Artist's illustration of Mars with its moons Phobos (middle) and Deimos (left). Scientists believe that the two moons originated from a common celestial body. Credit: NASA/JPL-Caltech/University of Arizona

Mysterious Origins of Mars’ Moons May Finally be Solved

Phobos and Deimos may have had a common ancient ancestor.


Phobos and Deimos may be the remnants of a larger body that collided with another body in orbit around Mars in the past and collapsed. This is the conclusion reached by planetary scientists who analyzed models of the evolution of the orbits of satellites. 

How did the moons of Mars form?

The origin of the small moons of Mars – Phobos, and Deimos still remains one of the unsolved problems in planetary science.

Today, there are two versions of their origin: these are bodies from the main belt of asteroids that were captured by Mars, or satellites were formed from matter ejected into the near-Martian orbit from the surface of the planet as a result of a powerful collision with a dwarf planet or a giant asteroid (this theory is explained at the bottom of this article).

More detailed information can be given by interplanetary stations (such as MMX or “Phobos-Grunt 2”), whose task is to deliver soil from Phobos to Earth, but for now, scientists are left to analyze data from orbiters operating near Mars or build computer models.

A group of planetary scientists led by Amirhossein Bagheri from the Institute of Geophysics of the Swiss Higher Technical School of Zurich decided to test the hypothesis about the asteroid nature of the moons of Mars using numerical simulations.

Scientists wanted to understand if it was possible to get around the problem of almost circular and equatorial orbits of satellites, which does not fit into the idea of ​​capturing Mars asteroids – in this case, the orbits should have been more elongated and have a random tilt.

It turned out that in the past (from 1 to 2.7 billion years ago) the orbits of Phobos and Deimos crossed, which means that both satellites could be fragments of a larger celestial body that was in near-Martian orbit and experienced a destructive collision with another body. More accurate temporal estimates can be given after clarifying the physical properties of Phobos and Deimos, in particular their porosity.

Later, Deimos remained in the immediate vicinity of the area of ​​his birth, and Phobos, under the influence of tidal forces, began to lower its orbit. Today, Deimos is very slowly moving away from Mars, and Phobos will continue to spiral on a spiral path towards the planet and will either collide with it or be destroyed by tidal forces.

A massive asteroid collided around Mars in the past

Several years ago, scientists concluded that Mars has experienced a collision with a giant asteroid in the past. As shown by a computer model, it was comparable in size to Ceres, the largest dwarf planet in the asteroid belt between Jupiter and Mars. The researchers’ article was published in the journal Geophysical Research Letters.

Several years ago, researchers studying Martian meteorites noticed that the Red Planet’s mantle contains unusually high siderophile elements. These include ruthenium, rhodium, palladium, osmium, iridium, molybdenum, platinum, as well as gold and rhenium (the latter two were excluded from the analysis).

These elements are involved in the formation of planetary nuclei, and after the gravitational differentiation of the metal core and the silicate mantle of Mars took place, they should have descended to the center under the influence of gravity.

However, their share in the mantle is much higher than expected (0.8 percent of the planet’s mass) and, moreover, this discrepancy is very similar to what is observed on Earth and the Moon. Some scientists explain the unusual composition of our planet and its satellite by a collision with a large celestial body, Theia; in this regard, the authors of the work suggested that Mars could also survive a major collision.

To test their theory, scientists ran a computer simulation. They randomly generated planetesimals with a diameter of 1 to 2,000 kilometers – the number of large and small bodies was calculated based on data on the main asteroid belt – and looked at how the composition of the mantle of the Earth, the Moon, and Mars would change when they collide with them. The starting point was the event that led to the formation of the moon.

It turned out that in addition to many small collisions, Mars should have experienced another large one about 4.43 billion years ago. To obtain a sufficient amount of siderophile elements in the mantle, an asteroid at least 1200 kilometers in size had to crash into the Red Planet.

If Mars really experienced a similar event in the past, then it must have left a mark on its surface. Scientists believe that the collision is indicated by the relief of the southern and northern hemispheres of the planet. While the south is an old highland with many craters, the north is a bit younger, and mostly smooth plains can be found.

On the other hand, this topography can be explained by the ancient northern ocean that existed in the Amazonian era. Scientists also believe that debris should have remained in orbit around the planet: in their opinion, they could have formed the moons of Mars – Phobos, and Deimos.

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Bagheri, A., Khan, A., Efroimsky, M., Kruglyakov, M., & Giardini, D. (2021, February 22). Dynamical evidence for Phobos and deimos as remnants of a disrupted Common progenitor.
Brasser, R., & Mojzsis, S. (2017, June 29). A colossal impact enriched MARS’ mantle with noble metals.
Choi, C. (2021, February 23). Were the mars moons Phobos and deimos born from ANOTHER Shattered moon?
Kellen Beck 2 days, 8. (2021, February 25). Mars’ 2 Weird Moons began as 1 large Moon, SUGGESTS STUDY.
O’Neill, M. (2021, February 27). Researchers say they’ve solved the puzzling mystery of the moons of mars.

Written by Vladislav Tchakarov

Hello, my name is Vladislav and I am glad to have you here on Curiosmos. As a history student, I have a strong passion for history and science, and the opportunity to research and write in this field on a daily basis is a dream come true.

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