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Phobos is Mars’s larger, closer moon; Deimos is smaller and orbits farther away. Phobos measures about 27 × 22 × 18 km and circles Mars in roughly 7.65 hours. Deimos measures about 15 × 12 × 11 km and takes about 30.3 hours. Both are irregular, cratered bodies, and both keep roughly the same hemisphere facing Mars. Their origin, however, is still unsettled: capture from the asteroid population and formation from material associated with Mars are among the possibilities, not confirmed explanations.
Phobos and Deimos compared
The clearest differences are size and orbital position. NASA’s dimensions show Phobos is larger along each of its three measured axes, while NASA Goddard Space Flight Center’s Mars Fact Sheet places it much closer to Mars than Deimos. The distances below are measured from Mars’s center—not heights above the planet’s surface.
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| 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 in the NASA Goddard fact sheet | 1.08° | 1.79° |
| Orbital eccentricity in the NASA Goddard fact sheet | 0.0151 | 0.0005 |
| Mass listed by NASA | 10.6 × 1015 kg | 2.4 × 1015 kg |
| Mean density listed by NASA | 1,900 kg/m3 | 1,750 kg/m3 |
| Notable surface appearance | Prominent grooves and the large Stickney crater | Appears smoother; lacks Phobos’s grooves and ridges |
The dimensions and appearance are described on NASA Science’s Phobos and Deimos pages; the orbital figures, masses, and densities are from the NASA Goddard Space Flight Center/NSSDC Mars Fact Sheet. These are not spherical moons, so one diameter cannot fully describe either body. The three-axis measurements make that irregularity visible; the fact sheet also lists axis radii of 13.0, 11.4, and 9.1 km for Phobos, and 7.8, 6.0, and 5.1 km for Deimos.
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Phobos is bigger on every listed axis
At 27 × 22 × 18 km, Phobos is larger than Deimos, whose corresponding dimensions are 15 × 12 × 11 km. The comparison holds across all three axes, not just a single approximate diameter. Phobos is also more massive: NASA lists its mass as 10.6 × 1015 kg, compared with 2.4 × 1015 kg for Deimos. Their mean densities are closer—1,900 kg/m3 for Phobos and 1,750 kg/m3 for Deimos—so the size and mass difference should not be mistaken for a large difference in density.
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How should the size numbers be read?
The three values are measurements along different axes, not three competing estimates of one diameter. A value such as 27 × 22 × 18 km indicates an elongated, uneven object. The listed radii tell the same story in another format: Phobos has radii of 13.0, 11.4 and 9.1 km, while Deimos has radii of 7.8, 6.0 and 5.1 km. Quoting all three axes avoids making either moon sound spherical and gives a fairer comparison of their actual shapes.
Both are cratered and irregular
Phobos is heavily cratered. Its most prominent landmark is Stickney, a large impact crater, and grooves are a conspicuous part of its surface. Deimos is also cratered, but appears smoother: some of its craters have been partly filled, and NASA notes that it lacks the grooves and ridges seen on Phobos. “Smoother” describes its observed appearance, not a lack of craters or a perfectly round shape.
NASA characterizes the moons’ surface materials as asteroid-like and associates both with C-type material. Its Deimos overview also compares dark surface materials with asteroids in the outer asteroid belt. That resemblance matters when considering possible origins, but it is not proof that either moon was captured from the asteroid population.
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How do their orbits differ?
Phobos is closer and moves around Mars faster
Phobos’s mean center-to-center distance from Mars is 9,378 km; Deimos’s is 23,459 km. Deimos therefore orbits at about two and a half times Phobos’s center distance. Because Phobos is the inner moon, its sidereal orbit is much shorter: 0.31891 days, or about 7.65 hours, compared with 1.26244 days, or about 30.3 hours for Deimos. Deimos’s period is almost four times as long. In everyday terms, Phobos completes roughly three orbits in an Earth day. NASA’s general overview rounds Deimos’s period to 30 hours.
A sidereal period is the time to complete an orbit relative to the stars. It is not the same as the time between successive appearances in the same position in the Martian sky, because Mars itself is rotating during the orbit. The precise periods in the comparison table are from the NASA Goddard fact sheet; the familiar hour figures are rounded for readability.
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Distance from the center is not altitude
The listed distances begin at Mars’s center. They are not distances above the surface, and they should not be described as such. To find an altitude above the surface, one would also need to account for Mars’s radius; the distance figures alone are orbital center distances. This distinction matters especially when comparing the moons’ reported distances with a map or a spacecraft’s height above Mars.
What do inclination and eccentricity show?
The fact sheet lists Phobos’s inclination as 1.08° and Deimos’s as 1.79°. These are both small numerical tilts under the reference convention used by the fact sheet, with Deimos’s listed value slightly larger. Eccentricity separates them more clearly: Phobos is listed at 0.0151, while Deimos is at 0.0005. Since zero represents a perfect circle, Deimos’s listed orbit is more nearly circular, whereas Phobos’s path departs a little more from a circle.
Both moons are tidally locked
Phobos and Deimos each rotate once during one orbit, so each keeps roughly the same hemisphere facing Mars. This is called synchronous rotation, or tidal locking. It does not mean the moons remain fixed above one point on Mars: they continue moving along their orbits, but their rotation keeps pace with that motion.
