The accretion, composition and early differentiation of Mars

The early development of Mars is of enormous interest, not just in its own right, but also because it provides unique insights into the earliest histo ry of the Earth, a planet whose origins have been all but obliterated. Mars is not as depleted in moderately volatile elements as are other terrestria l planets. Judging by the data for Martian meteorites it has Rb/Sr approxim ate to 0.07 and K/U approximate to 19,000, both of which are roughly twice as high as the values for the Earth. The mantle of Mars is also twice as ri ch in Fe as the mantle of the Earth, the Martian core being small (similar to 20% by mass). This is thought to be because conditions were more oxidizi ng during core formation. For the same reason a number of elements that are moderately siderophile on Earth such as P, Mn, Cr and W, are more lithophi le on Mars. The very different apparent behavior of high field strength (HF S) elements in Martian magmas compared to terrestrial basalts and eucrites may be related to this higher phosphorus content. The highly siderophile el ement abundance patterns have been interpreted as reflecting strong partiti oning during core formation in a magma ocean environment with little if any late veneer. Oxygen isotope data provide evidence for the relative proport ions of chondritic components that were accreted to form Mars. However, the amount of volatile element depletion predicted from these models does not match that observed - Mars would be expected to be more depleted in volatil es than the Earth. The easiest way to reconcile these data is for the Earth to have lost a fraction of its moderately volatile elements during late ac cretionary events, such as giant impacts. This might also explain the non-c hondritic Si/Mg ratio of the silicate portion of the Earth. The lower densi ty of Mars is consistent with this interpretation, as are isotopic data. Rb -87-Sr-87, I-129-Xe-129, Sm-146-Nd-142, Hf-182-W-182, Re-187-Os-187, U-235- Pb-207 and U-238-Pb-206 isotopic data for Martian meteorites all provide ev idence that Mars accreted rapidly and at an early stage differentiated into atmosphere, mantle and core. Variations in heavy xenon isotopes have prove d complicated to interpret in terms of Pu-244 decay and timing because of f ractionation thought to be caused by hydrodynamic escape. There are, as yet , no resolvable isotopic heterogeneities identified in Martian meteorites r esulting from Nb-92 decay to Zr-92, consistent with the paucity of perovski te in the martian interior and its probable absence from any Martian magma ocean. Similarly the longer-lived Lu-176-Hf-176 system also preserves littl e record of early differentiation. In contrast W isotope data, Ba/W and tim e-integrated Re/Os ratios of Martian meteorites provide powerful evidence t hat the mantle retains remarkably early heterogeneities that are vestiges o f core metal segregation processes that occurred within the first 20 Myr of the Solar System. Despite this evidence for rapid accretion and differenti ation, there is no evidence that Mars grew more quickly than the Earth at a n equivalent size. Mars appears to have just stopped growing earlier becaus e it did not undergo late stage (> 20 Myr), impacts on the scale of the Moo n-forming Giant Impact that affected the Earth.