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Planet Formation Group: Philip Carter

In the accepted model of planet formation, planets form via accretion from a disc of material orbiting their young host star. The Earth is believed to have formed from the same material that formed the chondritic meteorites, which are remnants of the same protoplanetary disc, and we therefore expect the Earth to have the same composition as these meteorites. As we cannot directly probe the majority of the Earth's interior, this model is very important to our understanding of the Earth.

This assumption has recently come under question as several indications of a non-chondritic composition for the Earth have been found. One possible solution to this problem is that the missing material is isolated in a hidden reservoir at the base of the mantle. Alternatively, the Earth's non-chondritic composition could be the result of the collisional erosion of differentiated planetesimals during its formation.

For example, if collisions of planetesimals and protoplanets generally strip mantle material from the growing Earth, but leave the core intact, the iron content of the Earth could be significantly altered from the composition of the primitive planetesimals.

In order to test this idea, we are performing a series of simulations that model the collisions of planetesimals as they accrete to form protoplanets similar to proto-Earth. These N-body simulations include a state-of-the-art collision model that we use to determine the outcomes of collisions, including fragmentation, allowing us to accurately model the collisional formation of protoplanets. We combine this with a model based on hydrodynamic simulations (Marcus et al. 2010), to calculate how these collisions affect the core to mantle balance of the differentiated planetesimals.

Collision Tree

Fig 1: A collision tree showing the final collisions involved in the construction of a protoplanet. (Note that there is no indication of relative time on this plot.)

Our first results indicate that it is possible for collisions to substantially alter the core-mantle balance, and hence composition, of the forming protoplanets (Bonsor et al. 2015).

Element Breakdown

Fig 2: Mg/Fe and Si/Fe ratios for the bulk Earth, chondrites, and the chondrites corrected for collisional evolution (coloured symbols). The range for bulk Earth and the corrected chondrites indicate the two extreme assumptions for the Si content of the Earth's core (0-10%). The blue and green symbols are for an initial core mass fraction of 0.22, the red and magenta for 0.35. From Bonsor et al. (2015).