Does Mars have moons?

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The Twin Attendants of the War God

For millennia, Mars has captivated the human imagination. A ruddy point of light in the night sky, it has been a symbol of conflict, a canvas for tales of alien civilizations, and a tantalizing destination for exploration. Yet for most of human history, this world, so similar to our own in its seasons and daily rhythms, appeared to travel through the cosmos alone. Unlike Earth with its constant, luminous companion, Mars seemed solitary. It wasn’t until the late 19th century that astronomers finally pierced the planet’s glare and discovered that the God of War does not travel unattended. He is flanked by two tiny, misshapen companions: Phobos and Deimos.

These are not moons in the conventional sense. They bear no resemblance to our own Moon, a majestic sphere that dominates the night. Instead, they are small, dark, lumpy bodies, more akin to asteroids than to the grand satellites of the gas giants. Phobos, the inner and larger of the two, is a potato-shaped rock just 27 kilometers across at its widest point. It hugs Mars so tightly that it completes an orbit in less than eight hours, racing across the Martian sky faster than the planet itself rotates. Deimos, its smaller sibling, is a mere 15 kilometers across and orbits much farther out, a faint, star-like point of light that takes over 30 hours to complete its journey.

Their names, drawn from Greek mythology, are fittingly ominous. Phobos and Deimos were the twin sons of Ares, the Greek god of war, and Aphrodite, the goddess of love. They were his attendants, driving his chariot into battle and personifying the terror and panic that accompany conflict. The names, meaning “Fear” and “Panic” respectively, perfectly capture the unsettling nature of these strange little worlds. They are dark, scarred, and locked in a complex gravitational dance with their parent planet that will ultimately seal their fates.

The story of the Martian moons is one of enduring mystery. Their discovery was a triumph of persistence, following centuries of speculation that was, by a strange twist of fate, remarkably accurate. Their physical nature presents a study in contrasts, with one moon battered and grooved, the other strangely smooth. Their ultimate destinies are a dramatic display of celestial mechanics in action: Phobos is spiraling inward to its eventual destruction, while Deimos is slowly drifting away, destined one day to escape Mars’s grasp entirely.

Above all, their existence poses a fundamental question that strikes at the heart of planetary science: where did they come from? Are they captured asteroids, wanderers from the main belt between Mars and Jupiter that were snared by the Red Planet’s gravity long ago? Or are they the children of Mars itself, born from the debris of a cataclysmic, planet-shattering impact in the solar system’s violent youth? The evidence is tantalizingly contradictory. Their composition screams “asteroid,” but their neat, orderly orbits suggest they were born right where they are.

Solving this puzzle is more than an academic exercise. These two small moons are not just cosmic curiosities; they are time capsules. They hold clues to the formation of Mars, the delivery of water and organic materials to the inner planets, and the chaotic processes that shaped our entire solar system. They have become primary targets for robotic exploration and are now being seriously considered as potential staging posts for the future human exploration of Mars. Phobos and Deimos, the tiny attendants of the war god, may yet become the stepping stones for humanity’s next giant leap.

Whispers in the Dark: From Literary Prophecy to Scientific Discovery

The journey to uncover the existence of Phobos and Deimos did not begin in an observatory. It started, improbably, in the pages of satirical fiction, more than 150 years before the moons were actually seen. This strange prelude to discovery is a fascinating tale of logical deduction, scientific error, and artistic license, a testament to how ideas can ripple through culture long before they can be verified by observation.

A Prescient Guess: The Moons in Fiction

In 1726, the Irish writer and cleric Jonathan Swift published his masterpiece, Travels into Several Remote Nations of the World, in Four Parts. By Lemuel Gulliver, First a Surgeon, and then a Captain of several Ships, better known today as Gulliver’s Travels. In the third part of the book, Gulliver visits the flying island of Laputa, a kingdom of absent-minded scientists and astronomers. It is here that Swift makes his astonishingly accurate “prediction.” He writes that the Laputan astronomers had:

“…discovered two lesser stars, or satellites, which revolve about Mars, whereof the innermost is distant from the center of the primary planet exactly three of his diameters, and the outermost five: the former revolves in the space of ten hours, and the latter in twenty-one and a half.”

When American astronomer Asaph Hall finally discovered the moons in 1877, their orbits proved to be uncannily similar to Swift’s description. Phobos, the inner moon, orbits at a distance of about 1.4 Martian diameters from the planet’s center and has a period of 7.7 hours; Swift’s figures were 3 diameters and 10 hours. Deimos, the outer moon, orbits at 3.5 Martian diameters with a period of 30.3 hours; Swift gave 5 diameters and 21.5 hours. While not exact, the figures were remarkably close for a work of fiction written a century and a half before the discovery. A few decades after Swift, the French philosopher Voltaire made a similar reference in his 1752 short story “Micromégas,” in which a giant visitor from a planet orbiting the star Sirius mentions that Mars has two moons.

This has led to speculation about how Swift could have “known” about the moons. The reality is less mystical but no less interesting. Swift’s guess was not a random shot in the dark but an educated one, likely based on two lines of reasoning popular at the time. The first was a simple, if flawed, sense of numerical harmony. In the early 18th century, it was known that the inner planets, Mercury and Venus, had no moons. Earth had one, and Jupiter, thanks to Galileo’s discoveries, was known to have four. A logical progression suggested that Mars, situated between Earth and Jupiter, ought to have two.

