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Phobos is Mars’s larger, closer moon; Deimos is smaller and farther out. NASA lists Phobos at 27 × 22 × 18 kilometers and Deimos at 15 × 12 × 11 kilometers. Phobos circles Mars in about 7.65 hours, compared with roughly 30.3 hours for Deimos. Both are tidally locked, but Phobos is slowly spiraling inward. Their origin is still unresolved: captured asteroids, material that formed around Mars, and debris from a major impact are all published possibilities.
Phobos and Deimos at a glance
The two moons are often discussed together because they are Mars’s only natural satellites, yet they differ substantially in scale, distance, orbital speed and surface appearance. The following figures come from NASA’s Mars material and Goddard Space Flight Center/NSSDC fact sheet. Distances are measured from the center of Mars, not from the surface.
| Feature | Phobos | Deimos |
|---|---|---|
| Approximate dimensions along three axes | 27 × 22 × 18 km | 15 × 12 × 11 km |
| Mean distance from Mars’s center | 9,378 km | 23,459 km |
| Sidereal orbital period | 0.31891 days, about 7.65 hours | 1.26244 days, about 30.3 hours |
| Orbital inclination | 1.08° | 1.79° |
| Orbital eccentricity | 0.0151 | 0.0005 |
| Mass | 10.6 × 1015 kg | 2.4 × 1015 kg |
| Mean density | 1,900 kg/m³ | 1,750 kg/m³ |
| Surface appearance | More irregular, with prominent grooves and Stickney crater | Less irregular and smoother-looking; no Phobos-like grooves and ridges |
Which moon is bigger?
Phobos is larger on every listed axis
Phobos is the larger moon by all three NASA-listed dimensions: 27 kilometers across its longest measured axis, 22 kilometers across the next, and 18 kilometers across the shortest. Deimos measures 15 × 12 × 11 kilometers. These are three-axis dimensions rather than a single diameter because neither moon is spherical.
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Using one “diameter” for either moon can therefore be misleading. A statement such as “Phobos is 27 kilometers wide” refers only to its longest listed dimension, not to a circular body with a uniform width. NASA’s fact sheet also gives the corresponding axis radii: 13.0, 11.4 and 9.1 kilometers for Phobos, and 7.8, 6.0 and 5.1 kilometers for Deimos.
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Mass and density reinforce the size difference
NASA lists Phobos’s mass as 10.6 × 1015 kilograms, compared with 2.4 × 1015 kilograms for Deimos. Their mean densities are relatively close—1,900 kg/m³ for Phobos and 1,750 kg/m³ for Deimos—so the mass difference is primarily consistent with Phobos having more volume, not with an enormous contrast in average composition.
How their surfaces and shapes differ
Phobos: irregular, grooved and dominated by Stickney crater
Phobos is heavily cratered and has a visibly irregular outline. Its most conspicuous feature is Stickney crater, a large impact crater that occupies a substantial part of the moon’s surface. Long grooves and ridges are another defining visual characteristic. The grooves are so prominent that they are useful for distinguishing Phobos in photographs from Mars orbit.
Deimos: smoother-looking despite its craters
Deimos is also cratered, but it generally appears smoother and less rugged. NASA notes that some of its craters have been partly filled, reducing the sharp relief seen on a heavily battered surface. Deimos lacks the prominent grooves and ridges associated with Phobos. “Smoother-looking” does not mean crater-free or spherical; it describes the visible surface relief and overall shape.
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NASA associates both moons with dark, C-type material and describes their surfaces as asteroid-like. Deimos’s dark materials have been compared with asteroids in the outer asteroid belt. That resemblance is relevant when scientists evaluate origin models, but it does not prove that either moon is a captured asteroid. Similar surface chemistry can result from different histories, and origin must be tested against orbital properties and other measurements as well.
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How far from Mars does each moon orbit?
Phobos is the inner moon
Phobos has a mean center-to-center distance of 9,378 kilometers. This number is measured from Mars’s center, so it is not an altitude above the Martian ground. Because Phobos is much closer to Mars than Deimos, it moves around the planet rapidly and appears to sweep across the Martian sky on a short cycle.
Deimos follows a wider orbit
Deimos’s mean distance is 23,459 kilometers from Mars’s center. Its orbit is therefore more than twice as far from the planet’s center as Phobos’s listed mean distance. The larger orbital radius helps explain why Deimos takes much longer to complete one circuit.
Inclination and eccentricity
NASA’s Goddard fact sheet gives Phobos an orbital inclination of 1.08° and eccentricity of 0.0151. Deimos has an inclination of 1.79° and a much smaller eccentricity of 0.0005. Inclination describes the tilt of an orbit relative to the reference plane used for the fact sheet; eccentricity describes how much the orbit departs from a perfect circle. Both moons therefore travel on nearly circular, low-inclination orbits, with Deimos’s listed path being especially close to circular.
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How long does each moon take to orbit Mars?
Phobos completes an orbit in about 7.65 hours
Phobos’s sidereal orbital period is 0.31891 Earth days, or approximately 7.65 hours. That is short enough for Phobos to complete roughly three orbits during one Earth day. The period is measured against the background stars, which is why it is called sidereal rather than solar.
