The Mysterious Origins of the Moons of Mars

Asteroids Pictures Gallery - Gaspra, Deimos and Phobos
This montage shows asteroid 951 Gaspra (top) compared with Deimos (lower left) and Phobos (lower right), the moons of Mars. The three bodies are shown at the same scale and nearly the same lighting conditions. Their different appearances may be due to the fact that Phobos and Deimos may or may not be captured asteroids. NASA, Jet Propulsion Laboratory

Mars has always fascinated humans. The Red Planet holds many mysteries, which our landers and probes are helping scientists solve. Among them is the question of where the two Martian moons came from and how they got there. Phobos and Deimos look more like asteroids than moons, and that has caused many planetary scientists to look for their origin somewhere else in the solar system. Others maintain that those moons may have formed when Mars did or are the result of some catastrophic event early in the history of the solar system. Chances are good that when the first missions land on Phobos, rock samples will tell a more definitive story about these mysterious companion moons.

Asteroid Capture Theory

One clue about the origins of Phobos and Deimos lies in their makeup. Both have many characteristics in common with two types of asteroids common in the belt: C- and D-type asteroids. These are carbonaceous (meaning they are rich in the element carbon, which bonds easily with other elements). Also, judging by the look of Phobos, it's easy to assume that it and its sister moon Deimos are both captured objects from the Asteroid Belt. This is not an unlikely scenario.  After all asteroids break free from the belt all the time. This happens as a result of collisions, gravitational perturbations, and other random interactions that affect an asteroid's orbit and send it off in a new direction. Then, should one of them stray too close to a planet, like Mars, the planet's gravitational pull could confine the interloper to a new orbit. 

If these ARE captured asteroids, then there are many questions about how they could have settled into such circular orbits over the history of the solar system. It's possible that Phobos and Deimos could have been a binary pair, bound together by gravity when they were captured. Over time, they would have separated into their current orbits. 

It's possible that early Mars was surrounded by many of these types of asteroids. They could have been the result of a collision between Mars and another solar system body in the early history of the planets. If this did happen, it could explain why Phobos's composition is closer to the makeup of Mars's surface than that of an asteroid from space. 

Large Impact Theory

That brings up the idea that Mars suffered a large collision very early in its history. This is similar to the idea that Earth's Moon is the result of an impact between our infant planet and a planetesimal named Theia. In both cases, such an impact caused a large amount of mass to be ejected into outer space. Both impacts would have sent a hot, plasma-like material into a concentric orbit about the infant planets. For Earth, the ring of molten rock eventually gathered together and formed the Moon. 

Despite the look of Phobos and Deimos, some astronomers have suggested that perhaps these tiny orbs formed in a similar way around Mars. Perhaps the best evidence for an asteroidal origin is the presence of a mineral called phyllosilicates on the surface of Phobos. It's very common on the surface of Mars, an indication that Phobos formed from the Martian substrate.

However, composition argument isn't the only indication that Phobos and Deimos may have originated from Mars itself. There is also the question of their orbits. They're nearly circular. They are also very near to Mars' equator. Captured asteroids likely wouldn't settle into such precise orbits, but material splashed out during an impact and then accreted over time could explain the orbits of the two moons.

Exploration of Phobos and Deimos

During the past decades of Mars exploration, various spacecraft have looked at both moons in some detail. But, more information is needed. The best way to get it is to do an in-situ exploration.  That means "send a probe to land on one or both of these moons". To do it right, planetary scientists would send a lander to grab some soil and rocks and return it to Earth for study). Alternatively, when humans start to explore Mars in person, part of a mission could be diverted to land people on the moons to do a more nuanced geological study. Either one would satisfy people's urge to know just how those moons came to be where they are in orbit around Mars.

Edited and updated by Carolyn Collins Petersen.