Phobos Is Doomed: Mars's Moon on a Collision Course With the Red Planet

Two Moons Unlike Any Other

Mars has two moons, Phobos and Deimos, discovered by American astronomer Asaph Hall in 1877. They are nothing like Earth's Moon — they are small, irregularly shaped, and heavily cratered, looking more like captured asteroids than geological siblings of their host planet. Phobos, the larger of the two, is approximately 27 kilometers by 22 kilometers by 18 kilometers — about the size of a city. Deimos is even smaller, roughly 15 by 12 by 11 kilometers. Neither is large enough for its own gravity to pull it into a spherical shape.

Whether Phobos and Deimos are captured asteroids or formed from debris blasted off Mars by an ancient impact is debated. Their appearance and composition are consistent with C-type asteroids from the outer asteroid belt, which would support the capture hypothesis. But captured asteroid moons typically end up in highly elliptical, inclined orbits, while both Phobos and Deimos orbit in nearly circular paths close to Mars's equatorial plane — more consistent with formation from a circumplanetary disc. The question of their origin remains open, and Japan's MMX (Martian Moon eXploration) mission, planned to collect a sample from Phobos and return it to Earth in the early 2030s, may help resolve it.

The Tidal Forces Destroying Phobos

Phobos orbits Mars at a distance of only about 9,376 kilometers from the planet's center — closer to its host planet than any other known moon in the solar system. At this distance, it orbits Mars in just 7 hours and 39 minutes, completing a full orbit faster than Mars rotates (a Martian day is about 24.6 hours). This means that from the surface of Mars, Phobos rises in the west and sets in the east, crossing the sky twice each night in the opposite direction from stars and the Sun.

The physics of its close orbit are working against Phobos. Because Phobos orbits faster than Mars rotates, the tidal bulge that Mars raises on Phobos is slightly ahead of the moon in its orbit. This configuration causes tidal friction to transfer angular momentum from Phobos's orbit to Mars's rotation — the exact opposite of what is happening with Earth's Moon, which is slowly moving away from Earth because the Moon orbits slower than Earth rotates. Phobos is therefore losing orbital energy and spiraling inward toward Mars at a rate of about 1.8 centimeters per year, or approximately 1.8 meters per century.

The Roche Limit and What Comes Next

In approximately 30 to 50 million years, Phobos will approach what is called the Roche limit — the distance at which a self-gravitating body is torn apart by the tidal forces of the planet it orbits. Inside the Roche limit, the difference in gravitational pull between the near and far sides of the orbiting body exceeds the body's self-gravity, and the material on the near side is pulled faster toward the planet than the body as a whole. The result is disintegration.

When Phobos crosses the Roche limit, it will be torn apart into millions of fragments. The most likely fate of this debris is the formation of a ring around Mars — perhaps comparable to Saturn's rings, though far less spectacular. The ring would persist for some period before the individual fragments gradually spiral inward and impact Mars's surface in a slow bombardment. Mars will briefly become a ringed planet before its former moon is entirely consumed.

Phobos's Present Surface

The present Phobos is already showing signs of tidal stress. Its surface is covered in a network of parallel grooves, many of them tens of meters deep and several kilometers long, that are thought to be fractures produced by tidal stretching. The most prominent feature on Phobos is Stickney Crater, a 9-kilometer-wide impact crater that is so large relative to the moon's 27-kilometer length that the impact that formed it nearly broke Phobos apart entirely. The crater's formation likely sent shockwaves through the entire body, contributing to the grooves observed on its surface. Phobos is a battered, fractured world being slowly torn apart by the gravity of the planet it circles — a moon in the process of becoming a ring, on a timescale that is long by human standards but brief in the context of the solar system's history.