What do many scientists believe lies beneath Europa's frozen surface?

There are only a few places in the solar system where scientists think life beyond the Earth might possibly be found. One of them is Europa, one of Jupiter's moons.

At first sight it seems an unlikely choice. Europa is a long way from the Sun, more than five times as far as the Earth is, and it is bitterly cold. It is also small, a little smaller than our own Moon. Its surface is frozen so hard that, where you could break the Earth's winter ice with an ice pick, on Europa you would need a jackhammer.

What makes Europa interesting is water. Water is one of the key ingredients for life, and scientists think Europa has a great deal of it, perhaps more than twice as much as the Earth. On the surface it is solid ice. Beneath that frozen shell, however, many scientists believe there may be a deep ocean of liquid water.

Nobody knows whether anything lives there. But a moon with a hidden ocean is exactly the kind of place scientists want to explore when they ask whether we are alone in the universe.