Two Moons, Two Mysteries

Earth has one moon. Jupiter has 95. Mars sits somewhere in between — with exactly two natural satellites, both discovered in 1877 by American astronomer Asaph Hall. But what Phobos and Deimos lack in number, they more than make up for in scientific intrigue. These aren't the round, luminous bodies of science textbook illustrations. They're battered, potato-shaped chunks of rock with orbits so unusual they've puzzled scientists for generations.

Phobos: The Doomed Moon

Phobos is the larger of the two moons, measuring roughly 27 × 22 × 18 kilometers — small enough that you could jog its longest axis in under three hours. It orbits Mars at an altitude of just 6,000 kilometers above the surface, closer to its parent planet than any other moon in the solar system. For context, Earth's Moon orbits at around 384,400 kilometers away.

That proximity has a dramatic consequence. Phobos orbits Mars faster than Mars rotates — completing a full orbit in just 7 hours and 39 minutes. This means that from the Martian surface, Phobos rises in the west and sets in the east, crossing the sky two or three times per day. No other moon in the solar system behaves this way.

But here's the real headline: Phobos is spiraling inward. Tidal forces from Mars are gradually pulling it closer at a rate of about 1.8 centimeters per year. In approximately 30 to 50 million years, Phobos will either collide with Mars or be torn apart by tidal forces to form a thin ring around the planet. Astronomers can already see evidence of this stress — a system of parallel grooves scoring the moon's surface, many of which are thought to be early fractures caused by tidal strain.

Phobos is also home to the Stickney Crater, a massive impact feature 9 kilometers in diameter — nearly half the moon's width. Whatever struck Phobos to create Stickney came close to shattering it entirely.

Deimos: The Quiet One

Deimos is smaller and more distant, measuring about 15 × 12 × 11 kilometers and orbiting Mars at roughly 23,460 kilometers. Unlike Phobos, Deimos is slowly drifting away from Mars — a more familiar dynamic, similar to how Earth's Moon is gradually receding from Earth. Deimos takes 30 hours and 18 minutes to complete one orbit, moving so slowly that from the Martian surface it appears to hang nearly motionless in the sky, taking about 2.7 days to go from moonrise to moonset.

Deimos has a smoother appearance than Phobos, its craters largely filled in with fine-grained regolith. Its two named craters — Swift and Voltaire, honoring writers who curiously predicted the existence of Martian moons in the 18th century — are the most prominent features on its surface.

Where Did They Come From?

The origin of both moons remains one of the open questions of planetary science. Three main hypotheses compete:

  • Captured asteroids: Their composition and irregular shapes resemble C-type asteroids from the outer main belt. Many scientists have favored this explanation, though the orbital mechanics of capturing two asteroids into nearly circular, near-equatorial orbits is difficult to explain.
  • Giant impact debris: A large collision with early Mars could have ejected material that coalesced into the two moons, similar to the leading theory for Earth's Moon. Recent modeling work has given this hypothesis growing support.
  • In-situ formation: The moons may have formed directly from a disk of material orbiting early Mars, though this scenario also has unresolved challenges.

Japan's Martian Moons eXploration (MMX) mission, scheduled to launch in 2026, aims to land on Phobos, collect samples, and return them to Earth by around 2031. Those samples could finally settle the origin debate — and help us understand not just Phobos, but the early history of Mars itself.

Why It Matters for Exploration

The moons of Mars aren't just scientific curiosities. Phobos and Deimos could serve as staging grounds for human missions to Mars, offering low-gravity platforms with no thick atmosphere to fight through. NASA and other space agencies have studied using Phobos as a base for remotely operating surface robots before astronauts make the final descent. Whatever their origin, these two strange little worlds may play an outsized role in humanity's future on the Red Planet.