The second, more scientific line of reasoning stems from a famous error made by the great German astronomer Johannes Kepler in 1610. When Galileo discovered what we now know to be the rings of Saturn, he wanted to secure his claim without revealing the details immediately. He did so by sending an anagram to his colleagues, including Kepler. The scrambled Latin message was: s m a i s m r m i l m e p o e t a l e u m i b u n e n u g t t a u i r a s. The correct solution was Altissimum planetam tergeminum observavi, meaning “I have observed the most distant planet to have a triple form.” Kepler unable to solve the puzzle, rearranged the letters into a different Latin sentence: Salue umbistineum geminatum Martia proles, which translates to “Hail, twin companionship, children of Mars.” He assumed Galileo had discovered two Martian moons. Although the truth about Saturn’s rings became known within a few decades, Kepler’s mistranslation endured as a persistent piece of astronomical lore.

Swift, an educated man with a keen interest in science, likely combined these two ideas. He took the plausible notion that Mars had two moons and, applying his understanding of Kepler’s laws of planetary motion, deduced what their orbits must be like. For the moons to have remained undiscovered for so long, they would have to be very small and orbit very close to the planet, resulting in short orbital periods. It was a brilliant piece of logical extrapolation based on a faulty premise, a scientific error that, through art, became a prophecy. In a fitting tribute to these literary forerunners, the two named craters on Deimos are now called Swift and Voltaire.

The Hunt for Martian Moons

The stage for the actual discovery was set in the summer of 1877. Mars was approaching one of its closest oppositions to Earth, an alignment that occurs roughly every two years when Earth passes between Mars and the Sun. This proximity makes the Red Planet appear larger and brighter in the sky, providing a prime opportunity for astronomers to study it in detail.

At the United States Naval Observatory in Washington, D.C., astronomer Asaph Hall III decided to undertake a systematic search for Martian satellites. Hall was a determined and meticulous observer, working with the observatory’s premier instrument: the 26-inch “Great Equatorial Refractor.” Commissioned in 1873, it was the largest refracting telescope in the world at the time, a technological marvel that gave Hall an edge over his contemporaries.

Even with this powerful tool, the task was immensely difficult. Any potential moons were expected to be incredibly faint and orbit very close to Mars. This meant Hall had to search for tiny specks of light buried within the overwhelming glare of the planet itself. It was akin to trying to spot a firefly next to a searchlight. To manage this, Hall employed a technique where he would position the telescope so that the bright disk of Mars was just outside the field of view, allowing his eyes to adapt to the darkness and scan the surrounding space for any faint objects moving along with the planet.

The Discovery by Asaph Hall

The search began in early August 1877 and was immediately met with frustration. Night after night, Hall scanned the area around Mars, finding nothing. The weather in Washington, D.C., was hot, humid, and frequently hazy, further hampering his efforts. On August 10, he caught a fleeting glimpse of a faint object, but bad weather rolled in before he could confirm it was a moon. The following night, he found nothing. By this point, Hall was disheartened and ready to abandon the search altogether.

It was at this critical juncture that his wife, Angeline Stickney Hall, intervened. Angeline was a remarkable woman in her own right, a gifted mathematician and former college professor who had tutored Asaph in mathematics. She had long supported his career, even assisting with his calculations in the early years. Seeing his frustration, she encouraged him to persevere for at least one more night. Hall later credited her directly for the discovery, writing, “The chance of finding a satellite appeared to be very slight, so that I might have abandoned the search had it not been for the encouragement of my wife.”

Heeding her advice, Hall returned to the telescope. On the night of August 11 (often recorded as August 12 in Universal Time), his persistence paid off. He spotted a faint object moving with Mars. He recorded his observation, noting, “I found a faint object on the following side and a little north of the planet. I had barely time to secure an observation of its position when fog from the River stopped the work.” Over the next few nights, despite more poor weather, he was able to track the object and confirm its motion. On August 16, he was certain: he had found a moon. This was the outer moon, Deimos.

The story wasn’t over. As he continued to observe his newfound moon on the night of August 17, another object appeared. This one was even closer to the planet and moving much faster. “On 17 August while waiting and watching for the outer moon, the inner one was discovered,” Hall wrote. He confirmed this second, inner moon on the following night. This was Phobos. With the character of both objects confirmed beyond doubt, the director of the observatory, Admiral John Rodgers, made a public announcement of the discovery on August 18, 1877. The news electrified the astronomical world, and Hall was celebrated for his achievement, receiving medals from scientific societies in France and England. The long-suspected attendants of Mars had finally been revealed.

Naming Fear and Panic

With the moons discovered, they needed names. Hall received many suggestions, but he ultimately chose the names proposed in a letter from Henry Madan, a science master at Eton College in England. Madan, a classical scholar, drew his inspiration from Homer’s Iliad. In Book XV, the god of war, Ares, prepares for battle by summoning his two sons, who also serve as his chariot attendants: Fear (Phobos) and Flight (Deimos).

The names were a perfect mythological fit. Mars, the Roman god of war, was the direct counterpart to the Greek Ares. It was only natural that his celestial companions should bear the names of his mythological sons. The names also capture the essence of war’s psychological toll. In Greek mythology, Phobos and Deimos were not just attendants but personified spirits, or daimones. Deimos represented the dread and terror one feels when facing a formidable foe – an external, objective fear. Phobos, from which we derive the word “phobia,” represented the internal, irrational panic that leads to rout and flight from the battlefield. As sons of Ares and Aphrodite (the goddess of love), they could also be interpreted as representing the fear of loss. The naming of the moons thus wove a thread of ancient mythology into the fabric of modern astronomy, forever linking the Red Planet to its terrifying attendants, Fear and Panic.

