Mars has two small, dark, potato-shaped moons called Phobos and Deimos. Neither looks anything like Earth’s smooth, round moon. They’re irregular, cratered lumps that resemble asteroids more than the bright celestial body you’re used to seeing in the night sky. Both are dark and reddish in color, and both are tiny: Phobos measures about 17 by 14 by 11 miles across, while Deimos is only about 7.5 miles in diameter.
Phobos: Battered and Grooved
Phobos is the larger of the two moons, and it looks like a rock that’s been through a war. Its most dramatic feature is Stickney crater, a 6-mile-wide impact scar that takes up nearly half the moon’s visible surface. Whatever hit Phobos to create Stickney nearly shattered the entire body. Surrounding Stickney and spread across the rest of the surface are thousands of smaller impact craters, giving Phobos a heavily pockmarked texture.
The surface itself has been pounded into a fine powder over billions of years of meteorite strikes. Images from the Mars Global Surveyor show dark trails running down the steep walls of craters, left behind by landslides triggered by impacts. Phobos also has a network of shallow grooves etched across its surface, lines that stretch for miles and give it a streaked, almost scratched appearance. The overall color is a very dark gray with reddish tones, similar to certain types of primitive meteorites found on Earth.
Deimos: Smaller and Smoother
Deimos looks noticeably different from its sibling. It’s much smoother, blanketed by a thick layer of loose, broken rock (called regolith) that fills in most of the craters and softens the terrain. Only the most recent impact craters poke through this dusty covering. High-resolution images from NASA’s Mars Reconnaissance Orbiter reveal subtle color variations across the surface: the smoothest areas appear redder, while fresh craters and ridgelines are less red and slightly brighter. That color difference comes from how long the surface material has been exposed to the space environment, which gradually darkens and reddens it over time. A fresh impact exposes lighter material underneath.
Despite its smoother look, Deimos is still an irregularly shaped, lumpy object. It’s about half the size of Phobos and orbits much farther from Mars.
How They Look From the Surface of Mars
If you stood on Mars and looked up, neither moon would look like the full moon you see from Earth. Earth’s moon has a diameter more than 100 times greater than Phobos, and it appears as a large, bright disk in our sky. Phobos, by contrast, would look like a small, oblong shape, clearly not round. It orbits only about 3,900 miles above the Martian surface, so it appears larger than Deimos, but still far smaller than Earth’s moon appears to us.
Deimos orbits about 12,800 miles out and would look like little more than a bright dot, roughly star-like in appearance. You’d need sharp eyes to make out any shape at all.
Phobos moves so fast that it completes three full orbits every Martian day. From the surface, it would rise in the west and set in the east, the opposite direction of what you’d expect, because it orbits faster than Mars rotates. Deimos takes about 30 hours per orbit, so it creeps slowly across the sky.
What Eclipses Look Like on Mars
NASA’s rovers have captured footage of both moons passing in front of the Sun, and these “eclipses” look nothing like the ones we experience on Earth. Phobos appears as a dark, potato-shaped silhouette crossing the Sun’s disk. It’s far too small to cover the Sun completely, so it creates a partial shadow rather than the dramatic total eclipses Earth’s moon can produce. Deimos appears even smaller during its transits, looking like a tiny dark speck moving across the bright solar disk. The irregular shapes of both moons are clearly visible during these events, reinforcing just how different they are from our own round, smooth satellite.
Why They Look So Different From Earth’s Moon
Both moons are thought to be captured asteroids, or debris left over from the early formation of the solar system. That origin explains their appearance. They never had enough mass to pull themselves into a sphere through their own gravity, the way Earth’s moon did. Instead, they retained the rough, lumpy shapes of the smaller rocky bodies they formed from. Their surfaces are extremely dark, reflecting very little sunlight, with an albedo (reflectivity) comparable to coal or asphalt. Spectral data from multiple spacecraft show that both moons share a similar composition, though scientists still debate whether they formed from material near Mars or were captured from elsewhere in the solar system.
Their small size also means gravity is almost nonexistent on their surfaces. On Phobos, you would weigh less than a pound. A strong jump could launch you into orbit, or even off the moon entirely. The fine powdery surface material stays in place only because there’s no wind or weather to disturb it.