New computer modeling suggests that the innermost planet endured at least one primordial collision with a bigger object. This encounter stripped proto-Mercury of most of its outer layers but left its huge iron core intact.
Any planetary scientist will tell you that Mercury is a small planet with big mysteries — despite the fact that NASA's Messenger spacecraft has returned vast troves of data since taking up orbit there nearly 3½ years ago.
No unanswered Mercurian puzzle is bigger than how it ended up with such a huge iron core, which takes up 80% of the planet's diameter and 65% of its mass. Basically, Mercury is pretty much a planet-scale cannonball and not much else.
Prior to Messenger's arrival, theorists has proposed three possible explanations for the inner planet's unique makeup:
#1: Mercury's initial composition was more Earthlike. But, thanks to being blasted by a young and energetic Sun, its outer layers got so hot that they largely vaporized.
#2: Mercury was never Earthlike but instead somehow assembled from metal-rich building blocks.
#3: The "big splat" gambit. Mercury was bigger when it formed, but then it endured a massive impact that stripped away most of its initial crust and mantle, leaving behind an iron-dominated core and not much else.
Option #2 has always seemed compositionally unlikely. Then, to everyone's surprise, Messenger discovered that Mercurian rocks contain substantial amounts of sulfur, potassium, and sodium — volatile elements that tend to disappear when things get too hot. This finding doomed scenario #1 and left #3 hanging by a thread.
Now theorists Eric Asphaug (Arizona State University) and Andreas Reufer (University of Bern, Switzerland) have put a new spin on the big-splat hypothesis that plausibly explains the Iron Planet's composition. Essentially, Mercury wasn't the target — it was the impactor.
Several years ago, Asphaug showed that many primordial collisions must have involved smaller objects running into bigger ones. The small fry weren't always simply gobbled up. About half the time, the impacts involved glancing blows — what he termed "hit-and-run" collisions — in which both objects survived but which left the smaller body stripped of its outer layers.
Assuming that all the protoplanetary combatants had already become differentiated, that is, with heavier elements like iron having settled into their cores, HRCs should have created numerous iron-rich bodies in the inner solar system. That's because 90% of the debris would eventually be swept up by the more massive object involved in each collision.
The way Asphaug and Reufer see it, Mars and Mercury are survivors — planetary embryos that managed to avoid being accreted into Earth or Venus. Perhaps because it orbited on the outer fringe of the chaotic terrestrial-planet construction zone, Mars somehow escaped any encounters with larger objects.
But, as the two researchers detail in July 6th's Nature Geoscience, Mercury could easily have endured one or more hit-and-run collisions and survived, progressively becoming more iron-rich after each wallop. Yet it managed to sweep up and hang onto enough silicates and volatile elements to reflect the composition it has today.
"It's like flipping a coin that lands heads two or three times in a row," Asphaug explains in a press release. "Lucky, but not crazy lucky. In fact, about one-in-10 lucky."
By the way, Asphaug and Reufer believe hit-and-run collisions could have a lot to do with how the asteroid belt ended up with dozens of iron-dominated "cores" (the source of Earth's iron meteorites) and yet only one object — Vesta — that still retains its original crust.