Phobos is the larger, inner moon of Mars; Deimos is smaller and farther out. Both are dark, irregular, heavily cratered bodies, but Phobos looks more rugged because of its grooves and enormous Stickney crater, while loose surface material makes Deimos appear smoother. Their origin is unresolved: NASA describes captured asteroids and debris left from early solar-system formation as competing possibilities, not established explanations.
Phobos and Deimos at a glance
The most useful comparison separates four things that are often mixed together: physical size, orbital distance and period, surface appearance, and origin. The table uses NASA reference values and keeps unlike measurements separate. Orbital semimajor axis is measured from Mars’s center; it is not altitude above the Martian ground.
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| Feature | Phobos | Deimos |
|---|---|---|
| Position | Inner moon | Outer moon |
| Approximate three-axis dimensions | 27 × 22 × 18 km | 15 × 12 × 11 km |
| Orbital semimajor axis (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 |
| NASA’s rounded description | About three orbits per Martian day | About 30 hours per orbit |
| Mean radius along the subplanetary axis | 13.0 km | 7.8 km |
| Mass | 10.6 × 1015 kg | 2.4 × 1015 kg |
| Mean density | 1,900 kg/m3 | 1,750 kg/m3 |
| Overall appearance | Dark, irregular, cratered, visibly rugged | Dark, irregular, cratered, smoother-looking |
| Distinctive surface features | Stickney crater and prominent grooves | Loose material partly fills craters; no comparable groove system is highlighted |
| Origin | Unknown; asteroid capture and early debris are proposed | Unknown; asteroid capture and early debris are proposed |
The dimensions are three measurements because neither moon is spherical. Calling either object “27 kilometers wide” or “15 kilometers wide” is a convenient shorthand, not a complete description of its shape.
What the numerical comparison shows
Phobos is larger along all three listed axes. Its longest dimension, 27 km, is 12 km greater than Deimos’s 15 km; its intermediate and shortest dimensions are also larger. The mean-radius figures provide another standardized reference: 13.0 km for Phobos versus 7.8 km for Deimos.
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The mass contrast is even more pronounced than a quick look at one dimension suggests. NASA lists 10.6 × 1015 kg for Phobos and 2.4 × 1015 kg for Deimos, so Phobos has roughly 4.4 times Deimos’s mass. These are reference values for the whole bodies, not measurements of a single rock or crater.
The listed mean densities are 1,900 and 1,750 kg/m3, respectively, a difference of 150 kg/m3. Density is mass divided by volume and is an average property of each irregular moon. It does not mean that either body is a perfectly uniform sphere or that every patch of ground has the same composition.
Orbit measurements show a similarly large contrast. Deimos’s semimajor axis of 23,459 km is about 2.5 times Phobos’s 9,378 km. The orbital periods differ by about a factor of four: 7.65 hours for Phobos compared with 30.3 hours for Deimos. In other words, the outer moon travels around Mars more slowly and on a larger orbit.
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Phobos has the greater dimensions
NASA gives Phobos dimensions of approximately 27 × 22 × 18 km. Deimos measures approximately 15 × 12 × 11 km. Phobos is therefore larger along every listed axis, not merely longer in one direction. Its mean radius along the subplanetary axis is 13.0 km, compared with 7.8 km for Deimos, according to NASA Goddard’s Mars Fact Sheet.
Those figures should not be treated as diameters of perfect spheres. An irregular moon has different lengths depending on which axis or region is measured. The three-axis values communicate the difference more honestly than one “average diameter.”
Mass and density add context
Phobos also has the greater mass: 10.6 × 1015 kg versus 2.4 × 1015 kg for Deimos. The same NASA fact sheet lists mean densities of 1,900 kg/m3 for Phobos and 1,750 kg/m3 for Deimos. Density is an average for each irregular body, not a claim that every location has identical material or that either moon is a uniform solid sphere.
The size difference helps explain why photographs of Phobos often show a more imposing, heavily sculpted body, while Deimos can look like a smaller, softer-edged object. The visual impression is also controlled by surface material and lighting, so apparent ruggedness is not a direct substitute for measuring dimensions.
