Phobos and Deimos: What You Are Actually Looking At

Asaph Hall almost gave up his search before he found them. His wife, Angelina, reportedly urged him to keep going one more night, and on August 12, 1877, he spotted Deimos. He found Phobos six days later, on August 18. He named them after the sons of Ares in Greek mythology: Phobos means fear, Deimos means dread.

The two moons could not be more different in behavior, even though they orbit the same planet.

Phobos: The Inner Moon on a Deadline

Phobos is the larger of the two, measuring about 26 by 23 by 18 kilometers across. It orbits Mars at an altitude of roughly 6,000 kilometers, closer to its planet’s surface than any other moon in the solar system. That proximity has a strange consequence: Phobos completes one full orbit in just 7 hours and 39 minutes. Mars itself rotates once every 24.6 hours. This means Phobos rises in the west and sets in the east, twice a day, moving visibly across the Martian sky in under four hours.

Its most striking surface feature is the Stickney crater, a 9-kilometer-wide impact scar that takes up a significant fraction of the entire moon’s face. The collision that formed Stickney nearly shattered Phobos entirely. Radiating outward from the crater are long, parallel grooves and troughs, some hundreds of meters wide, that scientists believe are either fractures from that ancient impact or evidence of surface material loosened by Mars’s gravitational pull.

Deimos: The Quieter Outer Moon

Deimos is smaller, measuring about 15 by 12 by 11 kilometers. It orbits much farther out, at around 23,460 kilometers from Mars, and takes 30.3 hours to complete one orbit. Because that period is slightly longer than the Martian day, Deimos rises slowly in the east and sets in the west, lingering in the sky for roughly 2.7 Martian days before it sets. From the Martian surface, it would look like a slightly brighter-than-average star, not a disc, because it is so small and distant.

Deimos has a smoother surface than Phobos, with fewer craters visible because loose regolith tends to fill them in over geological time. Its two largest craters are named Voltaire and Swift, after writers who, coincidentally, speculated about Martian moons before their actual discovery.

Phobos vs Deimos at a Glance

Where Did They Come From? The Science Is Not Settled

The origin of Phobos and Deimos is one of the genuinely open questions in planetary science. Two main hypotheses compete, and neither is without problems.

The Captured Asteroid Hypothesis

The dominant view for decades was that Mars captured two asteroids from the nearby asteroid belt long ago. Both moons have low albedo, meaning they reflect very little sunlight, which is consistent with carbonaceous chondrite composition, similar to a class of dark, carbon-rich asteroids. Their irregular shapes also fit. Asteroid capture is not impossible, but it requires a specific set of conditions: the incoming object needs to lose enough velocity at exactly the right moment to settle into orbit rather than fly past or crash.

The problem is that Phobos and Deimos orbit Mars in nearly circular, equatorial orbits. Captured asteroids typically end up in highly elliptical, inclined orbits. Getting from a chaotic capture trajectory to the tidy orbits these moons occupy requires a mechanism, and no fully convincing one has been demonstrated.

The Giant Impact Hypothesis

A competing model, gaining ground since the 2010s, proposes that a large body hit Mars early in solar system history, much like the giant-impact hypothesis for Earth’s Moon, and that debris from that collision accreted into Phobos and Deimos. This would naturally explain the equatorial, nearly circular orbits. A 2018 study published in Science Advances by Robin Canup and Julien Salmon modeled this scenario and found it could produce moon-forming discs consistent with the observed orbital properties.

If the giant-impact model is correct, Phobos and Deimos should be made of material similar to the Martian mantle, not asteroid belt rock. Japan’s Martian Moons eXploration (MMX) mission, scheduled for launch in late 2026 with an expected Mars arrival in 2027, is designed specifically to collect a sample from Phobos and return it to Earth in 2031. That sample will likely settle the debate. For now, both models remain scientifically live.

Phobos Is Spiraling Toward Mars and Will Eventually Form a Ring

Phobos is not going to be around forever. Because it orbits faster than Mars rotates, tidal forces are pulling it inward at a rate of about 1.8 centimeters per year. That number sounds small, but over geological time it adds up. Scientists calculate that within roughly 30 to 50 million years, Phobos will either crash into the Martian surface or, more likely, cross the Roche limit, the threshold inside which tidal forces exceed a moon’s own gravitational cohesion, and break apart.

