Mercury’s colossal iron core shrinks its crust into kilometer-scale cliffs
Mercury hides a massive metallic core that takes up roughly 85% of the planet’s radius, leaving a thin rocky shell whose cooling has contracted and buckled the surface into long lobate scarps hundreds of kilometres long. Data from NASA’s MESSENGER mission show a layered interior with a partly molten core, and while the full formation story remains debated, the observed contraction (up to about 7 km in radius since formation) records Mercury’s slow cooling. The upcoming BepiColombo mission, arriving in 2026, should refine estimates of the core and shell, and reveal whether there is any ongoing tectonic activity.