Collisions in solar system made Mercury metal-rich

The planet Mercury and other unusually metal-rich objects in the solar system may be relics left behind by collisions in the early solar system that built the other planets, says a study.

Mercury's unusual metal-rich composition has been a long-standing puzzle in planetary science.

The origin of Mercury has been a difficult question in planetary science because its composition is very different from that of the other terrestrial planets and the moon.

This small, innermost planet has more than twice the fraction of metallic iron of any other terrestrial planet. Its iron core makes up about 65 percent of Mercury's total mass. Earth's core, by comparison, is just 32 percent of its mass.

"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," said Erik Asphaug, a professor at Arizona State University in the US.

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

The researchers 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. These last two planets lucked out.

"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," Asphaug added.

The study appeared online in the journal Nature Geoscience.