Mars has two unique moons, which look more like asteroids than our own satellite. Phobos and Deimos were born about 4,000 million years ago, 600 million years after the formation of the Solar System, but its origin is still a mystery.
The moons of the red planet owe their names to the Greek twin gods Phobos ( panic ) and Deimos ( terror ), children of Ares – which the Romans knew as Mars. Now, a study by the Southwest Research Institute (SwRI) in Boulder (United States), points out that the two satellites are also children of Mars. As published this week Science Advances, the two moons were born mainly of Martian rocks ejected into space after the collision with another body 4 billion years ago
Currently, there are two theories about the origin of the small Phobos and Deimos, of only 22 and 12 kilometers in diameter, respectively. The first holds that they are really asteroids, coming from the belt between Mars and Jupiter, and captured by the gravity of the red planet. The way its crater-ridged surface reflects light, very similar to that of asteroids, supports this theory.
Phobos and Deimos were born mainly of Martian rocks ejected into space after the collision with another body 4 billion years ago
Another hypothesis is that they were born from an impact, as happened with our satellite, which is why the two moons orbit exactly in the same plane, something that would be a huge coincidence if they were two independent asteroids.
After performing computer simulations, the SWRI scientists have concluded that the most plausible scenario for the formation of Phobos and Deimos is that they are the result of the collision of a body of one thousandth of the mass of Mars, of a size similar to the asteroid Vesta or the dwarf planet Ceres, smaller than what other models had suggested so far.
The collision would have generated a disk of rocks, coming from both Mars and the other body, which would have given birth to several moons, including Phobos and Deimos. Those closest to the planet would have ended up disappearing, absorbed by the gravity of the red planet. The only survivors among the children of Mars would have been Phobos and Deimos, who orbit 9,000 and 23,000 kilometers from the Martian surface
If the object that impacted would have been larger, it would have given birth to huge moons that would not have allowed Phobos and Deimos to form, explains Julien Salmon, SWRI researcher and co-author of the study, by email.
The simulation also predicts that the material of the disk, and therefore the one that forms both satellites, would be 77% of Martian origin. So, Phobos and Deimos should have a composition very similar to Mars, says Salmon. They should also lack water, which would have evaporated completely in the collision.
The collision generated a disk of rocks, coming from both Mars and the other body, which would have given birth to several moons, including Phobos and Deimos
Both predictions will be put to the test by the Martian Moons Exploration (MMX) mission of the Japanese space agency (JAXA), planned for 2024. The spacecraft is expected to fly over the two moons and land at Phobos, where it will collect samples on the ground that it will return to the Earth in 2029. “We are looking forward to the analysis of the samples that the future JAXA mission will bring from Phobos, to see to what extent they confirm our results”, declares Julien Salmon.
The researchers also point out that the collision that gave birth to Phobos and Deimos could also form one of the gigantic craters on Mars, which are about 3,000 kilometers wide. Due to its magnitude, it is most likely that it gave rise to the Utopia crater in the northern hemisphere or to the Hellas crater in the northern hemisphere.
The JAXA Martian Moons Exploration mission will reveal whether the researchers’ conclusions are correct
“It could have been a great planetesimal [the embryo of a planet] that remained there after the formation of the planets, but it could also have been an object that was formed further away and that ended up approaching”, theorizes Julien Salmon.
“What our study shows is that all the moons of the rocky planets [of the Solar System] probably formed from a common and fundamental process: the accumulation of a disk of material generated by an impact towards the end of the formation of the planet, more than 4,000 million years ago, “says the SWRI researcher. “It is very interesting that this process can produce both a large satellite, in the case of the Moon and two small objects, Phobos and Deimos.”
All the moons of the rocky planets [of the Solar System] probably formed from a common and fundamental process: the accumulation of a disk of material generated by an impact towards the end of the formation of the planet, more than 4,000 million years”
“The simulation presented by this study allows us to explain the formation of the two satellites with an impact not very massive, and therefore in a simple and elegant way,” says Josep Maria Trigo , researcher at the Institute of Science of the Space (IEEC- CSIC) that has not participated in the investigation. The results “confirm that the impacts between different bodies were quite common and definitive when determining the final properties of each planet,” he explains by e-mail.