New simulations led by astrophysicist Robin Canup, who helped accredit the giant impact model to explain the origin of the moon, have now reinforced a proposed scenario to explain the origin of the moons of Mars. The Red Planet would have collided with a celestial body the size of Vesta or Ceres.
The two moons of Mars were discovered only in the second half of the nineteenth century, more precisely in 1877 and a few days apart during the month of August, by the American astronomer Asaph Hall.
Their names were originally suggested to Hall by Henry Madan, a professor at Eton College. Hall called them Phobos and Deimos, that is to say, the twins that the god Ares (Mars in Roman mythology) had from the goddess Aphrodite, and who are therefore minor deities of Greek mythology, meaning respectively in Greek Fear and Terror.
Although anecdotal, Kepler had claimed more than two centuries ago that the Red Planet had two moons. Pythagorean and Platonic, he had proposed a mathematical law to predict the occurrence of planetary moons by pointing out that the Earth had one moon and Jupiter four.
Astronomers and specialists in celestial mechanics have been, at least since Laplace, looking for cosmogonic models explaining the origin of the stars and they are of course asking questions about the origin of Phobos and Deimos.
Phobos and Deimos, a problematic composition and origin
Thus, it appears that Phobos and Deimos have more similarities with the asteroids than with the Earth’s moon because of their small sizes, their irregular shapes and the appearance of their surfaces. Possessing a very particular spectrum, they can even be compared with asteroids of type C or D.
However, the orbital characteristics of Phobos and Deimos do not fit with the hypothesis that would make them asteroids captured by Mars. They fit well however with the hypothesis that they are made of leftover materials from a ring of matter produced by a collision between Mars and a celestial body. A scenario analogous to that proposed to explain the origin of our own moon by a collision between the Earth and a small planet the size of Mars, called Theia.
This scenario has also been the subject of numerical simulations, notably a few years ago by French researchers. It has been revisited by members of the Southwest Research Institute in the United States who have just published an article in Science Advances. The team, which performed more powerful numerical simulations, was led by Robin Canup, the astrophysicist who made a name for herself in the early 2000s using numerical simulations that reinforced the collision hypothesis on the origin of our moon.
MMX, a Martian mission to test the hypothesis of the impact
According to Robin Canup, the new simulations give a particularly good account of the characteristics of Phobos and Deimos (the very low inclination and orbital eccentricity of the orbits of the two satellites), if we imagine an impact with a smaller celestial body than initially considered. It would be enough for a collision between Mars and an object whose size would be between that of Vesta and Ceres, that is to say between 500 and 1,000 kilometers in diameter.
But this implies that the two moons are essentially made of materials from Mars, which would not really stick with the spectral signature of Phobos and Deimos. However, scientists can explain this paradox by involving spatial erosion with the solar wind.
Another consequence of the collision hypothesis is that Phobos and Deimos should be very poor in water since they would come from materials heated at high temperatures by this collision. To really decide between the hypotheses, it would be much better to know the composition of the two moons.
And this is precisely what the MMX (Martian Moons Exploration) mission proposes to do, in particular by bringing back to Earth a sample of at least ten grams of Phobos soil, taken at a depth of at least two centimeters below the surface.