Mercury is a world barely larger than our Moon, yet it hides a disproportionately massive iron core that makes it incredibly dense. This episode explores the unresolved scientific mystery of how the innermost planet came to be a wounded, stripped remnant, examining the two leading theories: a catastrophic ancient collision that blew away its outer layers, and a prolonged baking period under the intense heat of the young Sun.

I kept returning to one question while working on this episode: how can a planet that sits so close to the fire simultaneously hold permanent deposits of water ice? The answer lies in the stark geometry of its cratered poles, but it is just one piece of a larger forensic puzzle. From the immense cliff-like scarps formed as the planet literally shrank in on itself, to its bizarre three-to-two spin-orbit resonance that ma ...