Mars has two confirmed natural moons: Phobos and Deimos. Both are tiny, irregularly shaped moons that look very different from Earth's large, nearly spherical Moon.
According to NASA, Phobos is the larger and closer of the two, while Deimos is smaller and orbits much farther from Mars.
Personally, I find Mars' moons more interesting than the number itself. We have one huge Moon dominating our night sky, while Mars has two objects that almost look like captured space rocks.
Here is a simple Phobos vs. Deimos comparison:
Feature | Phobos | Deimos |
|---|---|---|
Relative size | Larger | Smaller |
Mean radius | ~11 km | ~6 km |
Distance from Mars | Closer | Farther |
Orbital period | ~7.7 hours | ~30.3 hours |
Shape | Irregular | Irregular |
Both moons are extremely small compared with Earth's Moon, which has a radius of about 1,737 km.
Phobos
Phobos is the larger of Mars' two moons and orbits only about 6,000 km above the Martian surface. NASA describes it as the closest known moon to its planet in our solar system.
Its most noticeable feature is Stickney Crater, a massive impact crater relative to the moon's overall size.
Phobos also has a pretty dramatic future.
Its orbit is gradually shrinking, and NASA estimates that in tens of millions of years, Phobos could crash into Mars or break apart and potentially form a ring.
Honestly, that's the kind of space fact that makes "Mars has two moons" suddenly much less boring.
Deimos
Deimos is the smaller and more distant Martian moon.
Unlike heavily cratered-looking Phobos, Deimos has a somewhat smoother appearance because loose material partially fills some of its craters.
It takes about 30 hours to complete an orbit around Mars.
Its name comes from Greek mythology: Deimos means "dread," while Phobos means "fear."
Discovery
Both Martian moons were discovered in 1877 by American astronomer Asaph Hall at the U.S. Naval Observatory in Washington, D.C.
Hall discovered Deimos on August 12 and Phobos on August 18.
I find the timing fascinating. Astronomers knew Mars itself for thousands of years, yet its two tiny moons remained undiscovered until the late nineteenth century simply because they were so difficult to observe.
Origin
Scientists still don't have a completely settled explanation for how Phobos and Deimos formed.
Their dark appearance and irregular shapes once made captured asteroids an obvious possibility. However, NASA notes that their nearly circular, equatorial orbits complicate a straightforward capture explanation. Other hypotheses involve material produced by a giant impact around Mars.
Japan's MMX mission is designed to investigate this question by studying the Martian moons and returning samples from Phobos to Earth.
So even something as basic as "Where did Mars' moons come from?" remains an active scientific question.
FAQs
How many moons does Mars have?
Mars has two known moons: Phobos and Deimos.
Which is the largest moon of Mars?
Phobos is larger than Deimos.
Does Mars have more moons than Earth?
Yes. Mars has two moons, while Earth has one natural moon.
Will Phobos eventually hit Mars?
Its orbit is slowly shrinking. Scientists expect it eventually to crash into Mars or break apart, although this won't happen for millions of years.
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