Phobos: The Moon That Is Falling Apart

A potato-shaped moon you could jump off of

Forget the smooth silver disk of Earth's Moon. Phobos is a lumpy, cratered fragment roughly 27 × 22 × 18 km across — small enough that all of it would fit comfortably inside the Los Angeles metro area. Its mean radius is about 11.08 km, and its mass is a mere 1.06 × 10¹⁶ kg, about seven million times less than the Moon. It is not round because it is not massive enough for its own gravity to crush it into a sphere; below a few hundred kilometers in diameter, rock simply holds whatever irregular shape it was left in.

The surface gravity is almost comically weak — roughly 0.0057 m/s², about 1,700 times feebler than Earth's. The escape velocity is only about 11 m/s (~25 mph), which is roughly the speed of a gently tossed baseball. An astronaut standing on Phobos who jumped hard enough could, in principle, launch themselves off the moon entirely and into an independent orbit around Mars. Walking would be pointless; you would drift. A dropped hammer would take many seconds to fall a single meter.

Phobos is also dark. Its albedo — the fraction of sunlight it reflects — is only about 0.07, making it one of the darker bodies in the Solar System, comparable to fresh asphalt or coal. Its spectrum resembles that of D-type and carbonaceous (C-type) asteroids, the primitive, carbon-rich rubble found in the outer asteroid belt. That resemblance is at the heart of a long-running argument about where Phobos came from — a question we return to below.

The moon that rises in the west

Phobos orbits astonishingly close to Mars. Its semi-major axis is just 9,376 km from the planet's center, which means it skims only about 6,000 km above the Martian surface — closer to its planet than any other moon in the Solar System. From that altitude it completes a full orbit in 7 hours, 39 minutes (0.319 days). Mars itself takes 24.6 hours to spin once. Because Phobos races around faster than Mars turns beneath it, an observer on the Martian surface would watch it rise in the west, cross the sky in about four and a half hours, and set in the east — the opposite of every celestial body we are used to. It performs this backward journey roughly twice per Martian day.

Standing on Mars near the equator, Phobos would appear as a distinctly non-round object about a third the angular size of our full Moon, hurtling visibly across the sky. It also races through phases and slips into Mars' shadow every orbit, producing frequent eclipses. And because it hugs the planet so tightly, Phobos cannot be seen at all from Martian latitudes higher than about ±70°; it sits below the horizon, hidden by the curve of the planet.

Like our own Moon, Phobos is tidally locked: it keeps the same face turned toward Mars at all times, its rotation period exactly matching its orbital period. What sets it apart is not its locking but its doom — and to understand that, we have to look at the direction its orbit is slowly, inexorably changing.

Why Phobos is falling — the physics of the death spiral

The engine of Phobos' destruction is the same phenomenon that raises ocean tides on Earth: tidal interaction. Phobos' gravity pulls on Mars and raises a small tidal bulge in the solid planet. On Earth, our Moon orbits slower than Earth spins, so Earth's tidal bulge runs ahead of the Moon and tugs it forward, feeding it energy and pushing it outward (the Moon recedes about 3.8 cm per year).

Phobos is the mirror image of this. It orbits faster than Mars rotates, so the tidal bulge it raises lags behind it. That lagging bulge pulls Phobos backward, robbing it of orbital energy. Counterintuitively, a satellite that loses energy does not slow down — it drops to a lower, faster orbit. So Phobos spirals steadily inward. Measurements of its position over decades — refined by tracking from Viking, Mars Global Surveyor, and Mars Express — pin the decay at roughly 1.8 meters (about 2 m) closer to Mars every 100 years (an inward drift of order 10⁻⁹ m/s), and the rate accelerates as it descends.

Extrapolating that spiral forward, Phobos has only about 30 to 50 million years left — a blink in a Solar System that is 4.6 billion years old. But it will not survive intact all the way down. Phobos is already orbiting inside the Roche limit for a fluid or loosely-bound body — the distance within which a planet's tidal forces overwhelm the self-gravity holding a moon together. What keeps Phobos in one piece for now is its own material strength; it is a coherent rock pile, not a puddle. As it descends, the tidal stress climbs relentlessly, and the most likely ending is that the moon fractures and disperses into a debris ring around Mars before the remaining fragments rain down onto the surface over millions of years.

Grooves and Stickney — a moon showing its wounds

The most dramatic feature on Phobos is Stickney, an impact crater about 9 km across — nearly half the width of the entire moon. The collision that carved it must have come close to shattering Phobos completely; models suggest it flung debris across the whole surface and may have jolted the moon to its structural limits. The crater is named after Chloe Angeline Stickney Hall, the wife of Phobos' discoverer, Asaph Hall, who reportedly encouraged him not to abandon his search in 1877.

