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NASA's battle cry behind the small armada of orbiters, landers and rovers dispatched to Mars is "follow the water!" Where there's water, there could be life, which needs a solvent like water to assemble the complex macromolecules needed for living systems.

Mars is covered with geological evidence that it was once a soggy planet. But no longer. One of the most exciting findings to date from the roving field geologist, the Mars Science Laboratory Curiosity, was the detection of a dried up ancient stream where water once flowed billions of years ago.

The irony is that if you travel a couple hundred million miles beyond Mars' orbit you cross the solar system's frost line, the boundary beyond which there is plenty of water preserved from the planets' birth.

At least six outer moons have subsurface oceans that could potentially be cozy places for life: Europa, Ganymede, Callisto, Titan, Enceladus and Triton. Each of them could have as much if not more water than found in all of Earth’s oceans. In fact Earth is a comparatively dry world.

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