Far smaller and closer to the Sun than it should be, Mercury has long baffled astronomers because it defies much of what we know about planet formation. A new space mission arriving in 2026 might solve the mystery. At a cursory glance, Mercury might well be the Solar System's dullest planet. Its barren surface has few notable features, there is no evidence of water in its past and the planet's wispy atmosphere is tenuous at best. The likelihood of life being found amidst its scotched craters is non-existent. Yet, look closer and Mercury is a fascinating, improbable world that is shrouded in mystery. Planetary scientists remain flummoxed by the very existence of the closest planet to our Sun. This peculiar planet is tiny, 20 times less massive than Earth and barely wider than Australia. Yet Mercury is the second densest planet in our Solar System after Earth due to a large, metallic core that accounts for the majority of its mass.
Earth, Venus and Mars all have iron-rich cores that make up about half of their radius. On Earth, this is separated into a solid inner core and liquid outer core, which churns to produce our world's protective magnetic field. Above is the mantle and then the crust, where we live.
Mercury is completely different. Here the planet's core makes up about 85% of its radius, with only a thin rocky mantle and crust on top. This is what lies behind the planet's incredible density, but why its structure ended up like this isn't entirely clear. "The formation of Mercury is a major problem," says Nicola Tosi, a planetary scientist at the German Aerospace Centre in Berlin. "It's still unclear why Mercury looks like it does."
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