A Tale of Two Worlds: The Physical Nature of Phobos and Deimos

The first telescopic glimpses of Phobos and Deimos revealed them as little more than faint points of light. It would take the advent of the Space Age and a fleet of robotic explorers to unveil their true nature. What these missions found were not miniature versions of Earth’s Moon, but two distinct and peculiar worlds, each with its own unique character shaped by its size, composition, and relationship with Mars. They are a study in contrasts, from their battered surfaces to their divergent orbital paths.

Phobos: The Inner Guardian

Phobos is the larger and innermost of Mars’s two moons. It is a small, non-spherical body, often described as “potato-shaped,” with dimensions of roughly 27 by 22 by 18 kilometers. Its surface is one of the darkest in the solar system, reflecting only about 6% of the sunlight that strikes it, making it about as dark as a lump of coal. This low reflectivity, or albedo, combined with spectral data, suggests that Phobos is composed of material similar to carbonaceous chondrite meteorites – primitive, carbon-rich rocks that are among the oldest materials in the solar system.

One of the most striking properties of Phobos is its remarkably low density, measured at about 1.9 grams per cubic centimeter. This is significantly less dense than the rock of the Martian crust (around 3 g/cm³) and suggests that Phobos is not a solid, monolithic object. Instead, it is thought to be a “rubble pile” – a collection of rocks, gravel, and dust held together loosely by gravity. Models suggest that as much as 25% to 45% of its interior could be empty space, a porous network of voids and caverns between the constituent blocks. This fragile, rubble-pile structure has significant implications for its history and its ultimate fate.

The orbit of Phobos is as extreme as its physical nature. It circles Mars at a mean distance of only 9,378 km from the planet’s center, or less than 6,000 km above the Martian surface. No other known moon in the solar system orbits closer to its parent planet. This proximity results in a blistering orbital period of just 7 hours and 39 minutes.

This has a bizarre consequence for an observer on the surface of Mars. The planet itself rotates once every 24 hours and 37 minutes, a period known as a sol. Because Phobos orbits Mars more than three times faster than Mars spins, it exhibits a strange apparent motion in the sky. It rises in the west, speeds across the sky in just over four hours, and sets in the east. It can do this twice in a single Martian sol.

Like Earth’s Moon, Phobos is tidally locked with its parent planet. This means its rotational period is equal to its orbital period, so it always keeps the same face pointed toward Mars. An astronaut standing on the Mars-facing side of Phobos would see the Red Planet as a colossal, stationary disk dominating the sky, covering a full 42 degrees of view – over 80 times larger than the full Moon appears from Earth.

Deimos: The Outer Sentinel

Deimos is Phobos’s smaller, more distant sibling. It is also an irregular, potato-shaped object, measuring about 15 by 12 by 11 kilometers. Its composition appears to be similar to that of Phobos, with a very dark, carbonaceous surface, and its density is even lower, at around 1.5 grams per cubic centimeter, strongly supporting the idea that it too is a porous rubble pile.

Despite these similarities, Deimos presents a strikingly different appearance. While Phobos is heavily cratered and crisscrossed with grooves, Deimos is remarkably smooth. Its craters are subdued and appear partially filled in, as if a thick blanket has been draped over the landscape. This blanket is a deep layer of regolith – the unconsolidated dust, soil, and broken rock that covers the surface of airless bodies. The regolith on Deimos may be as much as 100 meters deep in places, effectively burying older craters and giving the moon its soft, rounded appearance.

The existence of this thick regolith is a scientific puzzle. Deimos has an incredibly weak gravitational field. Its escape velocity is a mere 5.6 meters per second (about 20 km/h). This is so low that a human could literally jump into orbit. Given this feeble gravity, it’s difficult to understand how the moon managed to hold on to the vast quantities of debris ejected by impacts over billions of years. The leading explanation is that while the ejecta easily escapes the moon’s own gravity, it doesn’t escape the gravity of Mars. The material enters orbit around the planet, forming a temporary dust torus along Deimos’s path. Over thousands of years, this material is slowly re-accreted by the moon, settling back down to create the thick, dusty layer we see today.

The orbit of Deimos is far more sedate than that of Phobos. It circles Mars at a mean distance of 23,460 km, over twice as far out as its sibling. Its orbital period is 30.3 hours, which is slightly longer than a Martian sol. This leads to another strange spectacle from the Martian surface. Deimos rises, as expected, in the east, but it moves across the sky with painstaking slowness. Because its orbital period is so close to the planet’s rotational period, it hangs in the sky for about 2.7 sols before finally setting in the west. To a Martian observer, Deimos would not look like a moon at all. With an angular diameter of only 2.5 arcminutes, it would appear as a very bright, star-like point of light, about as brilliant as Venus appears from Earth.

A Comparative Look

The stark differences and subtle similarities between Phobos and Deimos are best appreciated through a direct comparison. While both are small, dark, and irregularly shaped, their orbital dynamics and surface geology tell two very different stories. One is a world locked in a tight, fast embrace with its planet, its surface scarred by a violent history and showing the stresses of its impending doom. The other is a more distant, placid companion, its ancient wounds softened by a deep blanket of dust, slowly drifting away into the void. These contrasting characteristics are the primary clues scientists use to piece together their mysterious origins and predict their divergent futures.

