Which planet's largest moon is bigger than Mercury?

Jupiter's Ganymede, at 5,268 kilometres across - larger than Mercury and about three quarters the size of Mars, which makes it the largest moon in the Solar System. Saturn's Titan is also bigger than Mercury, so two moons outrank a planet.

Ganymede is the only moon known to generate its own magnetic field, which means it has a molten iron core and internal dynamo like a planet. That field carves a bubble inside Jupiter's own vastly larger magnetosphere - a magnetosphere within a magnetosphere, which exists nowhere else that anyone has found - and produces auroras, whose flickering was used by Hubble to infer a subsurface ocean.

That ocean is thought to hold more water than all of Earth's, buried under perhaps 150 kilometres of ice, and possibly layered between different phases of ice at different depths. The European mission JUICE, launched in 2023, will eventually enter orbit around it - the first spacecraft ever to orbit a moon other than our own - and arrives in the mid-2030s.