Earth is our only example of where we know for sure that life has evolved and it needs three things. Elements that are essential for life, such as carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur, an energy source which can fuel chemical reactions, for example the sun's energy on Earth, and liquid water, where chemicals can mix and bond together to form larger molecules. Almost everywhere we find water on Earth, we find life. But Earth is not the only place in our solar system where water may be found. In the outer solar system there are several moons which orbit planets such as Jupiter and Saturn. Since these moons are far away from the sun, they don't receive enough heat energy for water to exist as a liquid. Because of this, icy moons are characterized by thick icy shells. But is there any liquid water on icy moons? Many of these moons have very elliptical orbits, which means they travel around their parent planet in an oval shape. The more elliptical the moon's orbit, the stronger the gravitational pull from the planet. The force of gravity pulls and pushes the moons as they travel around their orbits, and if elliptical enough can generate enough heat to sustain oceans underneath their icy shells. This is called tidal heating. This then satisfies two of the requirements for life, energy from tidal heating and liquid water in huge oceans underneath the moon's icy crusts. We can then use telescopes and space missions to try and find out whether any of these moons also satisfy the third requirement for life, elements which can be used as building blocks for life. Around Jupiter, two of the most interesting moons for looking for life are Ganymede and Europa. Both of these moons have thick icy crusts. Underneath which, a liquid global ocean can be maintained because of tidal heating. Ganymede is the largest moon in our solar system, even bigger than the planet Mercury. It is also the only moon known to have its own magnetic field, which causes a rawer of glowing electrified gas which circle the north and south poles. Ganymede's solid icy crust is extremely thick, showing cracks because of tidal heating, and many craters caused by the impact of other objects onto the ice surface. Beneath the thick icy crust lies a huge subsurface ocean, which is estimated to be ten times deeper than the Earth's, which is around four kilometres on average and eleven kilometres at its deepest. On the icy shell of another of Jupiter's moons, Europa, water ice and reddish areas which indicate salts have been detected. These suggest Europa also has an ocean underneath its icy shell, and this global ocean is thought to contain two to three times more water by volume than Earth. This is a huge amount of liquid water, which could be an excellent place to look for evidence of life. Unlike Ganymede, Europa's ocean is probably in direct contact with warm rock at its sea floor. This could supply hydrogen and other essential elements into the water. This could produce chemical reactions, which may provide energy for life. If we go out even further into the solar system, Saturn has two moons that have particularly captured scientists' interest. Titan is an icy world whose surface is completely obscured by a golden hazy atmosphere. This atmosphere is a thick smog of nitrogen and methane. It's the only moon in the solar system with a dense atmosphere, and the only moon, aside from our own moon, onto which a spacecraft has landed. On the surface of Titan, the Huygens space probe found lakes, rivers and seas of liquid natural gas, including methane and ethane. No other world in the solar system, apart from Earth, has that kind of liquid activity on its surface. One of its most famous seas is called Kraken-Mare, named after the legendary sea monster. But Titan also has an ocean of liquid water mixed with salt and ammonia underneath its ice rock surface. It is possible that life might exist in Titan's sub-surface ocean, or even its seas of liquid methane, though any life there is likely to be very different from life on Earth. Another of Saturn's moons, in Saladus, is the smallest moon in our solar system, which shows signs of geological activity. At only 500 kilometres in diameter, it is roughly the size of France. At its South Polar region, there are huge cracks in the surface known as the Tiger Stripes. The cracks are around 130 kilometres long, 2 kilometres across, and 500 metres deep, and they are host to up to 100 jets that vent out plumes of icy material, thousands of kilometres into space. This icy material is sourced directly from a subsurface ocean. The plumes contain molecular hydrogen, suggesting that deep in the moon's interior, there is hydrothermal activity. On Earth, hydrothermal vents host many different life forms, and are even thought to be one of the most likely places where life originated, so the presence of hydrothermal activity on in Saladus is very exciting. Ganymede, Europa, Titan and Inceladus are only a few of the icy worlds in our solar system that might have liquid oceans. Others could be found as far away as the dwarf planet Pluto, and there is much more to discover around Neptune and Uranus. There are many spacecraft missions planned to study these icy worlds in more detail, which could help us to understand whether life could develop in a completely different environment to Earth, and whether Earth is indeed the only place where life can be found.