Scarred and Grooved Landscapes: The Geology of the Martian Moons

The surfaces of Phobos and Deimos are alien landscapes, shaped over billions of years by relentless bombardment and the subtle but persistent pull of Martian gravity. Close-up images from robotic spacecraft have transformed them from simple points of light into complex geological worlds. Yet for all their similarities in composition, their surfaces could not be more different. Phobos is a testament to violence, a body scarred by impacts and etched with mysterious lines. Deimos is a world of eerie smoothness, its history buried under a thick mantle of dust.

The Battered Face of Phobos

To look at Phobos is to see a history of violence. Its surface is ancient and saturated with impact craters of all sizes, from vast basins to tiny pits. There are an estimated 1,300 craters wider than 200 meters across its small surface. This intense cratering implies that the upper crust of Phobos is heavily fractured and pulverized, a shattered remnant of eons of collisions.

The most dominant feature on this battered landscape is Stickney crater. This enormous impact basin, located on the moon’s leading hemisphere, is about 9 kilometers in diameter. On a body as small as Phobos, this is a colossal feature, spanning nearly half the moon’s width. The impact that created Stickney was so powerful that it came perilously close to shattering Phobos completely. The crater is named in honor of Angeline Stickney Hall, Asaph Hall’s wife, whose encouragement was so instrumental in the moon’s discovery. High-resolution images of Stickney reveal a complex interior. Its walls are marked by streaks and lineations, evidence of landslides where loose material has slumped down into the crater’s bowl. Along its rim, massive boulders, some as large as a city block, are perched precariously – debris thrown out by the cataclysmic impact. Images have also revealed a distinct color difference, with material in and around Stickney appearing “bluer” (spectrally less red) than the rest of the moon’s surface. This is thought to be fresher, less space-weathered material excavated from the moon’s interior by the impact.

Beyond Stickney, the most enigmatic features on Phobos are the hundreds of long, linear depressions that crisscross its surface. These features, known simply as grooves, are typically 100 to 200 meters wide, up to 20 meters deep, and can run for kilometers. They are organized into intersecting families of parallel lines, giving the moon a strangely striated appearance. The origin of these grooves has been the subject of intense scientific debate for decades, a geological detective story with several compelling suspects.

The first and most obvious theory was that the grooves are fractures radiating from the Stickney impact. It seemed logical that an event powerful enough to create such a huge crater would also have sent deep fissures through the moon’s fragile, rubble-pile body. later mapping showed that the grooves don’t radiate perfectly from the center of Stickney but from a different focal point nearby, casting doubt on this simple explanation.

A second major hypothesis suggests the grooves are “stretch marks” caused by the tidal forces of Mars. Phobos orbits so close to Mars that the planet’s gravitational pull is not uniform across the moon; it tugs more strongly on the near side than the far side. This tidal stress is slowly deforming Phobos and pulling it apart. According to this model, the moon’s interior is a weak rubble pile, but it’s encased in a more cohesive outer layer of regolith. The tidal forces stretch this outer layer, causing it to fail and form the parallel stress fractures we see as grooves. This theory is particularly compelling because it connects the moon’s geology to its ongoing process of orbital decay and destruction. The grooves, in this view, are the first signs of the end.

A third set of theories proposes that the grooves are not fractures at all, but surface features carved by external processes. One model suggests that the grooves are chains of secondary impact craters, formed by streams of debris ejected from impacts on Mars itself. Another, more recent idea, uses computer simulations to show that boulders ejected by the Stickney impact could have rolled and bounced across the surface, carving out the long, linear tracks. This model cleverly explains many of the grooves’ oddities, such as why some are not perfectly aligned with the crater and why some areas of the moon are mysteriously groove-free – the boulders would have been launched over these “bald spots” like a ski jumper.

The surface of Phobos is a fascinating geological record. Its features tell a story of both a singular, catastrophic event in its distant past and a slow, inexorable process of destruction that defines its future. The debate over the grooves is a debate about which of these powerful forces – the ancient impact or the ongoing tidal stress – is the primary author of the moon’s most distinctive features. Phobos is a body literally scarred by its past and being torn apart by its future, and the evidence for both is written on its face.

The Smooth Mantle of Deimos

Traveling outward from Mars, one leaves the violent, grooved landscape of Phobos for the strange tranquility of Deimos. The two moons could not look more different. Where Phobos is sharp, angular, and deeply scarred, Deimos is soft, rounded, and eerily smooth. It is still a cratered body, but its features are muted and subdued, as if viewed through a soft-focus lens.

This smooth appearance is due to a remarkably thick blanket of regolith that covers the entire moon. This layer of fine dust and rock, perhaps as deep as 100 meters, has filled in most of the older, larger craters, leaving only the freshest, most recent impacts clearly visible. This gives Deimos a much more gentle topography than its inner sibling.

The presence of this deep regolith is a significant puzzle. Deimos’s gravity is so weak that debris from an impact should easily escape into space. The solution to this puzzle appears to lie in the gravitational influence of Mars. When an impact occurs on Deimos, the ejected dust and rock flies off the surface, but it doesn’t have enough velocity to escape the Martian system. It enters orbit around Mars, forming a diffuse ring or torus of material that follows Deimos’s path. Over long timescales, the moon sweeps through this debris cloud, and the material gently settles back onto its surface. This process, repeated over billions of years, has allowed Deimos to build up its thick, smooth mantle of dust.

