We know that the mighty Ganymede, one of Jupiter’s moons, is larger than the planet Mercury. It is also known that the make-up of the two objects is very different. So, what would happen if a celestial body with the composition and size of Ganymede took the place of Mercury?
Well, Mercury is a planet that is almost entirely made of its metal core with no geological activity; the planet is basically dead. Meanwhile, Ganymede is made of mostly water ice and liquid or slushy water that exists below the surface. The moon itself would have never formed in the way that it currently exists if it had formed in Mercury’s location because that many water molecules could never condense. But, if Ganymede was simply moved to Mercury’s position, it would still look different than its current state. The ice would melt and then that water and all the other liquid water would evaporate when facing the sun. Hypothetically, the ice could still exist on the side that isn’t facing the sun, because temperatures on the night side of Mercury drop to 100K which is basically the same temperature that exists on the surface of Ganymede (per this Wikipedia page). But the moon would spin, which would mean that within one rotation all its water and ice would be evaporated. Then Ganymede, too, would become a boring, dead rock.