From the November 2012 issue

Why does Mercury have such a large core?

Mercury's metallic core makes up more than 80 percent of planet's diameter. No other inner solar system planet compares.
By | Published: November 26, 2012 | Last updated on May 18, 2023

Planet-interiors
The solar system’s four terrestrial planets each holds a metallic interior, but Mercury is the oddball — its core spans more than 80 percent of the innermost world’s width. // Astronomy: Roen Kelly, after NASA

Why does Mercury have such a large core?

Philip Lynch

Tony, Wisconsin

One of the enduring enigmas of Mercury is why such a small planet, roughly 38 percent of Earth’s diameter, is so dense. Thanks to Mariner 10’s 1974–5 flybys of the innermost world and the orbiting MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, we now know Mercury has an average density of 5.43 grams per cubic centimeter. (Earth’s average density is 5.51 g/cm3.) 

This value means the planet must hold a high percentage of iron because the rocky surface material is much less dense than the planet’s average. Previous results from MESSENGER indicate that the metallic core likely makes up more than 80 percent of Mercury’s diameter. No other inner solar system planet has a core that constitutes so much of its interior. 

So how did Mercury end up this way? One possibility is that in the early solar nebula, gas drag affected the bodies that were richer in rocky silicate materials more than metal-rich bodies, essentially sorting them and leading to less rocky material closer to the Sun.

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