The Mysterious Origin 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. It was interesting in ancient times due to its mysterious red color and motion across the sky.  Today, people see pictures from the surface taken by landers and rovers, and see what an intriguing world it is. For the longest time, people thought there were "Martians", but it turns out there's no life there now. At least, none that anyone can see. There are other mysteries of Mars, among them the origins of its two moons: Phobos and Deimos.

Planetary scientists have many questions about them and are working to understand whether they came from somewhere else in the solar system, formed right along with Mars, or are the product of a catastrophic event in Mars's history.  Chances are good that when the first missions land on Phobos, rock samples will tell a more definitive story about it and its companion moon.

Asteroid Capture Theory

Judging by the look of Phobos, it's easy to assume that it and its sister moon Deimos are both captured asteroids from the Asteroid Belt.

It is not an unlikely scenario. After all asteroids break free from the belt all the time. This happens a sa  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 closely to a planet, like Mars, its gravitational pull could confine it to a new orbit.


Both Phobos and Deimos 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). 

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 Mars was once surrounded by many of these types of asteroids, maybe as a 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 that of Mars's surface than of an asteroid from space. 

Large Impact Theory

That brings us to the idea that Mars did, indeed suffer a large collision very early in its history. This is similar to the idea that Earth's Moon may have been 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. Well, it turns out that they might be at least partially right.

As mentioned above, the composition of Phobos is unlike anything found in the Asteroid Belt. So it if was a captured asteroid, it seems that it would have have an origin other than the belt.

Perhaps the best evidence so far gathered is the presence of a mineral called phyllosilicates on the surface of Phobos. This mineral is very common on the surface of Mars, an indication that Phobos formed from the Martian substrate. Beyond the presence of the phyllosilicates, the general mineral composition of both surfaces are in agreement.

But the composition argument isn't the only indication that Phobos and Deimos may have originated from Mars itself. There is also the question of orbit.

The near-circular orbits of the two moons are very near to Mars' equator, a fact which is difficult to reconcile in the capture theory.

However, a collision and re-accretion from a planetary ring of debris 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. The best way to know MORE about their chemical composition and densities 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 need to send a sample return mission (where a lander would land, grab some soil and rocks and return it to Earth for study), or -- in the very far future -- land humans there to do a more nuanced geological study. Either way, we'd have solid answers on the past of some very fascinating worlds. 

Edited and updated by Carolyn Collins Petersen.


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Your Citation
Millis, John P., Ph.D. "The Mysterious Origin of the Moons of Mars." ThoughtCo, Jul. 4, 2017, Millis, John P., Ph.D. (2017, July 4). The Mysterious Origin of the Moons of Mars. Retrieved from Millis, John P., Ph.D. "The Mysterious Origin of the Moons of Mars." ThoughtCo. (accessed January 16, 2018).