An image of Jupiter's moon Europa, rising over the planet, taken by the New Horizons spacecraft in 2011.

The planet Jupiter is a monster. It contains 2.5 times the matter of all of the other planets combined, its diameter is 11.2 times that of the Earth, it has a magnetic field 14 times and a surface gravity 2.5 times that of the Earth.

Jupiter is a gas giant, with an atmosphere composed mainly of hydrogen and helium but it has a very dense solid core and its powerful gravity holds 67 moons in its grasp as they orbit their giant master.

In 1610, shortly after Galileo had received a telescope, the latest scientific instrument to come out of the Netherlands, he worked out how to increase its magnification. When he looked at Jupiter through the modified telescope, he discovered to his delight, four moons orbiting the giant planet. One of these moons, Europa, is currently generating the most interest in the scientific community because the forces that this moon are subjected to make it one of the best candidates for finding life in the solar system.

Europa is slightly smaller than our Moon, it is composed largely of silicate rock but has a water-ice crust possibly up to 30km thick. It has one of the smoothest surfaces of any object in the solar system, in that it lacks mountains and craters. That means the surface is young. Nearly all thin atmosphered terrestrial objects in the solar system are pocked with craters.

In Europa's neighbourhood it would be expected that cometary bombardment would occur much more frequently than on Earth. Comets produce craters. No craters means the surface must be in motion and that means tectonic forces are active.

The surface of Europa is bizarre. The radiation from the sun and from the energetic particles produced by Jupiter's magnetic field would kill an unprotected human within a day.

Photographs from the Galileo probe in 1989 and from the Hubble Space Telescope, show a smooth, icy surface but covered by criss-crossing lines and striations. These are light and dark and up to 20kms across. Detailed analysis shows they have moved.

Geologists liken these "linaea" to ocean ridges found on Earth where continents are being rifted, pulled apart to produce valleys. On Europa the rifting opens up the crust and warm water erupts to the surface.

The source of this heat was puzzling. Europa is too small to generate its own heat and too small to retain much heat from its formation many billions of years ago. Its average surface temperature is minus 170 degrees Centigrade; so where is this heat coming from?

The gravitational force between two masses varies as the distance between them changes (it's actually an inverse square law) so the greater the separation the weaker the gravitational pull.

The gravitational force Jupiter exerts on Europa is not the same across the diameter of Europa. Because Jupiter is massive, the side of Europa nearest to Jupiter has a slightly larger gravitational pull than the other side. This difference causes a very slight distortion in Europa's shape.

On its own this would not be enough to account for Europa's internal heat, but large moons such as Ganymede and Callisto orbiting beyond its orbit, tug on Europa. The pull of Jupiter and the tug of Ganymede and Callisto cause tidal forces to alternately squeeze and stretch Europa's interior. The resulting friction heats the interior sufficiently to melt the icy regions beneath the crust. Astronomers think there is an ocean under the crust.

Heat and water are very important requirements for life. If the right mix of chemicals are present, then it could be that some form of life exists in Europa's ocean.

Astronomers are realistic in their expectations. It is thought complex life is highly unlikely, so the ocean will not be filled with whales or the like.

In May of this year, NASA took the first big step to find out if Europa has an ocean, by approving the instruments that will be carried by their planned Europa Clipper unmanned mission. The proposed launch date is 2025 and the travel time will be 6.4 years.

The plan is to orbit Europa 45 times using the instruments to gather as much data as possible, data which will hopefully confirm the existence of the ocean.

Proving there is life on Europa needs future missions to drill through the crust and sample the water beneath. If life were to be found, it would be a momentous discovery because the implications about the potential for life throughout the universe would be profound.