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Deimos takes about 30.3 hours
Deimos has a sidereal period of 1.26244 days, approximately 30.3 hours. NASA’s public overview rounds this to about 30 hours. Its farther orbit means a lower orbital speed and a much longer interval between successive circuits than Phobos’s.
Both moons are tidally locked
Phobos and Deimos each rotate once for every orbit around Mars. This tidal locking keeps approximately the same hemisphere facing the planet, just as Earth’s Moon keeps one familiar face toward Earth. Tidal locking describes the relationship between rotation and orbital period; it does not mean the moons remain fixed over one point on Mars.
What is happening to Phobos’s orbit?
Phobos is gradually moving inward
Unlike Deimos, Phobos is slowly losing orbital altitude. NASA describes its inward motion as about 1.8 meters per century. The change is extremely small on human timescales, but over geological time it is significant because Phobos orbits inside the distance at which tidal interactions would instead drive a moon outward.
The projected end state is a long-range estimate
NASA says Phobos may eventually collide with Mars or be torn apart into a ring on a timescale of roughly 50 million years. This is an evolutionary estimate, not a precise countdown to a known impact date. The eventual outcome depends on how tidal forces, structural strength and orbital evolution develop over millions of years.
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No equivalent long-term evolutionary claim is needed to distinguish Deimos for this comparison. Its important contrast is its wider, slower orbit and the absence of a stated inward-spiral forecast in the cited NASA material.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Where did Phobos and Deimos come from?
The origin of both moons remains unsettled. Their small sizes, dark asteroid-like surfaces and nearly equatorial orbits have to be explained together, and no single scenario is confirmed for both. NASA identifies three broad possibilities.
Hypothesis 1: captured asteroids
Under the capture scenario, Phobos and Deimos formed elsewhere—possibly as asteroids—and were later trapped by Mars’s gravity. Their C-type-like, dark surfaces are consistent with this idea, especially because such material is associated with the outer asteroid belt. However, surface resemblance alone is not enough to establish capture. Some evidence cited by NASA challenges a simple captured-asteroid explanation, and gravitational capture must also account for the moons’ current, relatively orderly orbits.
Hypothesis 2: formation from material around Mars
Another possibility is that the moons formed from material already in the region around Mars. In this view, debris or a disk near the planet gradually accumulated into satellites. A local formation model can naturally address why the moons orbit close to Mars and near its equatorial plane, but it must also explain their dark, asteroid-like composition.
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Hypothesis 3: debris from an impact on Mars
A major impact could have launched material from Mars into orbit, with some of that debris later assembling into one or both moons. This model links the satellites to Mars’s own geological history. It must account for the moons’ present composition and for the differences between Phobos and Deimos, including their unequal sizes, distances and surface appearances.
Why samples could help resolve the question
NASA reported in 2021 that ions escaping Mars may have been implanted into Phobos’s surface. The near side, which faces Mars, was estimated to have been exposed to 20 to 100 times more Martian ions than the far side. That creates a possible record of Mars’s atmosphere in addition to information about Phobos itself.
Quentin Nénon, lead author of the reported work and a researcher at the University of California, Berkeley’s Space Sciences Laboratory, explained the value of a sample this way: “With a sample from the near side, we could see an archive of the past atmosphere of Mars in the shallow layers of grain, while deeper in the grain we could see the primitive composition of Phobos.” A returned sample could therefore test whether Phobos is mainly primitive asteroid-like material, material derived from Mars, or a mixture shaped by both processes.
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Capture is a serious hypothesis, not a settled fact. The moons’ dark, C-type-like surfaces make an asteroid connection plausible, but NASA’s descriptions leave the broader origin question open. Formation near Mars and impact-generated debris remain viable alternatives. Until measurements—especially direct compositional and isotopic information from samples or more detailed spacecraft observations—can discriminate among those histories, it is more accurate to say that Phobos and Deimos may be captured asteroids than to state that they are.
What the differences tell us
- Size: Phobos is larger on all three listed dimensions and has roughly four times Deimos’s listed mass.
- Location: Phobos is the inner moon at 9,378 kilometers from Mars’s center; Deimos is farther out at 23,459 kilometers.
- Timing: Phobos orbits in about 7.65 hours, while Deimos needs about 30.3 hours.
- Shape and terrain: Phobos is more irregular, with grooves and Stickney crater; Deimos looks smoother and lacks those groove systems.
- Dynamics: Phobos is slowly spiraling inward and could eventually collide with Mars or become a ring, whereas no corresponding long-term claim is required for Deimos here.
- Origin: Captured asteroids, material formed around Mars and impact debris are all possibilities; none is confirmed for both moons.
Bottom line
Phobos and Deimos are not two versions of the same kind of moon. Phobos is the larger, closer, faster-orbiting and more geologically dramatic satellite, and its orbit is slowly decaying. Deimos is smaller, farther away, slower and smoother-looking. Their asteroid-like surfaces provide clues but not a final answer: the origin of Mars’s moons remains an open scientific question that future observations and samples may resolve.
