Discovered by the Mariner 10 probe, this field is just 1% as strong as Earth's. It shocked scientists who had assumed the small planet's core completely froze long ago.

Mercury's core makes up about 85% of its planetary radius, compared to Earth's core at just 55%. A giant ancient impact may have stripped away most of its original outer rock.

This thermal contraction creates giant cliff-like faults called lobate scarps. Some of these crustal wrinkles stretch for hundreds of miles and reach over a mile high.

Earth is the only planet denser than Mercury, mainly due to gravitational compression. Without this compression, Mercury's massive iron core would actually make it rank first.