Timing of Martian Core Formation from Models of Hf–W Evolution Coupled with N-body Simulations
Matthew Brennan, Rebecca Fischer, Francis Nimmo, David O'Brien
Determining how and when Mars formed has been a long-standing challenge for planetary scientists. The size and orbit of Mars are difficult to reproduce in classical simulations of planetary accretion, and this has inspired models of inner solar system evolution that are tuned to produce Mars-like planets. However, such models are typically not coupled to geochemical constraints. Analyses of Martian meteorites using the extinct hafnium-tungsten (Hf-W) radioisotopic system, which is sensitive to
... he timing of core formation, have indicated that the Martian core formed within a few million years of the solar system itself. This has been interpreted to suggest that, unlike Earth's protracted accretion, Mars grew to its modern size very rapidly. These arguments, however, generally rely on simplified growth histories for Mars. Here, we combine realistic accretionary histories from a large number of N-body simulations with calculations of metal-silicate partitioning and Hf-W isotopic evolution during core formation to constrain the range of conditions that could have produced Mars. We find that there is no strong correlation between the final sizes or orbits of simulated Martian analogs and their 182 W anomalies, and that it is readily possible to produce Mars-like Hf-W isotopic compositions for a variety of accretionary conditions. The Hf-W signature of Mars is very sensitive to the oxygen fugacity (fO2) of accreted material because the metalsilicate partitioning behavior of W is strongly dependent on redox conditions. The average fO2 of Martian building blocks must fall in the range of 1.10-1.35 log units below the iron-wüstite buffer to produce a Martian mantle with the observed Hf/W ratio. Martian 182 W isotopic signatures are more often reproduced if the planet's building blocks are sulfur-rich and exhibit a high degree of impactor metal equilibration, but the timing of accretion is a more important control. We find that while Mars must have accreted most of its mass within ~5 million years of solar system formation to reproduce the Hf-W isotopic constraints, it may not have finished accreting until >50 million years later. There is a high probability of simultaneously matching the orbit, mass, and Hf-W signature of Mars even in cases of prolonged accretion if giant impactor cores were poorly equilibrated and merged directly with the proto-Martian core.