Martians have a common ancestor

Mars ‘two moons, Phobos and Deimos, have amazed scientists since their discovery in 1877. They are very small: Phobos’ diameter of 22 kilometers is 160 times smaller than that of our moon, and Deimos is even smaller, with a diameter of only 12 kilometers. . “Our moon is essentially spherical, while the moons of Mars are very irregularly shaped – like potatoes”, says Amirhossein Bagheri, a doctoral student at the Institute of Geophysics at the ETH Zurich, adding: ‘Phobos and Deimos look more on asteroids than on natural moons. . ”

This led people to suspect that it could in fact be asteroids caught in Mars’ gravitational field. “But this is where the problems started,” Bagheri says. Captured objects are expected to follow an eccentric orbit around the planet, and the orbit will be randomly inclined. Contrary to this hypothesis, the orbits of the Martians are almost circular and move in the equatorial plane of Mars. So what is the explanation for the current orbits of Phobos and Deimos? To solve this dynamic problem, the researchers relied on computer simulations.

Calculating the past

“The idea was to track the orbits and their changes in the past,” says Amir Khan, a senior scientist at the Institute of Physics of the University of Zurich and the Institute of Geophysics at the ETH Zurich. It seems that the paths of Phobos and Deimos have crossed in the past. “This means that the moons were most likely in the same place and therefore have the same origin,” says Khan. The researchers concluded that a larger celestial body orbited Mars at the time. This original moon was probably hit by another body and disintegrated as a result. “Phobos and Deimos are the remnants of this lost moon,” says Bagheri, who is lead author of the study now published in the journal. Natural Astronomy.

Although easy to follow, these conclusions need extensive preliminary work. First, the researchers had to refine the existing theory that describes the interaction between the moons and Mars. “All the celestial bodies exert tidal forces on each other,” Khan explains. These forces lead to a form of energy conversion, known as dissipation, the scale of which depends on the size of the bodies, their inside composition and not least the distances between them.

Insights into the interior of Mars and its moons

Mars is currently being explored by NASA’s InSight mission, with the involvement of ETH Zurich: the electronics for the mission’s seismometer, which are recordings of marsquakes and possible meteorite impact, were built at ETH. “With these surveys, we can look at the Red Planet,” says Khan, “and these data are used to limit the Mars model in our calculations and the distribution that occurs within the red planet.”

Images and measurements by other Mars probes suggested that Phobos and Deimos were made of very porous material. At less than 2 grams per cubic centimeter, its density is much lower than the average density of the earth, which is 5.5 grams per cubic centimeter. “There are many cavities in Phobos, which can contain water ice,” Khan suspects, “and this is where the tides dissipate a lot of energy.”

Using these findings and their refined theory of the tidal effects, the researchers performed hundreds of computer simulations to track the orbits of the moons backwards in time until they reached the intersection – the moment Phobos and Deimos were born. Depending on the simulation, this time is between 1 and 2.7 billion years in the past. “The exact time depends on the physical properties of Phobos and Deimos, that is, how porous it is,” says Bagheri. A Japanese probe launched in 2025 will explore Phobos and return monsters to Earth. The researchers expect that these samples will provide the necessary details about the inside of the Martians, which will allow the accurate calculation of their origin.

The end of Phobos

Another thing their calculations show is that the common ancestor of Phobos and Deimos was farther from Mars than Phobos is today. While the smaller Deimos remained in the vicinity of the place where they originated, tidal forces caused the larger Phobos to approach Mars – and this process continues, as the researchers explain. Their computer simulations also show the future development of the moons’ orbits. It looks like Deimos will be moving very slowly away from Mars, just as our moon is slowly moving away from Earth. However, phobos will crash on Mars in less than 40 million years or be torn apart by the gravitational forces when it comes close to Mars.