Mercury's Iron Heart

Mercury's oversized core is a puzzle. Its iron core makes up about 85% of the planet's radius, far more than any other rocky planet. One theory suggests a massive collision early in its history stripped away most of the lighter rocky mantle, leaving behind the dense metallic interior.

Despite its tiny size, Mercury has a weak magnetic field — the only other rocky inner planet besides Earth to have one. The field is about 1% as strong as Earth's, too weak to block the solar wind but strong enough to show that Mercury's core is still partly liquid iron, churning after 4.6 billion years.