The surface of Deimos is not entirely featureless. It has two officially named craters: the 3-kilometer-wide Swift crater and the slightly smaller Voltaire crater. These names, of course, honor the two 18th-century writers who so presciently wrote of Mars’s two moons long before their discovery. High-resolution images have also revealed subtle variations in color and brightness across the surface. Smoother areas tend to be redder, while fresher craters and ridges appear slightly less red. These differences are likely due to space weathering, a process where exposure to solar wind and micrometeorites gradually darkens and reddens surface materials. The brighter, less-red areas are places where recent impacts or downslope movement of regolith have exposed fresher material from beneath the surface. Unlike Phobos, Deimos has no grooves. Its placid, dust-covered surface stands in stark contrast to the tortured geology of its sibling, a difference that remains one ofthe key mysteries of the Martian system.

The Great Debate: Where Did Phobos and Deimos Come From?

The most fundamental and fiercely debated question about the moons of Mars is their origin. For decades, planetary scientists have grappled with two competing hypotheses, each supported by a compelling line of evidence, yet each plagued by a significant flaw. Are Phobos and Deimos captured asteroids, cosmic wanderers from the outer solar system that were gravitationally snared by Mars? Or are they the native children of Mars, formed from the debris of a colossal impact that rocked the planet in its infancy? The answer to this question has implications that reach far beyond the Martian system, touching on the very processes that built the planets and distributed the building blocks of life throughout the solar system.

The Captured Asteroid Hypothesis

At first glance, the case for Phobos and Deimos being captured asteroids seems open-and-shut. Their physical characteristics are a near-perfect match for a class of asteroids known as C-types and D-types, which are common in the outer regions of the main asteroid belt and beyond.

The evidence for this hypothesis is primarily compositional. First, the moons are very dark, with low albedos that match these primitive asteroids. Second, their spectra – the way their surfaces reflect different wavelengths of light – are largely featureless and reddish, a characteristic signature of the carbon-rich, organic-laden materials that make up C- and D-type asteroids. Third, their very low densities are consistent with the porous, “rubble pile” structure expected of many asteroids. They simply look like asteroids. The idea that Mars, with its proximity to the asteroid belt, could have gravitationally captured one or two of these bodies seems entirely plausible.

this compelling compositional evidence runs headlong into a major, and perhaps fatal, problem: their orbits. The laws of celestial mechanics make capturing an asteroid into a stable orbit an exceptionally difficult feat. An asteroid approaching Mars from a solar orbit would be moving too fast to be captured. To enter orbit, it would need to lose a significant amount of energy at just the right moment. For the gas giants, this can happen through complex gravitational interactions with their large systems of existing moons. Mars has no such system.

The other primary mechanism for slowing down an object is aerobraking, using the drag of a planet’s atmosphere. But Mars’s present-day atmosphere is far too thin to capture a body the size of Phobos. Even if Mars had a much thicker atmosphere in the distant past, the process of aerobraking would likely have been too violent for a fragile rubble-pile body to survive intact. Furthermore, the capture process would almost certainly result in a highly eccentric (elongated) and inclined orbit. Phobos and Deimos occupy orbits that are almost perfectly circular and lie almost exactly in Mars’s equatorial plane. There is no widely accepted mechanism that can explain how captured asteroids could have settled into such neat, orderly orbits. The orbits suggest they were born there, while their composition suggests they were born somewhere else entirely. This is the central paradox of the Martian moons.

The Giant Impact Hypothesis

The second major theory proposes a much more violent and intimate origin story. In this scenario, Phobos and Deimos are not foreign bodies but are native to the Martian system, born from the aftermath of a giant impact. This model is analogous to the leading theory for the formation of Earth’s own Moon, which posits that a Mars-sized protoplanet collided with the early Earth, throwing a vast cloud of debris into orbit that eventually coalesced to form the Moon.

The primary strength of the giant impact hypothesis is that it elegantly explains the moons’ problematic orbits. The debris from a massive collision would naturally form a flattened disk of material orbiting around the planet’s equator. Moons that subsequently accreted from this disk would naturally form in the circular, equatorial orbits we observe today. This model neatly sidesteps the intractable orbital mechanics of the capture scenario. There is even circumstantial evidence for such an impact on Mars itself. The planet exhibits a stark “crustal dichotomy,” with the northern hemisphere being significantly lower in elevation and having a thinner crust than the southern hemisphere. This vast northern lowland, sometimes called the Borealis Basin, may be the scar from just such a planet-altering impact.

But just as the capture hypothesis struggles with orbits, the giant impact hypothesis struggles with composition. If the moons formed from a mixture of material from Mars and the impactor, they should bear a strong chemical resemblance to the Martian crust. They do not. As noted, their composition is a much better match for primitive asteroids. Why would debris from a rocky planet like Mars coalesce into bodies that look like carbon-rich asteroids?

Several explanations have been proposed. Perhaps the impactor itself was a primitive, carbonaceous body, and the moons formed primarily from its debris rather than from Martian material. Another possibility relates to the physics of the post-impact debris disk. Sophisticated computer models suggest that the inner part of this disk would have been a hot, molten ring, while the outer part would have been a cooler, gaseous region. It’s possible that the material in this outer disk condensed directly from a vapor phase into the kind of fine-grained, dark, primitive dust that makes up Phobos and Deimos, while any larger moons that formed in the hot inner disk quickly spiraled back into the planet and were destroyed.

Alternative and Hybrid Models

The stark conflict between the orbital and compositional evidence has led scientists to develop more nuanced, hybrid models that attempt to reconcile these two datasets. These theories seek to combine the best features of both the capture and impact scenarios.