Why “diameter” can be misleading
For a spherical moon, one diameter is usually an adequate first description. For Phobos and Deimos, it hides the shape that makes them scientifically interesting. A reader comparing “diameters” may also assume that a 27 km object is exactly twice as large as a 15 km object in every sense. The published three-axis dimensions show that each moon has a different elongated, uneven form, while the mass values show that total material does not scale from one headline dimension alone.
How do their orbits differ?
Phobos is much closer to Mars
Phobos’s orbital semimajor axis is about 9,378 km from Mars’s center. Deimos’s is about 23,459 km from Mars’s center. On this center-to-center measure, Deimos’s orbit is roughly two and a half times farther out than Phobos’s.
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“Distance from Mars” can mean several different quantities. The semimajor axis is the standard size of an orbit: for an elliptical path, it is the average of the closest and farthest orbital distances. It is measured from the planet’s center of mass. It is not the height above the surface.
NASA also describes Phobos as about 3,700 miles above the Martian surface. That is an altitude from the ground, whereas 9,378 km is a center-based orbital measure. Mixing those two numbers produces a misleading comparison.
Phobos completes an orbit in hours
NASA Goddard lists Phobos’s sidereal orbital period as 0.31891 days. Multiplying by 24 gives approximately 7.65 hours. NASA’s public description rounds this to about three orbits per day. A point on Mars would see Phobos move around the planet rapidly compared with ordinary lunar cycles.
Deimos takes about 30 hours
Deimos’s sidereal period is 1.26244 days, or approximately 30.3 hours. NASA commonly rounds that to about 30 hours per orbit. Its greater distance corresponds to a slower orbital motion around Mars than Phobos’s rapid inner orbit.
The periods are sidereal: they describe one complete revolution relative to the distant stars. They should not be confused with the interval between successive risings in a particular Martian location, which also depends on Mars’s own rotation and the observer’s position.
Why the inner moon moves faster
The paired values illustrate a basic orbital pattern without requiring a separate speed measurement: the satellite on the smaller orbit has the shorter period. Phobos covers one revolution in less than eight hours, while Deimos needs a little over 30 hours. Thus “closer” and “faster” describe related aspects of the same orbital arrangement, whereas “farther” and “slower” describe Deimos’s position in the system.
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What do the moons look like on the surface?
Both are dark, irregular, and cratered
Neither moon is a smooth, spherical satellite. Both are dark bodies with irregular outlines and many impact craters. Their heavily cratered surfaces record repeated impacts over long periods, but crater count alone does not make them look identical.
Phobos: grooves and Stickney crater
Phobos has the more visibly rugged appearance. Its most conspicuous landmark is Stickney crater, a very large impact feature relative to the moon’s overall size. Long grooves and ridges further mark the surface. In images, these structures make Phobos look sharply textured, with strong changes in relief and shadow.
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Stickney is not a separate moon or a smooth basin; it is an impact crater whose scale dominates photographs of Phobos. The grooves are surface features associated with the moon’s fractured, heavily modified terrain. Their presence is one of the quickest visual ways to distinguish Phobos from Deimos.
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Deimos is also heavily cratered, but it generally appears smoother. Loose surface material, or regolith, partly fills and softens the outlines of some craters. That infill reduces the contrast between crater rims and surrounding ground in many views.
“Smoother” does not mean Deimos avoided impacts or has no relief. Its craters remain evidence of impacts; the loose material changes how those features look. Deimos also lacks the prominent groove-and-ridge appearance that makes Phobos so distinctive.
Why photographs can exaggerate the difference
Image angle, illumination, camera processing, and viewing distance affect apparent roughness. Low-angle sunlight casts long shadows across grooves and crater rims, making Phobos’s relief stand out. More direct lighting can flatten shadows on either moon. A fair comparison therefore uses both measured dimensions and the recurring geological descriptions, rather than judging size or texture from one photograph.
Surface appearance also should not be turned into an unsupported composition claim. The dependable comparison is visual and geological: Phobos is rugged, grooved, and dominated by Stickney; Deimos is cratered but its loose material softens many features.
