The Martian moons Phobos and Deimos have a common ancestor

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A recent study suggests that Phobos and Deimos, the two moons of Mars, are the remains of an ancient common body, disintegrated between 1 and 2.7 billion years ago. This work also confirmed that the larger of the two, Phobos, is already doomed.

Two Martian “potatoes”

Mars now has two moons, Phobos and Deimos. The latter have quickly intrigued researchers since their discovery in 1877. On the one hand, they are tiny. The diameter of Phobos does not exceed twenty-two kilometers, while Deimos is approximately twelve kilometers wide. They also have very irregular shapes, similar to old potatoes.

This particular physique has already led researchers to suspect that these two moons could be asteroids once captured in the gravity field of Mars. But this idea does not hold. The captured objects should indeed follow an eccentric orbit around the planet, and this orbit would be at a random inclination. However, the orbits of Phobos and Deimos are almost circular and move in the equatorial plane of Mars.

But then, how to explain the current orbits of these two moons? To try to understand it, researchers at the University of Zurich relied on computer simulations. The idea: to retrace the orbits through time.

Phobos, the moon of Mars, covered with grooves. Credit: NASA / JPL-Caltech / University of Arizona

A common ancestor

All the stars exert tidal forces on each other. These forces lead to a form of energy conversion called “dissipation”, the scale of which depends on the size of the bodies involved, the distance between them, but also their internal composition.

Previous studies have already suggested that Phobos and Deimos are made of a porous material. They are therefore very sparse (less than two grams per cubic centimeter against 5.5 grams per cubic centimeter on average on Earth).

Based on these results, the researchers ran several computer simulations in an attempt to track the orbits of the moons backwards in time. Then there came a time when the two moons crossed in exactly the same place. In other words, it was at this precise moment that Phobos and Deimos were born. According to the simulation, this pivotal moment occurred there between 1 and 2.7 billion years.

The researchers conclude that a larger celestial body was in orbit around Mars at the time, and that this moon was likely struck by another body before it disintegrated. Phobos and Deimos would be none other than “The remains of this lost moon”, can we read in the review Nature astronomy.

We still lack data to refine these estimates. It all really depends on the physical properties of Phobos and Deimos. A Japanese probe, the launch of which is planned for 2025, could provide us with additional details. It will indeed be a question of bringing samples of Phobos to Earth. The analyzes that will follow will make it possible to better characterize the interior of these two Martian moons, and therefore to better identify their origin.

The moon Phobos. Credits: L G. Neukum (FU Berlin) et al., Mars Express, DLR, ESA; Acknowledgment: Peter Masek

Phobos, a doomed moon

It also emerged from this study that the common ancestor of Phobos and Deimos was further from Mars than Phobos is today. While the second has remained near the place where it was born, the tidal forces push the first to get closer to Mars…. dangerously.

Indeed, thanks to computer simulations, the researchers were able to go back in time, but also to “see the future” by estimating the future development of the orbits of the two moons. And if it appears that Deimos will move away from Mars very slowly, just as our moon is slowly moving away from Earth, Phobos will crash into Mars in less than 40 million years ago. It is also possible that she will find herself “torn” by the gravitational forces of her planet before even touching the ground.

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