Martian moons have a common ancestor

Artist’s impression of the collision between a primordial Martian moon and an asteroid, which could have led to the formation of Phobos and Deimos. Credit: Mark Garlick /

The two moons of Mars, Phobos and Deimos, have puzzled researchers since their discovery in 1877. They are very small: Phobos’s 22-kilometer diameter is 160 times smaller than our moon’s, and Deimos is even smaller. with a diameter of only 12 kilometers. . “Our moon is essentially spherical, while the moons of Mars have very irregular shapes, like potatoes,” says Amirhossein Bagheri, PhD student at the ETH Institute of Geophysics in Zurich, adding: “Phobos and Deimos are more alike. asteroids than natural moons. ”

This led people to suspect that, in fact, they could be asteroids captured in the gravitational field of Mars. “But that’s where the problems started,” says Bagheri. Captured objects would be expected to follow an eccentric orbit around the planet, and that orbit would have a random tilt. In contradiction to this hypothesis, the orbits of the 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 track the orbits and their changes in the past,” says Amir Khan, senior scientist at the Institute of Physics at the University of Zurich and the Institute for Geophysics at ETH Zurich. As it turned out, the orbits of Phobos and Deimos seemed to have crossed in the past. “This means that it is very likely that the moons were in the same place and therefore have the same origin,” says Khan. The researchers concluded that at that time a larger celestial body was orbiting Mars. This original moon was likely hit by another body and disintegrated as a result. “Phobos and Deimos are the remains of this lost moon,” says Bagheri, lead author of the study now published in the journal. Nature astronomy.

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

Information about the interior of Mars and its moons

Mars is currently being explored by NASA’s InSight mission, with the participation of ETH Zurich – the mission’s seismometer electronics, which records earthquakes and possibly meteorite impacts, were built at ETH. “These recordings allow us to look inside the Red Planet,” says Khan, “and this data is used to constrain the model of Mars in our calculations and the dissipation that occurs within the red planet.”

Images and measurements from other Mars probes have suggested that Phobos and Deimos are made of highly porous material. At less than 2 grams per cubic centimeter, its density is much lower than the average density on Earth, which is 5.5 grams per cubic centimeter. “There are many cavities within Phobos, which may contain icy water,” Khan suspects, “and that’s where the tides are causing a lot of energy to dissipate.

Using these findings and their refined theory of tidal effects, the researchers ran hundreds of computer simulations to track the orbits of the moons back in time until they reached the intersection, the moment Phobos and Deimos were born. Depending on the simulation, this point in 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 they are,” says Bagheri. A Japanese probe scheduled for launch in 2025 will explore Phobos and return samples to Earth. The researchers hope that these samples will provide the necessary details about the interior of the 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 smallest Deimos has remained in the vicinity of where it was born, the forces of the tides are causing the largest phobos to approach Mars, and this process is ongoing, as the researchers explain. Their computer simulations also show the future development of the moons’ orbits. It seems that Deimos will be moving away from Mars very slowly, just like our moon is slowly moving away from Earth. Phobos, however, will crash into Mars in less than 40 million years or be ripped apart by gravitational forces as it approaches Mars.

The orbit of the Martian Moon points to an ancient ring of Mars

More information:
Amirhossein Bagheri et al. Dynamic evidence of Phobos and Deimos as remnants of an interrupted common parent, Nature astronomy (2021). DOI: 10.1038 / s41550-021-01306-2

Citation: Martian moons have a common ancestor (2021, February 23) retrieved on February 23, 2021 from

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