Martian mountains have a common ancestor

An artist’s view of the collision between a Martian primordial moon and an asteroid, which may have led to the creation of Phobos and Deimos. Credit: Mark Garlick /

Mars ‘two moons, Phobos and Deimos, have been explorers since their discovery in 1877. They are very small: Phobos’ diameter of 22 kilometers is 160 times smaller than the moon’s diameter. we have, and Deimos is even smaller, with a diameter of just 12 kilometers. . “Our moon is largely spherical, while Mars’ branches are in a very irregular shape – similar to potatoes,” said Amirhossein Bagheri, a doctoral student at the Institute of Geophysics at ETH Zurich, adding: ” Phobos and Deimos look more like asteroids than natural branches. ”

This led people to suspect that they may have been asteroids captured in Mars’ gravity range. “But that’s where the problems started,” Bagheri says. Captured objects would be expected to follow a strange orbit around the planet, and that orbit would have a random inclination. Contrary to this notion, the orbits of the Martian branches are almost circular and moving in the equatorial plane of Mars. So what is the definition for the ordinary orbit of Phobos and Deimos? To solve this dynamic problem, the researchers relied on computer simulations.

Measuring the past

“The idea was to trace their orbits and their changes back in the past,” said Amir Khan, Senior Scientist at the University of Zurich Institute of Physics and ETH Zurich Institute of Geophysics. As it turned out, Phobos and Deimos’ orbits seemed to have crossed over in the past. “This means that the branches were very similar in the same place and therefore had the same origin,” Khan says. The researchers concluded that a larger celestial body orbiting Mars back. This original moon may have been hit by another group and collapsed as a result. “Phobos and Deimos are the remnants of this lost moon,” said Bagheri, who is the lead author of the study now published in the journal Astronomy of nature.

Although easy to follow, these decisions required extensive initial work. First, the researchers had to update the existing theory describing the interaction between the branches and Mars. “All the square bodies bring together tidal forces,” Khan explains. These forces lead to a type of energy conversion called dissipation, and their scale depends on the size of the body, their internal shape and especially the distances between them.

Views of the interior of Mars and its moon

Mars is currently being studied by NASA’s InSight mission, with the participation of ETH Zurich: the electronics for the mission’s seismometer, which records marsquakes and possibly meteorite effects, were built at ETH. “These recordings allow us to look inside the Red Planet,” Khan says, “and this data is used to limit the Mars model in our calculations and the distribution that takes place within the planet. red planet. “

Images and measurements by other Mars probes have suggested that Phobos and Deimos are made of highly 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 caves inside Phobos, which could be water ice,” Khan suspects, “and that’s where the tides cause a lot of energy to go down.”

Using these conclusions and their revised theory of tidal effects, the researchers ran hundreds of computer simulations to track the orbits of the branches back in time until they reached the inter- cut – the moment Phobos and Deimos are born. By simulation, this point is in the period between 1 and 2.7 billion years in the past. “The exact timing depends on the physical characteristics of Phobos and Deimos, that is, how porous they are” Bagheri says. A Japanese probe launched in 2025 will test Phobos and return samples to Earth. The researchers expect that these samples will provide the necessary detail about the interior of the Martian branches that will allow a more accurate calculation of the origin.

End of Phobos

Another thing their numerical figures show is that the common ancestor of Phobos and Deimos was farther from Mars than Phobos is today. Although the smaller Deimos have stayed close to where they came from, tidal forces are causing the larger Phobos to approach Mars – and this process continues, as the cargo explains research. Their computer simulations also show the future development of the lunar orbits. Deimos seems to be moving away from Mars very slowly, just as our moon is slowly slipping from Earth. Phobos, however, will fall into Mars in less than 40 million years or be torn apart by gravitational forces as they approach Mars.

Martian lunar orbit observations at an ancient ring of Mars

Further information:
Amirhossein Bagheri et al. Dynamical evidence for Phobos and Deimos as remnants of a broken common distributor, Astronomy of nature (2021). DOI: 10.1038 / s41550-021-01306-2

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

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