Answer: Phobos

Among moons in our solar system, Phobos, the largest and innermost of Mars’ two moons, is quite a curiosity. Phobos orbits just a scant 3,700 miles (6,000 kilometers) above the surface of Mars; for comparison, Earth’s moon has an average distance of approximately 239,000 miles (385,000 kilometers) from Earth (or nearly 65 times the distance between Phobos and Mars).

The tiny distance between Mars and its largest moon would be more than enough to make it noteworthy, but the close distance creates all sorts of equally noteworthy elements in the Martian/lunar relationship. Because Phobos is so close to Mars, it actually orbits the red planet faster than the planet itself rotates, such that Mars experiences two moonrises and moonsets per day.

Further, mathematical models suggest that, based on the fact that Phobos is quietly drifting closer to Mars by two meters every century, within 30-50 million years the moon will either smash into Mars or break apart and create a planetary ring like a smaller version of those seen around Saturn.