Scientists increasingly believe that liquid water is not limited to Earth or even to planets warmed directly by the Sun. In the outer Solar System, at least six moons are thought to hold deep oceans beneath thick shells of ice, turning them into major targets in the search for extraterrestrial life.
These hidden seas would stay liquid not because of sunlight, but because of internal energy. The leading idea is that a mix of factors, including gravitational tugging and heat inside the moons, can keep water from freezing solid even in extremely cold regions far from the Sun.
What makes the story more important is that the list of possible ocean worlds may still be growing. Researchers are revisiting older spacecraft observations with better tools, while improved computer models are helping scientists spot signs of buried oceans in places once considered less likely.
New missions are expected to sharpen that picture further. As more data arrives, scientists may find that ocean-bearing moons are more common and more varied than once thought, widening the range of environments where life might potentially exist in our Solar System.
