For over a century, scientists have puzzled over Mars’ two small, oddly shaped moons, Phobos and Deimos. Unlike Earth’s familiar round moon, these tiny companions look more like potatoes or cosmic pebbles, and their mysterious origins remain one of the biggest unanswered questions about the Red Planet. Understanding how Phobos and Deimos formed could unlock secrets not just about Mars, but potentially about the early solar system itself.
Back in 1877, American astronomer Asaph Hall discovered Phobos and Deimos while Mars was relatively close to Earth. He named them after the twin sons of Ares (the Greek equivalent of the Roman god Mars), meaning “terror” and “panic.” But the similarities to our own moon end there. Phobos and Deimos are incredibly small – Phobos is only about 14 miles (22 km) across on average, and Deimos is even smaller at about 7 miles (12 km). Imagine something roughly the size of a large city or a town orbiting a planet!
Compared to Earth’s moon, which is a massive sphere over 2,000 miles wide, Mars’ moons are dark, lumpy, and irregularly shaped – quite literally like bumpy potatoes floating in space, as astronomer Christopher S. Edwards describes them. Their unusual appearance has led many scientists to wonder if they are truly “moons” in the traditional sense or something else entirely.
So, where did these strange little objects come from? Two main theories dominate the scientific debate. One possibility is that Phobos and Deimos were originally asteroids drifting through space. As they passed too close to Mars, they might have been caught by the planet’s gravity and pulled into orbit. This idea fits their asteroid-like appearance and composition.
However, capturing two asteroids and getting them into their current orbits around Mars is surprisingly difficult to explain with physics. Mars has a weaker gravitational pull than Earth, and the specific paths Phobos and Deimos take around the planet aren’t easily achieved through simple gravitational capture. Plus, data from recent missions suggests the moons might have slightly different compositions, adding another layer of complexity.
This leads to the second major theory: that Phobos and Deimos formed from the debris of a giant impact on Mars billions of years ago. Much like how scientists believe our own moon was created when a Mars-sized object slammed into early Earth, a similar massive collision on Mars could have sent a cloud of rock and dust into orbit, which then clumped together to form the two moons. This theory could better explain their orbital paths.
As Nancy Chabot, a chief scientist at Johns Hopkins Applied Physics Laboratory, notes, the formation of Mars’ moons is “a major open question for science.” With conflicting evidence supporting both captured asteroid and impact theories, planetary scientists are eager for more data.
A composite image showing Mars' two small, irregularly shaped moons, Phobos on the left and Deimos on the right, against a starry background.
The good news is that answers may be on the horizon. The Japan Aerospace Exploration Agency (JAXA) is preparing a groundbreaking mission called Martian Moons Exploration (MMX), set to launch in 2026. MMX will travel to Mars, study both Phobos and Deimos up close, and crucially, collect a sample from the surface of Phobos to bring back to Earth.
Analyzing this precious sample will be key. If Phobos was originally an asteroid captured by Mars, its chemical makeup should be similar to other asteroids found in the solar system, particularly those from the outer asteroid belt. But if Phobos formed from an impact on Mars, the sample’s composition should bear a strong resemblance to the rocks and minerals found on Mars itself.
Solving the mystery of how Phobos and Deimos formed is exciting in itself, but the potential implications go even further. If the sample shows the moons came from Mars, it means scientists will hold in their hands pieces of the Red Planet from its very early history. These ancient Martian rocks could contain clues about whether early Mars had conditions suitable for life, helping us understand the planet’s past habitability.
JAXA’s MMX mission represents a major step toward unraveling these Martian mysteries. By bringing a piece of Phobos back to Earth, we could finally learn the origin story of these enigmatic moons and perhaps even touch upon the question of life beyond Earth.
Want to learn more about our solar system’s fascinating moons? Check out our look at how many moons are in the solar system or test your knowledge with our Mars quiz.
