The doomed moon

Phobos orbits just 6,000 km above the Martian surface — closer to its planet than any other moon in the Solar System. Our Moon, for comparison, keeps a respectful 384,000 km. That intimacy comes at a price: Mars's tides are slowly strangling it, dragging Phobos about 1.8 metres closer every century. The spiral only accelerates as it descends.

In roughly 50 million years — an afternoon, in Solar System terms — one of two things happens. Either Phobos slams into Mars, or, more likely, it crosses the Roche limit first: the altitude where the planet's gravity pulls harder on the moon's near side than the moon's own feeble gravity can resist, and the whole thing shreds into rubble. The debris would spread into a ring, making Mars, briefly, the only rocky planet with one — before the ring itself rains down as a planet-girdling meteor shower. There are hints this has happened before: some models suggest Phobos is itself the recycled remnant of an earlier, larger moon that already went through a crash-and-ring cycle.

Deimos faces the opposite fate. Orbiting beyond the synchronous altitude, it is slowly drifting outward, exactly as our own Moon is. Give it long enough and Mars's moon count goes from two to one to zero — the question is only the order of the funerals.

Fear and Dread: the names and the discovery

The names are a matched pair from Greek mythology: Phobos means fear or panic, Deimos dread or terror — the twin sons of Ares (the Greek Mars) who drove their father's chariot into battle. The suggestion came from a line in Book XV of Homer's Iliad, proposed by the English schoolmaster Henry Madan shortly after the discovery. For a planet named after the god of war, the escort is fitting.

Both moons were found in a single week in August 1877 by Asaph Hall at the US Naval Observatory in Washington, using what was then the world's largest refracting telescope. Hall had nearly given up the search; the story he told was that his wife, the mathematician Angeline Stickney, pushed him to try one more night. Deimos appeared on August 12, Phobos on August 18 — and the enormous crater that dominates Phobos, 9 km across on a 22 km moon, is named Stickney in her honour.

One more twist: the moons were "described" 151 years before anyone saw them. In Gulliver's Travels (1726), Jonathan Swift had his Laputan astronomers report that Mars has two small moons with short orbital periods — startlingly close to the truth. It was pure satirical luck, but astronomers enjoyed the coincidence enough to name a crater on Deimos after Swift.

Captured asteroids — or wreckage from a giant impact?

For a one-line trivia answer, the moons' origin is a genuine open fight in planetary science. Their dark, carbon-rich surfaces look exactly like asteroids from the nearby belt, which suggests Mars simply captured two passing rocks. But captured objects should end up on tilted, stretched-out orbits — and Phobos and Deimos travel in nearly perfect circles directly above Mars's equator, which is what you'd expect if they condensed from a debris disc thrown up by a colossal ancient impact, the same way Earth's Moon formed.

The tiebreaker is on its way. Japan's MMX (Martian Moons eXploration) mission is designed to orbit Mars, land on Phobos, grab a sample, and fly it home — the first material ever returned from the Martian system. If the sample is asteroid-stuff, capture wins; if it contains traces of Mars itself, the impact theory does. Either answer is good: a chip of Phobos may even carry ancient Martian surface material blasted onto it by meteorite strikes, making it a free sample of Mars into the bargain.

Meanwhile the moons are already photogenic. NASA's Perseverance rover has filmed Phobos crossing the face of the Sun from the Martian surface — a lopsided "googly eye" eclipse lasting about 40 seconds, over almost before it begins, because nothing involving Phobos happens slowly.

Two is unusual — the neighbourhood census

Among the rocky planets, owning any moon is the exception: Mercury and Venus have none, Earth has one, and Mars's pair is the inner Solar System record. The giant planets play a different game entirely.

Frequently asked questions

Seven questions about two moons

Sources

• NASA Mars Exploration Program — Phobos & Deimos overviews, the 1.8 m/century orbital decay and ~50-million-year fate.
• US Naval Observatory — Asaph Hall's August 1877 discovery of both moons.
• JAXA — MMX (Martian Moons eXploration) Phobos sample-return mission.
• NASA/JPL — Perseverance rover footage of Phobos transiting the Sun (2022).
• IAU Gazetteer of Planetary Nomenclature — Stickney and Swift craters, and the Iliad-derived moon names.

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