Metal-Rich Planet Mercury was Created by a Hit-and-Run Collision in our Early Galaxy

Mercury, one of the closest planets to our sun, has long puzzled scientists with its unusual metal-rich composition. Now, researchers have discovered the possible origins of Mercury and have revealed that the planet may be a relic left behind by collisions in the early solar system that built the other planets.

The small planet of Mercury is located the closest to the sun. It has more than twice the fraction of metallic iron of any other terrestrial planet. In fact, its iron core makes up about 65 percent of its total mass. In comparison, Earth's core is just 32 percent of its mass.

So why is Mercury such an anomaly? Scientists have come up with a new hypothesis involving hit-and-run collisions. They created a statistical scenario for how planets merge and grow based on the common notion that Mars and Mercury are the last two relics of an original population of about 20 bodies that mostly accreted to form Venus and Earth. But how did Mars and Mercury survive? It turns out they were lucky.

"How did they luck out? Mars, by missing out on most of the action-not colliding into any larger body since its formation-and Mercury, by hitting the larger planets in a glancing blow each time, failing to accrete," said Erick Asphaug, one of the researchers, in a news release. "It's like landing heads two or three times in a row-lucky, but not crazy lucky. In fact, about one in 10 lucky."

In fact, this scenario can actually explain why Mercury has a "missing" mantle and why it's so rich in metal.

"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," said Asphaug. "That is, the average unaccreted body will have been subject to more than one hit-and-run collision. We propose one or two of these hit-and-run collisions can explain Mercury's massive metallic core and very thin rocky mantle." In other words, Mercury's "mantle" is likely located on Venus or the Earth after a collision.

The findings reveal not only how Mercury formed, but also a bit more about planet formation in general. This could tell scientists a bit more about planets in other galaxies and which ones might be more likely to host life.

The findings are published in the journal Nature Geoscience.