Europa is like a spherical ice cube that has partially melted. In fact, if you shook this small moon of Jupiter, you might hear a sloshing sound.

That’s because a deep, global ocean lies beneath the moon’s frozen, criss-crossed crust – an ocean that might be as much as 100 kilometers deep. In fact, Europa’s ocean is so vast that it contains between two and three times as much water as all the oceans on Earth, combined.

What swims in that alien sea? We don’t yet know. Maybe nothing. But astrobiologists have placed Europa at – or near – the top of their visitation wish-list for decades. That massive ocean, which is likely filled with minerals