The other striking feature is a system of grooves — long, parallel troughs, typically 100 to 200 m wide and tens of meters deep, that stripe much of the surface in families of near-parallel lines. Their origin has been genuinely debated:

• Stickney ejecta: Early ideas held that the grooves were chains of secondary craters or fractures radiating from the Stickney impact.
• Rolling boulders: Some grooves appear to be trails plowed by boulders that bounced and rolled away from Stickney across the low-gravity surface, ending in the boulders themselves.
• Tidal stretching: A leading recent hypothesis proposes the grooves are stretch marks — surface failures produced as Mars' rising tidal stress slowly pulls the weakening moon apart. If true, the grooves are early symptoms of the very disintegration that will eventually destroy Phobos.

These explanations are not mutually exclusive, and which mechanism dominates remains an open research question. What is clear is that a low density of about 1.86 g/cm³ — well below that of solid rock — implies Phobos is porous, riddled with voids, quite possibly a loosely-bound rubble pile. A cracked, porous, internally weak body is exactly the kind of object that grooves easily and comes apart readily.

Where did Phobos come from? A captured asteroid or a Martian child?

Phobos' origin is one of the sharpest unresolved puzzles in planetary science, and it hinges on a contradiction between two lines of evidence.

• The 'captured asteroid' clue: Phobos is dark, spectrally resembles primitive carbonaceous and D-type asteroids, and has a low density consistent with such bodies. This strongly suggests it formed far out in the Solar System and was later gravitationally captured by Mars.
• The 'made at Mars' clue: But its orbit argues against capture. Phobos travels in a nearly circular path in Mars' equatorial plane (inclination only about 1.1°). Captured objects almost always arrive on tilted, elongated orbits, and it is dynamically very difficult to circularize and flatten a captured body's orbit so neatly. A near-circular equatorial orbit is instead the natural outcome of formation from a debris disk around Mars — the kind of disk a giant impact would have thrown up, echoing how Earth's Moon likely formed.

So the composition says 'asteroid' while the orbit says 'homegrown.' Proposed reconciliations include re-accretion of impact debris that happened to incorporate primitive asteroidal material, or capture followed by tidal reshaping of the orbit. There is currently no consensus. This is precisely why Phobos is such a high-priority target: a returned sample could settle the question directly by revealing whether its rock is Martian ejecta, primitive asteroid material, or a mixture.

From Asaph Hall to a sample-return mission

Phobos was discovered on 18 August 1877 by American astronomer Asaph Hall at the U.S. Naval Observatory in Washington, D.C., during a favorable close approach of Mars; he found the outer moon Deimos days earlier. He named them for the sons of Ares in Greek myth — Phobos (fear) and Deimos (dread) — the attendants of the god of war. For nearly a century they remained mere points of light.

The spacecraft era transformed them. Mariner 9 (1971) returned the first close images, revealing Phobos' irregular, cratered shape. The Viking orbiters (late 1970s) mapped it in detail and captured Stickney and the grooves. The Soviet Phobos 2 probe reached Mars in 1989 and returned data before contact was lost as it approached the moon. More recently, ESA's Mars Express has made repeated close flybys — some within tens of kilometers — refining the moon's mass, density, and the slow tightening of its orbit, while NASA's Mars Reconnaissance Orbiter has imaged Stickney and the grooves at high resolution.

The next leap is a sample return. Japan's space agency JAXA, with contributions from NASA, ESA-nation partners (Germany's DLR and France's CNES), leads the Martian Moons eXploration (MMX) mission, designed to orbit Phobos, land, collect surface material, and carry it back to Earth. If MMX succeeds, laboratory analysis of a genuine piece of Phobos could finally decide whether this doomed moon is a captured wanderer or a child of Mars — before, in geological time, it tears itself to pieces.

Phobos versus Deimos — the two very different moons of Mars

Property	Phobos (inner)	Deimos (outer)
Mean radius	11.1 km	6.2 km
Distance from Mars center	9,376 km	23,463 km
Orbital period	7 h 39 m	30 h 18 m
Orbit direction relative to Mars' spin	Faster than Mars rotates — rises in the west	Slower — rises in the east
Tidal fate	Spiraling inward, doomed in ~30–50 Myr	Slowly spiraling outward, escaping
Density	~1.86 g/cm³	~1.47 g/cm³