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Dr. Porco believes that the icy moon, with its underground liquid sea of water, organics, as well as an energy source, may potentially host life similar to that found in analogous environments on Earth. The March 2012 images of Cassini's "tiger stripes" revealed that these cracks widen and narrow, as was suspected from pictures taken previously. The fissures also change over time more frequently than was originally thought. The two opposite sides of the fissures move laterally relative to one another. This is analogous to the way two banks of the San Andreas Fault can move forward and back, as well as in opposite directions. The greatest slipping and sliding happens when Enceladus is closest to Saturn--as scientists expected. A billion years ago, our Moon was closer to Earth than it is now. As a result, it appeared to be a much larger object in the sky. During that ancient era, if human beings had been around to witness such a sight, it would have been possible to see the entire Moon--not merely the one near side face that we see now. A billion years ago, it took our Moon only twenty days to orbit our planet, and Earth's own day was considerably shorter--only eighteen hours long. Stupendous, almost unimaginably enormous tides, that were more than a kilometer in height, would ebb and flow every few hours. However, things changed, as the lunar orbit around our primordial planet grew ever wider and wider. Annually, Earth's Moon moves about 1.6 inches farther out into space. Currently, the lunar rate of rotation, as well as the time it takes to circle our planet, are the same. A moon is defined as a natural satellite that orbits a larger body--such as a planet--that, in turn, orbits a star. The moon is kept in its position both by the gravity of the object that it circles, as well as by its own gravity. Some planets are orbited by moons; some are not. Some dwarf planets--such as Pluto--possess moons. In fact, one of Pluto's moons, named Charon, is almost half the size of Pluto itself, and some planetary scientists think that Charon is really a chunk of Pluto that was torn off in a disastrous collision with another object very long ago. In addition, some asteroids are also known to be orbited by very small moons.