Mercury Born from Hit-and-Run Collisions

The planet Mercury, whose origins have been a long-standing puzzle to scientists, may have been born from ancient hit-and-run collisions in the early solar system, according to a new study.

Mercury is a particularly difficult planet to figure out because of its unusual metal-rich composition. Its iron core makes up about 65 percent of its total mass - by comparison, Earth's core is just 32 percent of its mass.

As described in the journal Nature Geoscience, scientists aimed to understand how such a planet accumulated from the dust, ice and gas in the early solar nebula.

A number of theories have been suggested, though none can explain how Mercury lost its mantle while retaining significant levels of volatiles - easily vaporized elements or compounds, such as water, lead and sulfur.

Researchers from Arizona State University (ASU) proposed that a hit-and-run collision was the answer. If a larger planet, like Venus or Earth, collided with an early Mercury, it may explain how it could have stripped the planet of its mantle and left behind mostly 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," ASU professor Erik Asphaug explained in a news release.

"We propose one or two of these hit-and-run collisions can explain Mercury's massive metallic core and very thin rocky mantle."

The team developed 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 maybe 20 bodies that mostly accreted to form Venus and Earth.

"The implication of the dynamical scenario explains, at long last, where the 'missing mantle' of Mercury is - it's on Venus or the Earth, the hit-and-run targets that won the sweep-up," Asphaug said.