Phobos and Deimos — Mars's Two Tiny Moons

Mars's two moons aren't round. They're shaped like potatoes.

That's not me being silly. Look at any photo of Phobos or Deimos and the comparison is hard to avoid. Both are small, lumpy, oddly-shaped lumps of rock, dark gray-brown and covered in craters. They look like cosmic spuds. And one of them is on a slow-motion crash course with the planet they orbit.

Here's the full story of Mars's two strange little moons.

Meet Phobos and Deimos

Phobos is the bigger of the two, but "bigger" is relative. It's only about 22 km across at its widest point — small enough to fit comfortably inside the Greater London area. Deimos is even smaller, just 12 km across.

Compare that to Earth's moon, which is 3,474 km wide. You could fit roughly 4,000 Phobos-sized moons inside our Moon, with room left over. Mars's moons are basically pebbles by Solar System standards.

Both of them orbit Mars at much closer ranges than our Moon orbits Earth. Phobos hovers only about 6,000 km above Mars's surface. (Our Moon is 384,000 km away.) Deimos is further out, at about 23,500 km, but still much closer than the Moon.

Both moons are also tidally locked, which means the same side always faces Mars — exactly the same way our Moon always shows us the same face.

Why Aren't They Round?

This one's actually a fun astronomy rule. To be round, a moon (or any object) needs to be massive enough that its own gravity squeezes it into a sphere. The bigger the object, the rounder it gets.

Anything below about 400 km wide usually isn't round, because its gravity is too weak to overcome the strength of the rock it's made of. Phobos at 22 km and Deimos at 12 km are well below that line. Their gravity isn't strong enough to crush them into a ball, so they keep whatever shape they had when they formed. Lumpy, irregular, and — yes — sort of potato-like.

Where Did They Come From?

This is still being argued about, surprisingly enough. There are two main ideas.

The captured asteroid theory. The most popular guess for a long time was that Phobos and Deimos used to be asteroids drifting around the inner solar system, and Mars's gravity captured them as they passed by. They certainly look like asteroids — their colors and chemical makeup match a class of asteroid called D-type, which is found in the outer asteroid belt.

The collision theory. But there's a problem. Captured asteroids would normally end up in odd, tilted orbits. Phobos and Deimos orbit very neatly around Mars's equator. So a newer theory says they might actually be debris from a giant collision a long time ago, when something massive smashed into Mars and threw a cloud of rock into orbit. Some of that rock then clumped together into the two moons we see today.

Whichever turns out to be right, they're definitely much older than human civilization — probably billions of years old.

Phobos Is Doomed

Here's where it gets dramatic.

Phobos isn't staying put. It's slowly spiraling closer to Mars, dropping by about 1.8 meters every hundred years. That sounds tiny, but over millions of years it adds up.

In about 50 million years, Phobos will get close enough that Mars's gravity rips it apart. Either Phobos will hit the surface (creating a crater bigger than anything on Mars today), or — more likely — it'll break into pieces just before impact and form a temporary ring around Mars.

If that happens, Mars will briefly become the second ringed planet in the inner solar system, joining Saturn-style company. The ring won't last forever (eventually the chunks will rain down on the surface), but for a few hundred million years, Mars would have a proper Saturn-like ring system.

None of this will happen during your lifetime. Or your great-great-grandchildren's lifetime. But astronomers studying Phobos can watch the slow descent in real time. The math is solid. Phobos is on borrowed time.

Phobos Rises in the West

This one is genuinely strange.

Because Phobos orbits Mars so quickly — once every 7 hours and 39 minutes — it actually laps Mars's rotation. Mars rotates once a day; Phobos goes around three times in that period. The result is that, from Mars's surface, Phobos appears to rise in the west and set in the east.

It also crosses the sky three times every Martian day, going the wrong way each time. If you stood on Mars and watched the sky for a sol, you'd see Phobos race across overhead three times, while the Sun and Deimos and the stars all moved the opposite way at their normal pace.

It's the only moon in the solar system that does this.

Deimos, the Quieter One

Deimos is the smaller, more distant moon, and it gets less attention. But it has its own oddities.

Deimos orbits Mars once every 30 hours and 18 minutes — slightly slower than Mars rotates. From the Martian surface, Deimos rises slowly in the east, drifts very slowly across the sky over about 2.5 sols, and then sets in the west. From any one spot on Mars, you'd see Deimos in the sky for several days at a time.

Deimos is also so small and so distant that, from Mars's surface, it looks more like a bright star than a moon. There would be no visible disc, just a slow-moving point of light.

Deimos has the opposite future of Phobos: instead of falling into Mars, it's slowly drifting away. In billions of years it might escape Mars's gravity entirely.

Who Discovered Mars's Moons?

An American astronomer named Asaph Hall found both of them in 1877, just six days apart. He'd been searching for Martian moons for weeks at the U.S. Naval Observatory in Washington, D.C., and was about to give up when his wife told him to keep trying. (He named one of the largest craters on Phobos after her: Stickney, her maiden name.)

Hall named the moons after the Greek god Ares's two sons. Phobos means "fear," and Deimos means "terror" or "dread." (Ares was the Greek version of Mars, the god of war, so the gloomy theme runs in the family.)

Have We Visited Them?

Not directly, no. Many spacecraft have flown past Phobos and Deimos and photographed them in detail — most recently the European Space Agency's Mars Express orbiter and NASA's HiRISE camera on Mars Reconnaissance Orbiter. But no probe has ever landed on either moon.

That might change soon. Japan's space agency JAXA is preparing a mission called MMX (Martian Moons eXploration), which is scheduled to land on Phobos, collect a soil sample, and bring it back to Earth in the early 2030s. The samples would tell us, finally, whether Phobos is a captured asteroid or a piece of Mars itself — settling a question that's been open for over a century.

Why So Few Moons?

It's worth asking. Earth has one big moon. Jupiter has 95. Saturn has over 270 (including ring particles). Why does Mars only have two tiny ones?

Probably because Mars doesn't have enough gravity to hold on to large moons. Capturing a big object would require a much stronger pull than Mars can manage. The two small moons we see today might be all that ever stuck around.

Or — if the collision theory is right — Mars used to have more moons, and most of them either crashed into the surface or got flung off into space, leaving just these two ancient survivors. Plus eventually, of course, none.

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