Dr Philip J Carter

University of Bristol

Research: Giant Impacts

The assembly of rocky planets like the Earth ends with a series of giant embryo–embryo impacts. A giant impact with the proto-Earth is prevailing theory for the origin of the Moon. Giant impacts may also play crucial roles in the formation of Mercury, Jupiter, Uranus, and Pluto.

The canonical Moon‐forming impact model is often considered the archetype of a giant impact; however, recent works have shown it has several substantial difficulties in explaining the Earth and the Moon. A wide range of impacts with substantially different outcomes are possible.

Giant impacts are a key part of planet formation. I am interested in how the thermodynamics of giant impacts and the chemical makeup of the surviving bodies and ejecta affect the growth, composition, and evolution of planets.

Carter et al., 2020, JGR Planets, 125, e06042.

Example giant impacts

Figure 1: Time sequences of giant impacts showing density of the SPH simulations in the equatorial plane. (a) A canonical Moon‐forming impact; (b) a similar mass impactors Moon‐forming scenario; (c) a pre‐spinning proto‐Earth Moon‐forming impact; and (d) a partial accretion impact. The time in hours is shown in the top right corner of each panel. The density of the fluid is shown using interpolation rather than showing the individual SPH particles. The dashed white circles in the final column indicate the present‐day size of the Earth, the black circles show the size of the corotating regions at the end of these simulations. (From Carter et al., 2020).

Figure 2: Interactive time sequence of the similar mass impactors Moon‐forming giant impact (row b in Figure 1) showing density in 3D. The size of the cube is 68x68x68 Mm. Yellow shows the highest density material, blue shows the lowest density material.

We examined the total energies involved in giant impacts that form Earth‐like planets and find that there are large differences across the wide range of possible impacts.

Energy components of example giant impacts

Figure 3: Energy components in example Moon‐forming giant impact simulations. The left hand panels show the first 2 hr in more detail. The canonical model is shown as a solid blue line, the similar‐mass impactor scenario as a dashed purple line, the pre‐spinning proto‐Earth scenario with a dotted magenta line, and the partial accretion example with a solid orange line. Due to a large contribution from gravitational potential energy the total energy is negative in all the examples. (From Carter et al., 2020).

The internal energy increases cause large portions of each body to vaporize as the result of impacts. Giant impacts produce planetary bodies that are significantly inflated in size compared to condensed planets of the same mass, and there are substantial differences in their potential, kinetic, and internal energies. As a result, how planets and their cores evolve after different impact scenarios may vary widely.

Figure 4: Midplane specific entropy for the example partial accretion impact. The number in the top right corner of the panel indicates the time in hours since the start of the simulation. The color scale indicates the entropy of the material, and transparency indicates the density, where the lowest density material is almost entirely transparent. The diffuse yellow material is vaporized rock. More animations can be found here.