During a chaotic period early in the Solar System's history, the inner planets were formed by repeated collisions between smaller bodies, with the resulting debris merging into a single body. But our own planet suggests that these collisions don't always have to end neatly in a merger; the Moon was formed by the impact of a Mars-sized body that smashed into the larger Earth.

In fact, simulations of a mixed population of small bodies show that about half of the collisions are hit-and-run affairs, where "the projectile... bounces off or plows through" the object it's impacted. Now, researchers are suggesting that these hit-and-run collisions may explain one of the oddest properties of the planet Mercury: 70 percent of its mass is its iron-rich core.

This abundance of iron is difficult to explain through other models of planet formation. If a body were gradually built up by collisions, its composition should reflect the starting materials of the Solar System. And based on the asteroids we've sampled, these starting materials were mostly silicates with a bit of iron mixed in, varying slightly depending on where they're located within the Solar System. This neatly explains why the Earth, Mars, and Venus have roughly similar compositions.

The new model would suggest that Mercury also formed with a similar composition to the rest of the planets. And as with the Earth, Mars, and Venus, this differentiated so that the iron and other metals sunk to the core, and this was surrounded by a mantle of silicate rocks.

That's where the similarities end, though. In Mercury's case, rather than continuing to grow larger through mergers, it started to shrink through hit-and-run collisions. In their simulations, the two authors (Erik Asphaug at Arizona State and Andreas Reufer at the University of Bern) show that a hit and run collision with a larger planet can strip much of the mantle from the smaller body, leaving it behind in the orbit of the larger one. Eventually, this stripped mantle ends up part of the larger planet. Meanwhile, the smaller body can re-condense in a different orbit, lacking a fraction of its original mantle.

In a simulation with 20 original bodies orbiting the Sun, the authors would end up with a large variety of possible outcomes. Within these scenarios, they looked at the history of the last two surviving small bodies. One of them was likely to have survived either without a collision or while having a single hit-and-run event. This could explain Mars, which has a terrestrial composition. The second, however, is likely to have endured more than two hit-and-run impacts and is likely to have had its mantle severely depleted. In other words, it should look like Mercury.

These collisions would also vaporize any of the volatile elements and chemicals on both bodies involved. But the authors argue that these should remain in orbits near their respective sources, and thus re-accrete back to the bodies they originated from. This would explain another puzzling feature of Mercury, which has more volatiles than you might expect (and more than our Moon was left with, given the Earth was nearby to Hoover them up).

These sorts of processes aren't limited to planet-sized bodies. The authors suggest that it could also explain the composition of some metal-rich asteroids such as Psyche. If they're right, it should also apply to exosolar systems as well, which may explain some of the odder planets we're finding in them.

Nature Geoscience, 2014. DOI: 10.1038/NGEO2189 (About DOIs).