The formation of Mercury has perplexed planetary scientists for quite some time, as it has a wide variety of unique features when compared to other terrestrial planets in the solar system. A new hypothesis by Erik Asphaug of Arizona State University proposes that Mercury could have been the result of a series of glancing blows early in the history of the solar system, causing these differences. The paper was published in Nature Geoscience.

Mercury’s iron core is an astonishing 65-70% of the planet’s total mass, while Earth’s core represents only 32% of total mass. The incredibly high metal content of the first planet from the Sun is an anomaly among rocky planets in our solar system, as it is also lacking a bulky mantle.

There have been a wide variety of hypotheses trying to explain how Mercury could have been such an oddball following planetary formation, though none of them were able to adequately explain why the planet’s mantle was absent but there is still a large quantity of volatiles. Volatiles are elements and compounds that have low boiling points, such as water. A massive collision during formation would have eliminated the mantle, but it would have gotten rid of the volatiles as well. 

This new model explores an alternative view of how the aggregation materials in the proto-solar system would have created a planet with so many distinct properties. The hypothesis states that rather than getting blasted by one large collision, Mercury grazed other large planetary bodies. This hit-and-run model suggests that the mantle was chipped away in pieces, but without the force required to leave lasting shock features. 

Previously, this type of model was discounted because it was assumed that the planetary object that would eventually accreted into the larger body, and would not be able to survive on its own.

“The surprising result we have shown is that hit-and-run relics not only can exist in rare cases, but that survivors of repeated hit-and-run incidents can dominate the surviving population. That is, the average unaccreted body will have been subject to more than one hit-and-run collision,” explains Asphaug in a press release. “We propose one or two of these hit-and-run collisions can explain Mercury’s massive metallic core and very thin rocky mantle.”

So what happened to Mercury’s mantle? The most likely scenario is that the remains stayed with the larger planetary body after the collision, which would be Venus or Earth. This model is a bit more convoluted than others, as two full bodies wouldn’t always merge after a collision; it could be done in chunks. This helps account for the wide range of composition seen among planets and asteroids. 

“Protoplanets do merge and grow, overall, because otherwise there would not be planets,” continues Asphaug. “But planet formation is actually a very messy, very lossy process, and when you take that into account, it’s not at all surprising that the ‘scraps,’ like Mercury and Mars, and the asteroids are so diverse.”