reduced
proto-Earth produced a Moon-forming debris ring with mostly a proto-Earth's
mantle composition. Chalcophile and some siderophile elements in the silicate
Earth added by the Mars-like impactor were extracted into the core by a sulfide
melt. In contrast, the composition of Mars indicates its rapid accretion of
lesser amounts of chondrules under nearly uniform oxidizing conditions. Mars'
rapid cooling and early loss of its dynamo likely led to the absence of plate
tectonics and surface water, and the present-day low surface heat flux. These
similarities and differences between the Earth and Mars made the former
habitable and the other inhospitable to uninhabitable.
