The relationship between a planet and its moon can be a beautiful one, but sometimes things take a turn for the worse. According to researchers at NASA, grooves on the surface of Phobos, Mars' largest moon, are evidence that the celestial body has "structural failure" and is slowly falling apart. Slowly is the operative word, as scientists estimate the complete destruction of the moon won't happen for another 30 to 50 million years. Interestingly, the force destroying Phobos is the very planet that it orbits.
Mars's gravity is pulling Phobos closer to the planet at a rate of around 6.6 feet every century. The mutual gravitational pull of the planet and the moon (tidal force), combined with what scientists suspect is the moon's weak internal structure, is causing stress fractures on Phobos that will continue to grow over time. Scientists believe the interior of Phobos is "a rubble pile, barely holding together, surrounded by a layer of powdery regolith about 330 feet (100 meters) thick." Eventually the stress will be too much for the moon to handle, and it will reach its literal breaking point.
Similarly fractured Triton, Neptune's largest moon, may suffer the same fate as it slowly spirals towards and collides with the gas giant, but it has roughly 3.6 billion years before that will happen.