Hit-and-run planetary collisions may have left Mercury 70% iron

NOAA

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 impact. 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.