Title:
The curious case of Mars' formation

Abstract:
Recent isotopic studies of the martian meteorites suggests that the bulk
composition of Mars is significantly different from Earth, with Mars
being composed of a higher fraction of ordinary chondrite-like material.
Here, we evaluate this compositional difference in more detail by
comparing output from two N-body planet formation models: the
"Classical" case wherein Jupiter and Saturn are kept in their current
orbits, and the "Grand Tack" case in which Jupiter and Saturn migrate
through the primordial asteroid belt. Our results show that both models
tend to yield Earth and Mars analogues whose accretion zones overlap.
Assuming a disc with a composition change (break) at 1.3 AU reproduces
the isotopic differences between Earth and Mars in both models. At this
break location, the Classical case fares better in forming Mars with its
documented composition (29% to 68% enstatite chondrite plus 32% to 67%
ordinary chondrite) though the Mars analogues are generally too massive.
However, if a restriction of mass on the Mars analogues is included such
as requiring that its mass only differ from its current value by at most
50%, the Classical model does not work better. We further calculate the
isotopic composition of 17O, 50Ti, 54Cr, 142Nd, 64Ni, and 92Mo in the
martian mantle from the Grand Tack simulations. We find that it is
possible to match the calculated isotopic composition of all the above
elements in Mars' mantle with their measured values, but the resulting
uncertainties are too large to place good restrictions on the early
dynamical evolution and birth place of Mars. These large uncertainties
are caused by both the chaotic evolution of the forming planets and the
large uncertainties in the measured isotopic deviations of some elements
in martian samples. Hence, increase in both the resolution of N-body
simulations and the precision of isotopic measurement from martian
meteorites are required to solve the origin of Mars and the other
terrestrial planets.