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.

Portrayal of two planets colliding
An artist's impression of the collision between two massive protoplanets early in solar-system history. Such a "big splat" might have left Mercury with a thin silicate mantle overlying a huge, iron-rich core.

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.

Hit-and-Run Collisions

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.

Computer model of Mercury's formation
In this computer simulation, proto-Mercury (orange, with turquoise denoting its iron-rich core) strikes a more massive protoplanet (red, with dark-blue core) with a glancing blow.
E. Asphaug & A. Reufer / Nature Geoscience

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.

Computer model of Mercury's formation
The shock of the glancing collision strips and vaporizes much of smaller impactor's outer layers. What remains is dominated by the iron-rich core — an outcome very much like the makeup of modern-day Mercury.
E. Asphaug & A. Reufer / Nature Geoscience

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.

Want to hold the innermost planet in your hands and study its surface up close? Now you can, thanks to Sky & Telescope's Mercury globe, created using 18,000 images from the Messenger spacecraft.


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Anthony Barreiro

July 10, 2014 at 5:44 pm

Is it possible that Mercury might be the surviving core of the protoplanet that hit the Earth, leading to the formation of the Moon? The Moon and Mercury seem complementary in so far as the Moon has a relatively small core and Mercury has a relatively big core.

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July 11, 2014 at 10:44 am

I think it is a reasonable working hypothesis now. The moon has to have been formed by some close interaction with another protoplanet. The range of possibilities includes the survival of a remnant of the protoplanet (=Theia). Working out the consequences of this to yield observable confirmatory data in today's solar system is dauntingly complicated. One difficulty that I see now is that Mercury seems to have lost a lot of orbital energy in the process to get it down into its present orbit. It doesn't seem likely to have happened in a single step with the earth encounter and formation of the moon. Mark Twain's comment about rich dividends in speculation from such a small investment in fact is especially relevant here.

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Peter Wilson

July 10, 2014 at 11:04 pm

As I recall, the core of the protoplanet that hit the Earth merged with Earth’s in the process. Only the less-dense, rocky material of either body was blasted into space, most of it "only" about 10,000 miles high, where the Moon formed. It is dynamically impossible that the dense core could have escaped to Mercury’s orbit, while the lighter “fluff” that became the Moon got left behind.

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Peter Wilson

July 11, 2014 at 11:32 am

Sorry, Anthony, I misunderstood the proposed mechanism, and therefore your question. The list of suspects for the larger planet has to include Venus or Earth, so it would seem entirely possible. Yet the dynamics are still tricky. The angle-of-impact would have to be such that it transferred a substantial amount of orbital angular momentum: Earth would get boosted to a higher orbit, with Mercury falling to a lower one. As diagrammed, this would mean the sun was at upper right, with the larger body moving towards lower right. The impact would then impart a clockwise rotation, whereas bodies in the solar-system tend to rotate counterclockwise...Venus being an exception. So is Venus the more likely "target"?

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July 13, 2014 at 5:33 pm

Well, I'll be damned! Velikovsky was right after all!

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