Dense Metal Core
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

 
Mercury is so dense, scientists believe its heavy iron core accounts for two-thirds of the planet's mass, more than twice the ratio of core to mass for Earth, Venus or Mars. Scientists aren't sure what caused this incredibly high density, but suggest it might have started off with more mass that got scraped off by collisions. Researchers hope MESSENGER's geology measurements can shed light on how the planet formed, and how it got to be so dense.
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