Computer simulations suggest Mars moons result from ancient collision

According to new research, the two tiny moons of the Red Planet – Phobos and Deimos – could have formed after an ancient collision. It’s an intriguing possibility, but not everyone is convinced by the evidence.

Phobos and Deimos look like potatoes (although that could actually be considered an insult to potatoes). The origin of these malformed moons is not entirely clear, but their odd shape, combined with their small size, has led to speculation that they were captured asteroids. Indeed, Phobos is 23 kilometers wide and Deimos 7 miles (11 kilometers) wide, so it’s not a totally extravagant idea.

Other factors must be taken into account, however, such as their unusual compositions (they are very different from Mars from a geological point of view) and their unexpected orbits. This is because the captured asteroids must have elongated orbits and random tilt angles, which do not apply to either Phobos or Deimos. Instead, the two moons feature unusually circular orbits that are aligned above the equatorial plane of the Red Planet.

Another possibility is that the two moons are the shattered remains of an ancient collision, a hypothesis envisioned by Amirhossein Bagheri, a doctoral student at ETH Zurich and lead author of New Nature Astronomy. paper on the subject.

Bagheri and his colleagues ran computer simulations of the two moons, but instead of running their models forward from a presupposed set of conditions, scientists ran them backwards to track the historical movements of the moons over the course of time. time. Sure enough, the simulations showed that Phobos and Deimos actually crossed paths.

This implies that “the moons were most likely in the same place and therefore have the same origin,” said study co-author Amir Khan, senior researcher at the University of Zurich and ETH Zurich, in a report. declaration. By “same origin,” Khan refers to a single parent object – a larger Martian moon that no longer exists – that shattered after being struck by a celestial object, such as an asteroid or comet. Bagheri said that Phobos and Deimos “are the remnants of that lost moon,” which they said was in a near-synchronous orbit (i.e. an orbital period that corresponds to the rate of rotation of the planet) around Of March.

To run these simulations, however, the team had to understand how Mars and the two moons interacted over eons, including the tidal forces involved and the resulting energy dissipation.

Thanks to NASA’s InSight and its ability to monitor seismic activity on Mars, scientists have a better understanding of what is going on beneath the Martian surface. The same cannot be said of the Martian moons, but scientists have photos and measurements collected by remote sensing. Phobos and Deimos are probably like Swiss cheese, filled with many cavities, part of which may contain water ice.

Equipped with their updated variables, the scientists ran the models, showing that the birth of the two moons occurred at some point between 1 billion and 2.7 billion years ago. The fairly large difference is linked to the uncertainties about the porosity of the two moons. Better data could clarify this, and the good news is that the Japanese space agency is planning a mission to Phobos, called Exploration of the Martian moons, in which a probe will return surface samples at some point later this decade.

While the study offers an intriguing look at the history of the moons, Matija Ćuk, a researcher at the SETI Institute, was not convinced by the results.

“The authors have a detailed model of the tides on Mars, but are really expanding the physics regarding tides in moons,” he said in an email. “More importantly, their scenario makes no sense as they project the orbits of Phobos and Deimos to overlap billions of years ago to [a very] high relative velocity – much more than one would expect from the failure of a combined body. “

Ćuk said the idea of ​​a single Martian moon in a near-synchronous orbit about 2-3 billion years ago is “probably not plausible,” as the moon would quickly move away from such a position, and ultimately “does not help to solve questions that people have on the origin of Phobos and Deimos, ”he said.

To which he added: “I am surprised that this article has been accepted in a high level journal like Nature Astronomy.”

Ćuk, along with his colleague David Minton from Purdue University, have their own ideas about Phobos and Deimos. In search published last year, scientists provided further evidence that Phobos is caught in a cycle of death and rebirth that periodically and temporarily produces rings around the red planet. These rings ultimately spawn whole new moons, in what is an alternate explanation for how Mars got its moons.