The Martian moons have a common ancestor

The artist’s impression of the collision of the Martian ancestor and the asteroid, which could have led to the formation of Phobos and Deimos. Credits: Mark Garlick /

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

This has led people to suspect that it could actually be asteroids trapped in Mars ’gravitational field. “But that’s where the problems started,” Bagheri says. It would be expected that the captured objects would follow an eccentric orbit around the planet and that orbit would be at a random angle. Contrary 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 take orbits and their changes back to the past,” says Amir Khan, a senior scientist at the Institute of Physics at the University of Zurich and the Institute of Geophysics at ETH Zurich. It turned out that the paths of Phobos and Deimos crossed in the past. “That means the months were very likely to be in the same place and therefore have the same origin,” Khan says. The researchers concluded that a larger celestial body was then orbiting Mars. This original month was probably hit by another body and disintegrated as a result. “Phobos and Deimos are remnants of this lost month,” says Bagheri, lead author of the study, which is now published in the journal Astronomy of nature.

Although easy to follow, these conclusions required extensive preliminary work. First, researchers had to perfect an existing theory describing the interaction between the moon 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 extent of which depends on the size of the body, their internal composition, and not the smallest distance between them.

An insight into the interior of Mars and its moons

Mars Mars is currently being explored by NASA’s InSight mission, with the participation of ETH Zurich: electronics for the mission’s seismometer, which records markers and possible meteorite impacts, was built at ETH. “These images allow us to look inside the Red Planet,” says Khan, “and these data are used to limit the Mars model in our calculations and the scattering that occurs within the Red Planet.”

Images and measurements of other probes on Mars suggest that Phobos and Deimos are made of highly porous material. With less than 2 grams per cubic centimeter, their density is much lower than the average density of the Earth, which is 5.5 grams per cubic centimeter. “There are a lot of cavities in Phobos where water ice can be found,” Khan suspects, “and there the tides cause a lot of energy to be wasted.”

Using these discoveries and their sophisticated theory of tidal effects, the researchers conducted hundreds of computer simulations to track the orbits of the moon back in time until they reached the crossroads – the moment Phobos and Deimos were born. Depending on the simulation, this moment lies in the past between 1 and 2.7 billion years ago. “The exact time depends on the physical properties of Phobos and Deimos, that is, how porous they are,” Bagheri says. A Japanese probe scheduled for launch in 2025 will explore Phobos and return samples to Earth. The researchers expect that these samples will provide the necessary details about the interior of the Martian months that will allow more accurate calculations of their origin.

Near Phobos

Another thing their calculations show is that the common ancestor of Phobos and Deimos was further away from Mars than Phobos is today. Although the smaller Deimos remained near where it originated, tidal forces are causing the larger Phobos to approach Mars – and this process is ongoing, the researchers explain. Their computer simulations also show the future development of the moon’s orbits. It seems that Deimos will move away from Mars very slowly, just as our moon is slowly receding from Earth. However, Phobos will collapse on Mars in less than 40 million years, or gravitational forces will tear it apart as it approaches Mars.

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More information:
Amirhossein Bagheri et al. Dynamic evidence for Phobos and Deimos as remnants of a disturbed common ancestor, Astronomy of nature (2021). DOI: 10.1038 / s41550-021-01306-2

Citation: Martian months have a common ancestor (2021, February 23) downloaded February 23, 2021 from

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