Mercury’s mean density sits just below Earth’s, making it the runner-up among planets even though it is much smaller. A disproportionately huge iron core packed inside a thin silicate shell drives that ranking.
Impacts or early vapor loss may have stripped lighter outer rock, leaving metal-rich leftovers. MESSENGER’s gravity and composition data tightened the density story and the oversized-core picture.
Density is comparative planetology in one number. Mercury and Earth both shout “iron-rich,” while the giants whisper “light gases and ices.”
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