When that breakup happens, the debris will spread into a ring system around Mars. Saturn’s rings are the famous example of what planetary rings look like, but Mars will eventually have a version of its own, made from the wreckage of a moon that was already showing signs of structural stress. The parallel grooves visible on Phobos today may already be early evidence of that tidal stretching, though this interpretation is still debated among researchers.

Deimos, by contrast, is slowly moving away from Mars. Its orbit is above the synchronous orbit altitude, which means tidal forces push it outward rather than inward, the same mechanism that causes Earth’s Moon to recede from us at about 3.8 centimeters per year.

How Phobos and Deimos Compare to Earth’s Moon

The comparison is almost comic in scale. Earth’s Moon is 3,474 kilometers in diameter. Phobos, at roughly 22 kilometers across its longest axis, is about 160 times smaller. If you placed Phobos inside Washington DC, it would fit inside the city limits.

Earth’s Moon is massive enough to drive tides, stabilize our axial tilt, and create total solar eclipses by almost exactly covering the solar disc. Phobos and Deimos can produce partial eclipses on Mars (NASA’s Curiosity and Perseverance rovers have both photographed these), but nothing like what we see from Earth’s surface during totality.

The Moon also formed through a well-understood giant-impact event roughly 4.5 billion years ago. Mars’s moons lack that clear origin story. They are, in a sense, unresolved objects, scientifically speaking, even after 150 years of study.

You can follow ongoing space science coverage as the MMX mission progresses and new data arrives.

Could Mars Have Had More Moons?

Some models suggest Mars may have had additional moons in the distant past that have since either crashed into the planet or been ejected from orbit. A 2021 paper by Amirhossein Bagheri and colleagues, published in Nature Astronomy, used tidal dissipation modeling to trace Phobos and Deimos’s orbital histories backward and concluded both moons likely share a common origin, consistent with the giant-impact model. The study also suggested a third, larger moon may have formed and since been destroyed.

How plausible is that scenario? The orbital mechanics are internally consistent. If Phobos and Deimos both formed from the same impact debris disc, a larger, closer body would have spiraled inward first and impacted Mars long before either current moon reached its present position. The Bagheri model sets rough timeframes: such a predecessor body would have been gone within hundreds of millions of years of the original impact, leaving behind the two smaller survivors we observe today.

Mars’s connection to broader planetary system dynamics is something researchers track alongside Earth’s own environmental history. Our environment coverage includes how planetary science informs our understanding of habitability and climate on a geological scale.

Frequently Asked Questions

How many moons does Mars have?

Mars has exactly two natural satellites: Phobos and Deimos. Both were discovered in 1877 by astronomer Asaph Hall. They are irregularly shaped and much smaller than Earth’s Moon, with Phobos measuring about 26 kilometers at its longest and Deimos about 15 kilometers.

Are Phobos and Deimos captured asteroids?

Possibly, but this is not certain. Their dark, low-reflectivity surfaces resemble carbonaceous asteroids from the outer asteroid belt. However, their nearly circular, equatorial orbits are difficult to explain through capture dynamics. The competing giant-impact hypothesis, which would make them debris from a collision with Mars, is also scientifically credible. Japan’s MMX mission aims to return a Phobos sample to Earth in 2031 to resolve this.

Will Phobos crash into Mars?

Phobos is spiraling inward at approximately 1.8 centimeters per year due to tidal forces. Within an estimated 30 to 50 million years, it will either impact Mars or, more likely, cross the Roche limit and disintegrate into a debris ring around the planet. Current calculations favor ring formation over direct impact, but the outcome depends on the moon’s internal structure and composition.

What is the Stickney crater on Phobos?

Stickney is a roughly 9-kilometer-wide impact crater on Phobos, named after Angelina Stickney Hall, the wife of the moon’s discoverer. It is one of the largest craters relative to body size in the solar system. The impact that created it came close to shattering Phobos entirely. Grooves radiating across much of the moon’s surface may be fractures caused by that same collision.

Can you see Phobos and Deimos from Mars?

Yes, but neither looks like Earth’s Moon from our surface. Phobos appears about one-third the angular diameter of our full Moon and moves visibly across the Martian sky in just a few hours. Deimos looks like a bright star with a barely discernible disc. NASA’s Curiosity and Perseverance rovers have photographed both moons, including partial solar eclipses when Phobos transits the sun.

Does Mars have a ring?

Not currently. Mars has no ring system today. However, when Phobos crosses the Roche limit in tens of millions of years and breaks apart under tidal stress, the resulting debris is expected to form a thin ring around Mars. This would make Mars one of the few inner solar system planets to have a ring system, at least temporarily on geological timescales.