  The abundance of highly siderophile elements (HSEs) inferred for Mars' mantle
from martian meteorites implies a Late Veneer (LV) mass addition of ~0.8 wt%
with broadly chondritic composition. Late accretion to Mars by a differentiated
Ceres-sized (~1000 km diameter) object can account for part of the requisite LV
mass, and geochronological constraints suggests that this must have occurred no
later than ca. 4480 Ma. Here, we analyze the outcome of the hypothetical LV
giant impact to Mars with smoothed particle hydrodynamics simulations together
with analytical theory. Results show that, in general about 50% of the
impactor's metallic core shatters into ~10m fragments that subsequently
fragment into sub-mm metallic hail at re-accretion. This returns a promising
delivery of HSEs into martian mantle compared to either a head-on and
hit-and-run collision; in both cases,<10% of impactor's core materials are
fragmented and finally embedded in the martian mantle. Isotopic evidence from
martian meteorites, and interpretations from atmospheric mapping data show that
a global surface water reservoir could be present during the early Noachian
(before ca. 4100 Ma). The millimeter-sized metal hail could thus react with a
martian hydrosphere to generate ~3 bars of H2, which is adequate to act as a
greenhouse and keep early Mars warm. Yet, we also find that this atmosphere is
transient. It typically survives shorter than 3 Myr based on the expected
extreme ultraviolet (EUV) flux of the early Sun; if the Sun was a slow rotator
an accordingly weaker EUV flux could extend this lifetime to >10 Myr. A dense
pre-Noachian CO2 atmosphere should lower the escape efficiency of hydrogen by
IR emission. A more detailed hydrodynamic atmospheric model of this early
hydrogen atmosphere is warranted to examine its effect on pre-Noachian Mars.
