How Mars got its two small moons Phobos and Deimos

No planet stirs human imagination quite like Mars. As non-stop land-based missions continue to probe its past and search for possible traces of life, another intrigue—this time high above its skyline—had long baffled scientists: how did Mars end up with its two small moons, Phobos and Deimos, first spotted in 1877? This riddle may have just been solved by a multidisciplinary study combining French, Belgian and Japanese expertise.
Scientists have long hesitated between two hypotheses. The first suggests that the moons are asteroids like those found in the belt between Jupiter and Mars; but why they should have been trapped around Mars remains unclear. An alternative theory posits that the moons formed from the debris of a collision between Mars and a protoplanet—a planet in the making; here though, uncertainty has hovered over the mechanism producing two small satellites. A major difficulty has been to explain why a giant impact on Mars would have left two moons so different from our own Moon, a huge single mass, that also formed from Earth undergoing such an impact.
The researchers used astrophysics, planetary science, mathematics and computer science to create complex models running a range of hydrodynamic and numerical simulations able to recreate the sequence of past events. Their findings back up the second hypothesis of an almighty collision while also, for the first time, coherently filling in the gaps to explain how two moons emerged from the crash debris.
According to the simulations, Mars suffered a colossal impact with a body three times smaller some 4 to 4.5 billion years ago. Debris from the collision initially accumulated into a long disk around Mars, resembling one of Saturn’s rings. Within this disk, an enormous moon a thousand times the mass of Phobos gradually formed—similar to the way in which our Moon amassed from debris created by Earth’s impact.

https://news.cnrs.fr/articles/solved-the-mystery-of-the-martian-moons