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Where did Phobos and Deimos come from?
Asteroid capture is one possibility
NASA’s Mars fact material says the moons may be captured asteroids. This idea treats Phobos and Deimos as bodies that formed elsewhere and were later gravitationally acquired by Mars. Their dark, irregular, cratered appearances are consistent with that broad possibility, but appearance alone does not prove capture.
Formation from debris is another possibility
NASA also discusses formation from debris associated with the early solar system. In that scenario, material already near Mars could have accumulated into the two small moons after major impacts or during the planet’s early history. The public evidence does not establish that this explanation is correct either.
The origin remains unsettled
NASA states that the origin of Phobos and Deimos is still being debated. The same uncertainty applies to both moons: neither a captured-asteroid origin nor an early-debris origin should be presented as confirmed. A careful comparison says what is known—their sizes, orbits, masses, densities, and surfaces—and labels origin statements as hypotheses.
Because the two moons share a system and broadly similar dark, irregular appearances, researchers often discuss their origins together. Similarity does not by itself show that they formed in exactly the same way, and NASA’s public summaries do not settle that question.
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What the evidence can and cannot establish
Measured dimensions and orbital values describe the moons as they are now. Surface descriptions explain why images differ. Neither category, by itself, identifies where the moons formed. The most accurate wording is therefore conditional: capture and early-debris formation are proposed explanations, while the origin of both moons remains unknown.
What will happen to Phobos over very long timescales?
Phobos is spiraling inward
NASA describes Phobos as slowly moving inward toward Mars. Its long-term path is not stable indefinitely. The forecast is that, on an approximately 50-million-year timescale, Phobos could either collide with Mars or be pulled apart and form a ring of debris.
Fifty million years is an approximate estimate, not a precise countdown to a known event. It describes a projected long-term outcome, not an imminent hazard for people or spacecraft operating today.
Why the same warning is not used for Deimos
The comparison does not require a matching near-term fate claim for Deimos. NASA’s commonly cited long-term forecast concerns Phobos’s inward evolution. Deimos is the farther, slower outer moon, but NASA’s public summaries do not give an equivalent impending-impact or ring-formation timetable for it.
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Keep the forecast separate from today’s orbit
The inward spiral is a change over geological time, not a correction to the present semimajor axis. Reporting 9,378 km describes Phobos’s current reference orbit; reporting roughly 50 million years describes a possible future evolution. Putting the two facts in separate sentences prevents readers from mistaking a long-term forecast for a current distance or an exact deadline.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to compare the moons without mixing measurements
- Start with the three-axis dimensions. Record 27 × 22 × 18 km for Phobos and 15 × 12 × 11 km for Deimos. This preserves their irregular forms.
- Identify the orbital measure. Use semimajor axis from Mars’s center: 9,378 km for Phobos and 23,459 km for Deimos. Do not substitute a surface altitude for one of these values.
- Compare periods in the same unit. Phobos takes 0.31891 days, about 7.65 hours; Deimos takes 1.26244 days, about 30.3 hours.
- Describe appearance separately from size. Both are dark, irregular, and cratered. Add Phobos’s grooves and Stickney crater, then explain Deimos’s smoother look as the effect of loose material partly filling craters.
- Treat origin as an open question. Mention captured asteroids and early debris as proposed explanations, and state that NASA considers the debate unresolved.
- Add the future forecast only where it belongs. Phobos’s inward spiral and approximate 50-million-year possible collision-or-ring outcome are long-term context, not a measurement of its present orbit.
Common comparison mistakes
- Calling the moons spherical: Their three-axis dimensions and irregular outlines make a single exact diameter misleading.
- Confusing orbital distance with altitude: A semimajor axis is measured from Mars’s center; an altitude is measured from the surface.
- Describing Deimos as uncratered: It is heavily cratered. Loose material partly fills some craters and makes them look less sharp.
- Calling Phobos’s future certain: The approximately 50-million-year collision-or-ring scenario is an estimate of possible long-term outcomes.
