Dr Philip J Carter

University of Bristol

Research: Composition of planetary embryos

Carter et al., 2015, ApJ, 813, 72.

Planets form via accretion from a disc of material orbiting their young host star. The Earth is believed to have formed from material similar to that which formed the chondritic meteorites, which are remnants of the same protoplanetary disc, and we therefore expect the Earth to have a similar composition to these meteorites, at least in terms of refractory elements.

This assumption of a chondritic bulk composition 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 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. This could explain the Earth's super-chondritic iron-magnesium ratio (Earth Fe/Mg ≈ 2.1, O'Neill & Palme 2008; chondritic Fe/Mg 1.92, Palme & Jones 2003).

In order to test this idea, we carried out 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 mantle stripping law 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 types of the final collisions involved in the construction of a protoplanet. (Note that there is no indication of relative time on this plot. From Bonsor et al. 2015.)

Our results indicate that it is possible for collisions to alter the core-mantle balance, and hence composition, of growing protoplanets (Bonsor et al. 2015, Carter et al. 2015). We find that embryos formed during the intermediate stages of planet formation (from planetesimals with a uniform core fraction) exhibit a range of core mass fractions. We also see that remnant planetesimals show a large variation in core fraction, with both iron-rich and silicate-rich fragments being produced via collisions.

Fig. 2: Evolution of the core fractions of planetesimals and embryos in a calm protoplanetary disc simulation. The upper panel shows embryos only (Carter et al. 2015).

Simulations that include significant dynamical excitation from Jupiter during the "Grand Tack" (in which Jupiter and Saturn migrate inward and then outward through the protoplanetary disc, see Walsh et al. 2011) show a much greater variation in the compositions of planetary embryos.

Fig. 3: Evolution of eccentricity vs semi-major axis for growing terrestrial planet embryos in the Grand Tack model.

Our simulations show that the variations in core mass fraction induced by collisional evolution during the intermediate stages of planet formation can be sufficient to account for the Earth's non-chondritic Fe/Mg ratio. See Carter et al. (2015) for further details. More animations can be found here.

Fig. 4: Bulk Fe/Mg ratios for collisionally processed embryos. The dotted lines indicate the Fe/Mg ratios for the five chondrite types, and the solid line indicates bulk Earth.