Martian moons have a common ancestor

The two moons of Mars, Phobos and Deimos, have puzzled researchers since their discovery in 1877. They are very small: the diameter of Phobos 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 irregular in shape – like potatoes,” explains Amirhossein Bagheri, a doctoral student at the Institute of Geophysics at ETH Zurich, adding: “Phobos and Deimos are alike more to asteroids than natural moons. ”

This has led people to suspect that it could in fact be asteroids captured in the gravity field of Mars. “But that’s where the problems started,” Bagheri says. The captured objects would have to follow an eccentric orbit around the planet, and that orbit would be at a random tilt. Contrary to this hypothesis, the orbits of Martian moons 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 trace the orbits and their changes in the past,” explains Amir Khan, senior scientist at the Institute of Physics at the University of Zurich and at the Institute of Geophysics at ETH Zurich. It turned out that the eye sockets of Phobos and Deimos appeared to 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 was orbiting Mars at the time. This original moon was probably struck by another body and disintegrated as a result. “Phobos and Deimos are the remnants of that lost moon,” says Bagheri, lead author of the study now published in the journal Nature astronomy.

Although easy to follow, these findings required extensive preliminary work. First, the researchers had to refine the existing theory describing the interaction between the moons and Mars. “All celestial bodies exert tidal forces on each other,” Khan explains. These forces lead to a form of energy conversion called dissipation, the scale of which depends on the size of the bodies, their interior composition and especially the distances between them.

Insight 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 of the mission’s seismometer, which records mars tremors and possibly meteorite impacts, was built at ETH . “These recordings allow us to look inside the Red Planet,” Khan says, “and this data is used to constrain the Mars model in our calculations and the dissipation occurring inside the Red Planet.”

Images and measurements taken by other Mars probes have suggested that Phobos and Deimos are made of a very porous material. At less than 2 grams per cubic centimeter, their density is much lower than the Earth’s average density, which is 5.5 grams per cubic centimeter. “There are a lot of cavities inside Phobos, which could contain water ice,” Khan suspects, “and this is where the tides disperse a lot of energy.”

Using these results and their refined theory of the effects of tides, the researchers ran hundreds of computer simulations to track the moons’ orbits back in time until they reach the intersection – the point at which Phobos and Deimos were born. According to the simulation, this moment 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, their porosity,” Bagheri explains. A Japanese probe slated for launch in 2025 will explore Phobos and return samples to Earth. The researchers expect these samples to provide the necessary details of the interior of Martian moons that will allow more precise calculations of their origin.

The end of Phobos

Another thing their calculations show is that the common ancestor of Phobos and Deimos was further from Mars than Phobos is today. While the smaller Deimos has remained close to where it was created, tidal forces are pushing the larger Phobos to approach Mars – and this process is ongoing, as the researchers explain. Their computer simulations also show the future development of the orbits of the moons. It looks like Deimos will move away from Mars very slowly, just as our moon is slowly moving away from Earth. Phobos, however, will crash into Mars in less than 40 million years or be torn apart by gravitational forces as it approaches Mars.