Mars is lonely, but it’s not alone. If you stood on the dusty surface of the Red Planet tonight, you wouldn’t see one big, romantic moon like ours. Instead, you’d see two lumpy, terrified-looking space rocks zipping across the stars at totally different speeds. Phobos and Deimos aren’t your typical moons. They look more like giant potatoes than celestial spheres, and honestly, their origin story is one of the biggest "whodunnits" in modern astronomy.
NASA scientists have been scratching their heads over these two for decades. Are they captured asteroids? Did a massive collision blow them off the Martian surface eons ago? We’re still arguing about it. But one thing is for sure: Phobos is doomed. It’s falling toward Mars at a rate of about six feet every hundred years. Eventually, it’s going to get ripped apart, giving Mars a temporary ring before the whole mess slams into the dirt.
The Terror and the Dread: A Quick History
Asaph Hall discovered these tiny companions back in 1877 at the U.S. Naval Observatory. He almost gave up, too. It was his wife, Angeline, who kept pushing him to keep looking. He named them after the sons of Ares (the Greek version of Mars)—Phobos (Fear) and Deimos (Dread).
They are tiny. Phobos is only about 14 miles across. Deimos is even smaller, barely 8 miles wide. For context, you could basically bike across Phobos in an afternoon, assuming you didn't accidentally launch yourself into orbit with one bad pedal stroke. The gravity there is basically non-existent.
Why Phobos and Deimos Defy Explanation
The "Captured Asteroid" theory used to be the gold standard. They look like D-type asteroids—dark, carbon-rich, and weirdly shaped. If Mars’ gravity just snagged them as they drifted by from the asteroid belt, it would make perfect sense. Except for one huge problem: their orbits.
Both moons orbit almost perfectly above Mars' equator in near-circular paths. If they were captured, their orbits should be elongated and tilted, like a lasso thrown haphazardly. To get them into those perfect circles, you’d need a thick atmosphere or some kind of "brake" that just doesn't exist anymore.
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The Giant Impact Hypothesis
Then there’s the "Big Splash" idea. This suggests a massive object slammed into Mars—similar to how our own Moon was formed—and the debris eventually clumped together to form Phobos and Deimos. Research published in Nature Geoscience by Pascal Rosenblatt and his team suggests this might be the only way to explain those circular orbits.
But wait. If they came from Mars, why don't they look like Mars? The spectral data—the "fingerprint" of the light reflecting off them—doesn't match the Martian surface. It’s a total contradiction that keeps planetary scientists like Bethany Ehlmann at Caltech busy.
Up Close With Phobos: The Doomed Potato
Phobos is the "inner" moon. It’s so close to Mars—about 3,700 miles away—that it orbits the planet three times a day. From the surface of Mars, you’d actually see it rise in the west and set in the east. Most moons don't do that.
The most striking feature of Phobos is the Stickney Crater. It’s a massive hole, nearly 6 miles wide. The impact that caused it was so violent it nearly shattered the entire moon. You can see grooves radiating away from the crater like stretch marks. For a long time, we thought these were "scars" from the impact.
Recent modeling suggests something much creepier. Those grooves might be structural "stretch marks" because Phobos is literally being pulled apart by Martian gravity. It’s a "rubble pile" moon—not a solid rock, but a collection of boulders held together by a thin crust. As it spirals closer to the Roche limit, the tidal forces are stretching it. In about 30 to 50 million years, Phobos will be a ring of debris.
Deimos: The Distant, Drifting Outlier
Deimos is the chiller sibling. It hangs out much further away, taking about 30 hours to complete one orbit. It’s moving away from Mars, slowly escaping into the void, which is the exact opposite of what Phobos is doing.
Because it’s further away, it looks more like a bright star than a moon from the Martian surface. It’s also much smoother than Phobos. It has craters, sure, but they’ve been filled in by a thick layer of regolith (space dust). It’s basically a giant, dusty bean floating in vacuum.
The Future of Exploration: MMX and Beyond
We haven't had much luck landing on these things. The Soviet Phobos 1 and 2 missions in the late 80s were plagued by bad luck and technical failures. More recently, the Russian Fobos-Grunt mission didn't even make it out of Earth's orbit.
The big hope now is JAXA’s Martian Moons eXploration (MMX) mission. Scheduled to launch in the mid-2020s, it aims to land on Phobos, grab a sample, and bring it back to Earth. This is a game-changer. If we get a piece of Phobos in a lab, we can finally settle the "Captured vs. Impact" debate.
Why We Care About These Rocks
If humans are ever going to live on Mars, Phobos and Deimos are going to be our gas stations.
Because their gravity is so low, it takes almost no fuel to land or take off. You could set up a base on Phobos, use it as a shielded observation deck, and even mine it for water ice (if there’s any tucked away in the shadows). It’s much easier to land on Phobos than it is to land on Mars itself because you don't have to deal with the "seven minutes of terror" involved in atmospheric entry.
Common Misconceptions About the Martian Moons
- They are solid rocks: Nope. They are likely porous "rubble piles" with lots of empty space inside.
- They are permanent: Phobos is a cosmic goner. It’s a temporary feature of the solar system.
- They are easy to see: Deimos is so small it barely looks like a moon to the naked eye from Mars.
- They formed with Mars: Probably not. Most evidence points to them being "add-ons" from later in the solar system's life.
How to Track Mars and Its Moons Today
You don’t need a multi-billion dollar rover to appreciate this. While you can't see the moons with a backyard telescope (they’re too small and drowned out by Mars' glare), you can track Mars itself.
- Find Mars: Look for the non-twinkling orange "star" in the night sky. Use an app like Stellarium to pinpoint its current position relative to the constellations.
- Follow JAXA MMX: Keep tabs on the Martian Moons eXploration mission updates. This mission will provide the first high-definition 4K images of the surface of Phobos.
- Monitor the "Mars Ring" Theory: Read up on the latest papers regarding how Phobos' eventual destruction will create a ring system. It’s a fascinating look at the long-term evolution of planets.
- Explore HiRISE Imagery: The Mars Reconnaissance Orbiter (MRO) has taken incredible high-resolution photos of both moons. You can browse the public archives to see every boulder and groove on Phobos.
Phobos and Deimos aren't just boring rocks. They are a ticking clock and a scientific puzzle wrapped into one. Whether they are pieces of the ancient Martian crust or stray travelers from the edge of the solar system, they hold the key to understanding how rocky planets evolve over billions of years. Keep an eye on the MMX mission—we’re about to get the answers we’ve been waiting for since 1877.