Mars has two known natural moons: Phobos and Deimos. They are far smaller and lumpier than Earth’s Moon. Phobos travels close to Mars and circles it in roughly eight hours, while Deimos is farther out and takes roughly 30 hours.
Phobos: the close, fast moon
Phobos is the larger of the two, measuring roughly 27 by 22 by 18 kilometers along its three main axes. It is heavily cratered, including a huge scar called Stickney crater. Because Phobos orbits faster than Mars rotates, it would appear to move across a Martian sky in the opposite direction from many familiar moonrises on Earth.
Deimos: smaller and farther away
Deimos is smaller and orbits at a greater distance from the planet. From the ground, it can appear more like a bright point than a large disk. Both moons were discovered by American astronomer Asaph Hall in 1877. Their names come from Greek mythological figures associated with fear and dread.
Why are both moons irregular rather than round?
Their gravity is too weak to force the rigid material into nearly spherical shapes the way gravity does for much larger planetary bodies. Impact craters and fractured surfaces contribute to their irregular silhouettes. Phobos’s famous crater does not mean it is a perfect rocky potato; it is a heavily modified small world.
Where did Phobos and Deimos come from?
Scientists are still investigating their origins. Some evidence resembles primitive asteroid material, raising the possibility of capture; other models suggest debris produced by an ancient collision around Mars. Neither simple explanation should be presented as settled fact. Spacecraft observations and future sample studies may help distinguish competing hypotheses.
Could Phobos eventually break apart?
Phobos is slowly moving closer to Mars as tidal interactions alter its orbit. NASA describes a long-term possibility that it might break apart into debris forming a temporary ring or strike the planet tens of millions of years from now. Those are distant projections, not near-term hazards to Earth.
FAQ
Which moon is bigger?
Phobos is larger than Deimos, although both are tiny compared with Earth’s Moon.
Can you see them in a Mars rover photo?
Yes. NASA rovers and orbiters have photographed them, including views of Phobos passing in front of the Sun.
Does Earth have two moons too?
Earth has one large permanent natural moon; occasional temporarily captured small bodies are a different category.
Sources
NASA: Moons of Mars and NASA JPL: Phobos solar transit, August 2026 explain the moons and recent observations. For more, see facts about Mars.
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