- Presenting an origin theory as fact: Capture and early-debris formation remain competing hypotheses.
- Comparing rounded and precise values without labeling them: “Three orbits per day” and “about 30 hours” are rounded public descriptions, while 0.31891 and 1.26244 days are NASA fact-sheet values.
- Using “distance” without a reference point: Always say whether the number is a semimajor axis from Mars’s center or an altitude above the surface.
- Inferring geology from brightness alone: A photograph’s lighting and processing can change apparent texture; use the documented grooves, crater infill, and dimensions together.
Which moon is the better example of a rugged Martian satellite?
Phobos is the clearer example if the comparison is about dramatic relief: it is larger, much closer to Mars, completes an orbit in roughly 7.65 hours, and displays Stickney crater, grooves, and ridges. Deimos is the better example of how a cratered body can appear comparatively smooth when loose material softens crater forms. It is smaller, farther out, and takes about 30.3 hours to orbit Mars.
Neither description changes the central scientific qualification. Both moons are small and irregular, both are cratered, and neither has a settled origin. The strongest comparison combines geometry, orbital dynamics, surface geology, and explicit uncertainty instead of relying on a single photograph or a single distance figure.
A concise way to explain the difference
If a reader wants one sentence, say this: Phobos is the larger, inner moon, about 27 × 22 × 18 km, orbiting roughly 9,378 km from Mars’s center every 7.65 hours; Deimos is the smaller, outer moon, about 15 × 12 × 11 km, orbiting roughly 23,459 km from the center every 30.3 hours. Both are dark, irregular, cratered bodies, but Phobos has conspicuous grooves and Stickney crater while loose regolith makes Deimos look smoother. NASA lists their origins as unresolved possibilities involving captured asteroids or early debris.
That formulation answers the practical comparison while preserving the essential qualifications: three-axis dimensions instead of pretend spherical diameters, center-based orbital distances instead of ambiguous heights, sidereal periods in common units, and hypotheses clearly separated from established measurements.
Frequently Asked Questions
Is Phobos bigger than Deimos?
Yes. NASA gives Phobos dimensions of about 27 × 22 × 18 km and Deimos dimensions of about 15 × 12 × 11 km. Phobos is larger along every listed axis and has the greater mass.
Which moon is closer to Mars?
Phobos is closer. Its orbital semimajor axis is about 9,378 km from Mars’s center, compared with 23,459 km for Deimos. These are center-based orbital measures, not surface altitudes.
How long does each moon take to orbit Mars?
Phobos takes 0.31891 days, or about 7.65 hours. Deimos takes 1.26244 days, or about 30.3 hours.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsAre Phobos and Deimos spherical?
No. Both are small, irregular bodies. NASA’s three-axis dimensions are more informative than treating either moon as a perfect sphere with one exact diameter.
Why does Phobos look rougher than Deimos?
Phobos has prominent grooves, ridges, and the large Stickney crater. Deimos is also heavily cratered, but loose surface material partly fills craters and makes its terrain look smoother.
Are the moons captured asteroids?
That is one proposed explanation, not a confirmed conclusion. NASA also discusses formation from debris associated with the early solar system and says the origin of both moons remains debated.
Will Phobos hit Mars?
NASA estimates that on an approximately 50-million-year timescale Phobos could collide with Mars or break apart into a ring of debris. This is an approximate long-term forecast, not a precise countdown.
Does Deimos have grooves like Phobos?
NASA’s comparison highlights Phobos’s prominent grooves, while Deimos is described as smoother-looking because loose material partly fills its craters. No comparable groove system is highlighted for Deimos.
The Bottom Line
Bottom line: Phobos is the larger, closer, faster-orbiting moon, measuring about 27 × 22 × 18 km and circling Mars in roughly 7.65 hours. Deimos is smaller and farther out at about 15 × 12 × 11 km, taking about 30.3 hours per orbit. Both are dark, irregular, cratered bodies; Phobos is more rugged because of Stickney crater and its grooves, while Deimos looks smoother because loose material softens its craters. NASA considers both moons’ origins unsettled, with asteroid capture and early-debris formation still proposed.
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