Why are Mars moons not round?

Why Mars’ Moons Look Like Lumpy Potatoes (and Not Smooth Balls)

Ever looked up at the Moon and thought, “Wow, what a perfect sphere!”? Well, Mars’ moons, Phobos and Deimos, are… not like that. Instead of being nice and round, they’re oddly shaped, kind of like lumpy potatoes floating in space . Seriously, take a look at the pictures. What’s the deal?

The answer boils down to gravity, that invisible force that shapes the cosmos.

Gravity: The Ultimate Sculptor (When It’s Strong Enough)

Think of gravity as a cosmic sculptor . For big objects, like planets and our own Moon, gravity is a powerful artist. It pulls everything inward, squishing the material until you get the most compact shape possible: a sphere . As NASA’s Anjali Tripathi puts it, self-gravity pulls a celestial body into a spherical form . Bruno Merín from the European Space Agency uses a great analogy, comparing it to water swirling down a drain, everything trying to get as close to the center as possible .

But here’s the catch: this sculpting process only works if you have enough “clay,” or in this case, mass . Smaller objects? Not so much.

Phobos and Deimos: Too Tiny to be True Spheres

Phobos, the bigger of the two Martian moons, is only about 27 x 22 x 18 kilometers across . Deimos is even tinier, just 12.6 km in diameter . To put that in perspective, that’s about the size of a small city! Because they’re so dinky, their gravity is super weak. I mean, the gravitational force on Phobos is a measly 1/1,000th of Earth’s . That’s not nearly enough to force them into a spherical shape. So, they stay lumpy, bumpy, and full of craters.

Cosmic Leftovers: Where Did They Come From?

Their weird shapes also give us clues about where they came from in the first place. One popular idea is that they’re captured asteroids, basically space rocks that wandered a little too close to Mars and got snagged by its gravity . They’re made of stuff similar to C-type asteroids, like carbon-rich rock mixed with ice, which supports this theory . Maybe Jupiter gave them a gravitational shove, sending them toward Mars .

But here’s a wrinkle: Phobos and Deimos have pretty circular orbits, which is unusual for captured asteroids. Usually, captured objects have wilder, more erratic paths. So, another theory is that they formed from debris blasted into orbit when something big smacked into Mars . Imagine a giant cosmic fender-bender! All that debris could have formed a ring around Mars, and Phobos and Deimos eventually clumped together from that material.

Interestingly, a recent study suggests that Phobos and Deimos might have actually been a single “protomoon” that split apart a long time ago . The simulations show that Phobos, after breaking away from Deimos, would have started with a more oval-shaped orbit, which gradually smoothed out over time.

Phobos’ Doomed Future

Here’s a slightly morbid fact: Phobos is slowly spiraling towards Mars, getting about six feet closer every century . At this rate, in about 50 million years, it’s either going to crash into the planet in a blaze of glory or break apart and form a spectacular ring around Mars . Deimos, on the other hand, is slowly drifting away.

Even though we’re still piecing together the exact story of Phobos and Deimos, their oddball shapes tell us a lot about the early solar system and the power (or lack thereof) of gravity. Hopefully, future missions to these strange little moons will finally solve the mystery . Who knows what secrets they’re hiding?