The search for life is also a search for water - and one of the most water-rich bodies in the Solar System is thought to be Europa, one of Jupiter's satellites.

The rapid movement of the cloud belts that circle Jupiter itself, and the huge bolts of lightning within them, indicate that this planet is very turbulent and inhospitable. However, its satellite Europa is a much better prospect.

Europa is one of the four satellites of Jupiter first described by Galileo Galilei in 1610. It is similar in size to Earth's Moon. Information and images from NASA's Galileo mission, which orbited Jupiter and the Galilean satellites between December 1995 and October 2003, indicated that Europa is made of metal and silicates, but that it has a thin icy crust. It is this ice, which reflects sunlight, which makes Europa's surface one of the brightest in the Solar System.

The Galileo probe also recorded data on Europa's magnetic field, which are thought to show that there is a salty ocean below the ice. The water is kept liquid by internal heat generated from the combined gravitational pull and push from Jupiter and the innermost Galilean satellite, Io. This heat may well up from the ocean floor through hydrothermal vents similar to those found on Earth's ocean floors. Europa, therefore, possesses the fundamentals necessary for life: water and energy.

	This is an extract from 'Anybody out there', chapter 5 of the Voyage in
	Space and Time booklet