From Earth all natural celestial objects rise in the east and set in the west.  The same is not true at Mars.  Phobos goes in reverse.  

If you were to stand on the surface of the Red Planet, Phobos would rise in the west.  It would appear about one-third the apparent size of our Moon as seen from Earth’s surface, and it would cross the Martian sky against the flow of the other celestial objects before setting in the east.  What makes Phobos so different?

Simple: it orbits unusually close to Mars, at an average altitude of around 9400 km (compared with our Moon’s distance from Earth of around 385 000 km).  To maintain such a low orbit, Phobos has to move so quickly that it out-paces the planet’s rotation.  Mars rotates once ever 24 hours 37 minutes, whereas Phobos completes an orbit in just 7 hours 39 minutes.

Hence, Phobos is constantly overtaking the surface of the planet.  All other natural celestial objects, including Mars’s second moon Deimos, are moving more slowly in relation to the surface of the planet and so are brought into view as the surface turns to face them. This causes them to rise in the east and set in the west.

From Earth, only artificial space-borne objects, such as the International Space Station, are in such low orbits and travelling so fast that they appear to rise in the west.

Back on Mars, the Phobos weirdness does not stop with its backward journey through the sky.  Because the moon is in such a low orbit above the equator, it can never be seen from inside the Martian polar circles. -- Stuart