One of the most promising of these is the “disrupted asteroid” model. In this scenario, a single, larger body – either a captured asteroid or a small planetesimal from the early solar system – did not collide directly with Mars but passed so close that it crossed the planet’s Roche limit. This is the distance within which a planet’s tidal forces are strong enough to tear a smaller body apart. The asteroid would have been shredded by Mars’s gravity, creating a debris ring around the planet. Phobos and Deimos would then have formed from this ring material. This clever model accounts for both the asteroid-like composition (the material came from the disrupted body) and the circular, equatorial orbits (they formed in-situ from a disk).

Another intriguing idea is the “moonlet cycle” theory. This model suggests that Mars may have had rings in the past and that Phobos is merely the latest in a long line of recycled moons. In this scenario, a moon forms from the outer edge of a debris ring. Tidal forces cause it to spiral inward toward Mars. Once it reaches the Roche limit, it is torn apart, replenishing the ring system. A new, smaller moon then begins to form from the new ring’s outer edge, and the cycle repeats. This theory suggests that Phobos is a relatively young object, perhaps only a few hundred million years old, and that Deimos, orbiting farther out, is the sole survivor of an earlier generation of Martian moons.

Ultimately, the origin of Phobos and Deimos is more than just a local Martian mystery. It serves as a important test case for our broader understanding of how planetary systems form and evolve. The Earth-Moon system provides our primary model for how a giant impact can create a single, large satellite. The Martian system, with its two tiny moons, offers a contrasting outcome. Resolving why a similar process – if it occurred – produced such different results will refine our models of planet formation. If the moons are indeed captured asteroids, they represent pristine samples of material from the outer solar system, delivered to our doorstep. Studying them can tell us about the composition and dynamics of the early solar system. The stubborn contradiction between their composition and their orbits forces scientists to develop more sophisticated models of planetary evolution that can be applied not just to Mars, but to moons around other planets and even to distant exoplanetary systems. The final answer, which may only come from analyzing a returned sample, will have ripple effects across the entire field of planetary science.

A View from the Red Planet: The Martian Sky

For a hypothetical observer standing on the dusty, ochre plains of Mars, the sky would offer a celestial display utterly alien to our own. The familiar comfort of a single, large moon tracing a predictable monthly path would be replaced by the strange and hurried motions of two small, swift attendants. The experience of watching Phobos and Deimos would be one of contrasts: one a frantic dash, the other a stately procession.

Phobos would be the more dramatic performer. Because it orbits Mars faster than the planet rotates, it appears to move backward in the sky. It would rise not in the east, but in the west, appearing as a small, dark, and distinctly non-circular disk. At its largest, it would appear about one-third the angular diameter of our own full Moon. It would not linger. It would race across the Martian sky in just four hours and fifteen minutes before setting in the east. On many days, an observer could watch this spectacle twice, as Phobos completes more than two full circuits for every Martian sol. Its surface features, particularly the gash of Stickney crater, might even be visible to the naked eye as it sped overhead.

Deimos, by contrast, would be a model of patience. It rises, as expected, in the east, but its 30.3-hour orbit is so close to Mars’s 24.6-hour rotational period that it appears almost stationary. It would look not like a moon, but like a bright star, shining with the brilliance of Venus as seen from Earth. It would take a full 2.7 days for Deimos to slowly trace its path across the sky before finally setting in the west. For days at a time, it would be a constant, slow-moving companion in the Martian night.

Neither moon is large enough to cause a total solar eclipse as our Moon does. Instead, they produce what are called transits, where their small, dark silhouettes pass across the face of the Sun. Mars rovers like Curiosity and Perseverance have captured stunning images of these events. A transit of Deimos is an unremarkable event, with the tiny moon appearing as little more than a small, slow-moving sunspot. A transit of Phobos is a more dramatic affair. Its larger, irregular shape takes a noticeable bite out of the Sun as it rapidly scuds across the solar disk. These transits, observed from the surface, have allowed scientists to refine their knowledge of the moons’ orbits with incredible precision.

A History of Robotic Encounters

Our modern understanding of Phobos and Deimos has been built incrementally, through a series of robotic missions that have visited the Martian system over the past half-century. Each spacecraft, from the earliest orbiters to the sophisticated platforms of today, has peeled back another layer of mystery, transforming these moons from faint astronomical points into complex geological worlds.

First Glimpses: Mariner & Viking

The first spacecraft to provide a close-up view of the Martian moons was NASA’s Mariner 9. Arriving at Mars in 1971, it became the first artificial satellite to orbit another planet. After waiting for a planet-encircling dust storm to subside, Mariner 9 turned its cameras on Phobos and Deimos. The images it returned were revolutionary. For the first time, their irregular, non-spherical shapes were clearly visible. They were revealed to be dark, heavily cratered bodies, confirming that they were ancient objects. The data also allowed for the first accurate measurements of their size and confirmed that, as expected, they were tidally locked, always keeping one face towards Mars.

The next great leap in understanding came with NASA’s twin Viking orbiters in 1976. The Vikings provided images with much higher resolution, allowing for the creation of the first detailed surface maps. The Viking 1 orbiter captured the iconic image of Phobos that revealed its most dominant feature: the massive Stickney crater. This single image showed that Phobos was a world that had survived a near-catastrophic impact. The Viking images also revealed the second great mystery of Phobos’s surface: the strange, linear grooves that crisscross the landscape. For Deimos, the Viking observations were equally revealing. They showed a world with a dramatically different appearance – a smooth, muted surface where craters appeared to be filled in by a thick layer of dust, or regolith. The Viking missions established the fundamental geological dichotomy between the two moons that continues to drive scientific inquiry today.

