A giant impact four billion years ago could have resulted in Mercury losing most of its mantle to deep space, potentially explaining one of the planet’s key mysteries.

Mercury is unusual compared with other rocky planets in the solar system as its iron core accounts for more than 80 per cent of its radius, compared with just 50 percent for Earth. This makes it very dense with just a thin mantle, about 500 kilometres thick, above the core. Astronomers have long wondered why this is the case …