Why is Phobos moving inward?
Phobos is slowly spiraling toward Mars. NASA describes its inward motion as about 1.8 meters per century. Over a very long timescale—roughly 50 million years—NASA says Phobos may either collide with Mars or break apart and form a ring. Those are possible outcomes of a gradual orbital evolution, not a precise forecast of when or how the end state will occur.
This estimate is best understood as a warning against treating the current orbit as permanent, not as a countdown relevant to human timescales. It also applies to Phobos; the cited material does not make a comparable long-term evolutionary claim about Deimos. The inward trend is consequently one of the strongest dynamical distinctions between the two moons in this comparison.
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Where did Mars’s moons come from?
No origin explanation is established for both moons. NASA describes three broad possibilities: Phobos and Deimos could be asteroids captured by Mars’s gravity; they could have formed from material in the region around Mars; or they could have formed from debris produced by an impact on Mars. The observations constrain these ideas, but the cited NASA material does not identify one as confirmed.
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The moons’ dark, asteroid-like surfaces and association with C-type material are relevant to a capture explanation. In this scenario, bodies that formed elsewhere would have been drawn into orbit around Mars. But similarity of surface material alone does not demonstrate capture. NASA notes that some evidence challenges this explanation, so it should be presented as a possibility rather than the settled answer. A reader asking, “Were Mars’s moons captured asteroids?” therefore deserves the precise answer: perhaps, but that remains a hypothesis.
Formation from material around Mars
Another possibility is that the moons formed from material in the region around Mars. This is distinct from capture: instead of an already formed asteroid being trapped, the bodies would have taken shape from material associated with Mars. The observations described here do not settle how such material would have produced both moons, so this remains a broad hypothesis rather than a complete account.
Debris from an impact on Mars
A third proposed route is formation from debris generated by an impact on Mars. This, too, connects the moons’ history to the planet rather than requiring them to have arrived as intact asteroids. It is one of the possibilities NASA names; the evidence summarized here does not establish that an impact origin is correct or explain the event in detail.
These hypotheses aim to account for the moons’ observed properties and their relationship to Mars. Surface resemblance can inform the comparison, but does not settle it by itself. The origin question remains open because the same broad observations can be interpreted in more than one way.
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Why samples from Phobos could help
Surface material may preserve clues beyond the moon’s own formation. In a 2021 NASA report, researchers described evidence that ions escaping from Mars can be implanted in Phobos’s surface. NASA reported that the near side may have been exposed to an estimated 20 to 100 times more Martian ions than the far side. The estimate concerns exposure to these ions; it is not a measurement of the amount of Martian material in every grain.
Quentin Nénon, lead author of the reported study and a researcher at UC Berkeley’s Space Sciences Laboratory, explained the potential value of a sample: “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 sample could therefore help investigate both Mars’s past atmosphere and Phobos’s own composition. That makes sampling relevant to the origin question, but the possibility of useful evidence is not itself a confirmed answer.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.The practical takeaway
Phobos is the larger, faster-orbiting inner moon; Deimos is smaller, farther out, and follows a more nearly circular orbit according to the listed eccentricities. Both are irregular and cratered, and both keep approximately one side facing Mars. Phobos’s orbit is slowly shrinking, while the origin of the pair remains unresolved. Asteroid capture is one hypothesis, not a proven explanation.
Frequently Asked Questions
Which is bigger, Phobos or Deimos?
Phobos is bigger on all three listed axes: about 27 × 22 × 18 km, versus 15 × 12 × 11 km for Deimos. NASA also lists Phobos as more massive, although their mean densities are relatively close.
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How long does each moon take to orbit Mars?
Phobos takes about 7.65 hours, or 0.31891 days, to complete a sidereal orbit. Deimos takes about 30.3 hours, or 1.26244 days. Phobos therefore makes roughly three orbits in one Earth day.
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How far are Phobos and Deimos from Mars?
Their mean distances from Mars’s center are about 9,378 km for Phobos and 23,459 km for Deimos. These are center-to-center distances, not altitudes above the Martian surface.
Are Phobos and Deimos tidally locked?
Yes. Each moon rotates once per orbit and keeps roughly the same hemisphere facing Mars. They still travel around Mars; tidal locking does not hold either moon stationary over one surface location.
Were Mars’s moons captured asteroids?
Asteroid capture is one NASA-listed possibility, supported in part by their dark, C-type, asteroid-like materials. It is not a settled explanation, and NASA also discusses formation from material around Mars or impact debris.
What will happen to Phobos?
Phobos is moving inward at about 1.8 meters per century. NASA estimates that on a roughly 50-million-year timescale it could collide with Mars or break apart into a ring. This is a long-range estimate, not an exact countdown.
The Bottom Line
Bottom line: Phobos is the larger inner moon, measuring 27 × 22 × 18 km and orbiting Mars in about 7.65 hours. Deimos measures 15 × 12 × 11 km, lies farther out, and takes about 30.3 hours. Both are irregular, cratered and tidally locked. Phobos is spiraling inward, while the moons’ origin—captured asteroids, Mars-associated material or impact debris—remains unresolved.
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