Ganymede is about 5,270 km across, compared with about 4,880 km for Mercury, making it the largest moon in the solar system. It's also the only moon known to generate its own magnetic field, complete with faint auroras. Yet Mercury is twice as massive, because Ganymede is largely made of ice.
A large share of Earth's water molecules formed in the interstellar cloud that existed before the Sun was even born, more than 4.6 billion years ago. You are literally drinking a cosmic fossil.
By day, the Martian sky looks butterscotch because of fine dust hanging in the air. But at dusk, those same grains scatter blue light forward, creating a bluish glow around the Sun. Rovers have photographed these blue sunsets: the exact reverse of what we see on Earth.