The Phobos Program and Mars Global Surveyor

In the late 1980s, the Soviet Union undertook the ambitious Phobos program, sending two spacecraft, Phobos 1 and Phobos 2, to Mars. The program was unfortunately plagued by misfortune. Contact was lost with Phobos 1 while it was still en route to Mars due to a ground control error. Phobos 2 successfully entered Mars orbit in January 1989 and began its observations. It studied Mars and returned 37 images of Phobos, providing new data on the moon’s surface and thermal properties. just as it was maneuvering to make a close approach and deploy two landers onto the moon’s surface, contact with Phobos 2 was also lost, likely due to an onboard computer failure.

It wasn’t until the arrival of NASA’s Mars Global Surveyor (MGS) in 1997 that the moons were studied in detail again. MGS made several close passes of Phobos, using its Mars Orbiter Camera (MOC) to capture some of the highest-resolution images to date. These images were sharp enough to resolve individual boulders on the rim of Stickney crater and to show clear evidence of landslides on the crater walls. The Thermal Emission Spectrometer (TES) on MGS measured the surface temperature of Phobos, finding extreme variations between the sunlit side (as warm as -4°C) and the shadowed side (as cold as -112°C). This rapid heat loss confirmed that the moon’s surface is covered in a layer of fine, powdery dust that cannot retain heat, much like Earth’s Moon.

The Modern Era: Mars Express & Mars Reconnaissance Orbiter

The 21st century has brought an even more detailed understanding of the moons, thanks to two long-lived and powerful orbiters. The European Space Agency’s Mars Express spacecraft, which arrived at Mars in 2003, has been particularly valuable. Its highly elliptical orbit allows for regular close flybys of Phobos, sometimes passing within 100 kilometers of its surface. These close encounters have allowed for the most precise measurements of Phobos’s mass and volume, which in turn confirmed its low density and provided the strongest evidence yet for its porous, “rubble pile” interior. The MARSIS radar instrument on Mars Express has even peered beneath the moon’s surface, searching for signals that might reveal its internal structure.

NASA’s Mars Reconnaissance Orbiter (MRO), in orbit since 2006, carries the most powerful camera ever sent to another planet, the High Resolution Imaging Science Experiment (HiRISE). While MRO’s lower orbit makes close flybys of the moons impossible, HiRISE has captured stunning long-range images of both Phobos and Deimos. These images have revealed subtle color variations across their surfaces, which provide clues about surface composition and the effects of space weathering. The incredible detail in HiRISE images has been important for mapping the groove systems on Phobos and for studying the fine texture of the regolith on Deimos, helping scientists to refine their theories about how these enigmatic features formed. Together, these modern missions have provided the rich dataset that fuels the ongoing debate about the moons’ origins and drives the planning for the next generation of exploration.

The Fates of Fear and Panic: A Look to the Future

The story of Phobos and Deimos is not confined to the past. They are dynamic worlds with active futures, locked in a gravitational tug-of-war with Mars that will ultimately determine their destinies. One is doomed to a fiery end, while the other is destined for a lonely exile. These dramatic fates, combined with their enduring scientific mysteries, have made them compelling targets for future exploration, both robotic and, eventually, human.

Phobos’s Doomed Spiral

Phobos is living on borrowed time. Its incredibly close and fast orbit places it inside what is known as the synchronous orbit radius – the distance at which a satellite’s orbital period matches the planet’s rotational period. Moons orbiting inside this radius, like Phobos, are dragged backward by the planet’s tidal forces. The gravitational bulge that Mars raises on Phobos pulls the moon forward in its orbit, but the bulge that Phobos raises on Mars lags slightly behind due to the planet’s slower rotation. This lagging bulge pulls back on Phobos, stealing its orbital energy.

The result is a slow but inexorable orbital decay. Every year, Phobos spirals about 2 centimeters closer to Mars. This may not sound like much, but over geological timescales, it adds up. In approximately 30 to 50 million years, this inward spiral will reach its catastrophic conclusion.

Two possible fates await Phobos. As it gets closer, the tidal stresses exerted by Mars will intensify. If Phobos is strong enough to hold together, it will eventually crash into the Martian surface, creating a massive impact crater. given its likely rubble-pile structure, a more dramatic end is probable. It will reach the Roche limit, the critical distance at which the planet’s tidal forces overcome the moon’s own gravitational cohesion. At this point, Mars will tear Phobos apart. The shattered remains of the moon will spread out into a narrow, dense ring of debris orbiting the Red Planet. Mars, for a time, will have a ring system to rival Saturn’s, a temporary monument to its destroyed moon. This ring would not be permanent; over a few million years, the debris would continue to spiral inward, raining down onto the Martian surface in a series of “moon showers.”

Deimos’s Slow Escape

Deimos, orbiting far outside the synchronous orbit radius, is experiencing the opposite effect. In its case, the tidal bulge raised on Mars rotates ahead of the moon, pulling it forward and giving it a small boost of orbital energy. This process, known as tidal acceleration, is the same mechanism that is causing Earth’s Moon to slowly recede from our planet.

The effect is very small, but over hundreds of millions of years, the orbit of Deimos is gradually getting larger. Its ultimate fate is a quiet one. It will continue to drift farther and farther away from Mars until, in the distant future, it will finally escape the planet’s gravitational pull altogether. Deimos will then become a solitary object, a tiny asteroid once more, embarking on its own independent orbit around the Sun.

The Next Chapter: The MMX Mission

The key to finally solving the great debate over the moons’ origin lies in analyzing a physical sample in a laboratory on Earth. That is the primary goal of the upcoming Martian Moons eXploration (MMX) mission, a bold new venture led by the Japan Aerospace Exploration Agency (JAXA) with significant international collaboration from NASA and European space agencies.

Scheduled to launch in 2026, the MMX spacecraft will travel to Mars and enter orbit around the planet in 2027. It will then spend a significant amount of time performing detailed remote sensing of both Phobos and Deimos, mapping their surfaces and analyzing their composition in unprecedented detail. The main event of the mission will be a daring landing on the surface of Phobos. The spacecraft will touch down, collect at least 10 grams of regolith, and then lift off again. After collecting its precious cargo, MMX will perform several close flybys of Deimos for comparative science before beginning its long journey home. In 2031, a small return capsule containing the sample of Phobos will re-enter Earth’s atmosphere and land, delivering the first-ever material from the Martian system to be returned to Earth.

The analysis of this sample is expected to provide a definitive answer to the origin question. If the sample’s composition and isotopic signatures match those of Martian meteorites, it will be strong evidence for the giant impact hypothesis. If it closely matches carbonaceous chondrite meteorites, it will favor a capture or disrupted asteroid origin. MMX will also deploy a small rover, contributed by the French and German space agencies, to explore the surface of Phobos, providing ground truth and context for the returned sample.

Stepping Stones to Mars: The Moons in Human Exploration

Beyond their scientific value, Phobos and Deimos are increasingly seen as key strategic assets for the future of human exploration. The immense challenge of landing humans on the surface of Mars and returning them safely to Earth has led many mission planners to consider using the moons as an intermediate step – a “base camp” in the Martian system.

This “moons first” approach has several compelling advantages. One of the biggest is the ability to conduct real-time teleoperation of robotic assets on the Martian surface. The communication delay between Earth and Mars can be up to 24 minutes each way, making direct control of a rover a slow and cumbersome process. An astronaut crew stationed on Phobos or Deimos could operate rovers, drones, and other robots on Mars with the immediacy of a video game, dramatically increasing the efficiency and scientific return of surface exploration.

The moons could also be a source of vital resources. If, as suspected, they contain water ice beneath their regolith, this could be mined and processed to produce breathable air, drinking water, and, critically, rocket propellant. This concept, known as in-situ resource utilization (ISRU), could dramatically reduce the mass and cost of a human Mars campaign, as the fuel for the return journey would not have to be launched all the way from Earth.

Finally, the moons offer a less demanding environment for the first human missions to the Martian system. Traveling to and from the moons requires significantly less energy than descending into and ascending out of Mars’s deep gravity well. The moons’ mass could also provide natural shielding from the harsh radiation environment of deep space. A mission to Phobos or Deimos would allow NASA and its partners to test out the transportation, habitation, and life support systems needed for a Mars mission in a deep-space environment, but without the added complexity and risk of a planetary landing. There are significant challenges, of course. The extremely low-gravity environment would make landing and surface operations difficult, and the presence of usable resources is still unconfirmed. Nevertheless, these tiny, battered worlds may hold the key not only to Mars’s past but also to humanity’s future in the solar system.

Summary

Orbiting the Red Planet are two of the most unusual satellites in the solar system, Phobos and Deimos. Discovered in 1877 by Asaph Hall after centuries of literary speculation, these tiny, misshapen moons are worlds of stark contrast and enduring mystery. Named for the mythological sons of the war god Mars – Fear and Panic – they are dark, heavily cratered bodies that appear more like captured asteroids than conventionally formed moons.

Phobos, the larger and inner moon, is locked in a tight, frantic orbit, racing around Mars three times a day. Its surface is a testament to a violent history, dominated by the giant impact crater Stickney and crisscrossed by enigmatic grooves that may be the first signs of its structural failure. Deimos, its smaller and more distant companion, presents a placid, smooth face, its ancient craters softened by a deep blanket of dust. Its slow, stately procession across the Martian sky makes it appear more like a bright star than a moon.

Their existence presents a fundamental scientific puzzle. Their asteroid-like composition suggests they are captured wanderers, yet their neat, circular, equatorial orbits defy the mechanics of capture and suggest they were born in orbit around Mars, likely from the debris of a giant impact. This central contradiction has spurred the development of new, hybrid theories and has made the moons a prime target for robotic exploration, with each successive mission providing more detailed clues to their nature.

Their futures are as divergent as their present appearances. Phobos is caught in a gravitational death spiral, destined to be torn apart by Mars’s tidal forces in the next 50 million years, its remains briefly forming a spectacular planetary ring. Deimos, in contrast, is slowly drifting away, fated to one day escape Mars’s gravity and wander the solar system on its own.

The next chapter in their story will be written by JAXA’s MMX mission, which will return the first-ever samples from Phobos to Earth, promising to finally solve the riddle of their origin. Looking further ahead, these small worlds may play a pivotal role in human exploration, serving as strategic outposts and resource depots for the first human missions to the surface of Mars. Phobos and Deimos, the small attendants of the war god, continue to fascinate, holding keys to understanding the violent birth of planets and perhaps serving as the stepping stones to humanity’s future on the Red Planet.

Last update on 2026-01-09 / Affiliate links / Images from Amazon Product Advertising API