The Red Planet’s two tiny moons—Phobos and Deimos—could have formed after an ancient collision, according to new research. It’s an intriguing possibility, but not everyone is convinced by the evidence.
Phobos and Deimos resemble potatoes (though this may actually be seen as an insult to potatoes). The origin of these malformed moons isn’t entirely clear, but their strange shape, combined with their diminutive size, has led to speculation that they’re captured asteroids. Indeed, Phobos measures 14 miles (23 kilometers) wide and Deimos 7 miles (11 kilometers) wide, so this isn’t an entirely outlandish idea.
Other factors need to be considered, however, like their unusual compositions (they’re very unlike Mars from a geological perspective) and their unexpected orbits. Indeed, captured asteroids should have elongated orbits and random angles of inclination, neither of which apply to Phobos or Deimos. Instead, both moons feature exceptionally circular orbits which are aligned above the Red Planet’s equatorial plane.
Another possibility is that the two moons are the shattered remnants of an ancient collision, a hypothesis considered by Amirhossein Bagheri, a doctoral student at ETH Zurich and the lead author of a 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, the scientists ran them backwards to track the historical movements of the moons over time. Sure enough, the simulations showed that Phobos and Deimos did in fact cross paths.
This implies that the “moons were very likely in the same place and therefore have the same origin,” explained study co-author Amir Khan, a senior scientist at the University of Zurich and ETH Zurich, in a statement. By “same origin” Khan is referring to a single parent object—a larger Martian moon that no longer exists—which splintered after getting struck by a celestial object, like an asteroid or comet. Bagheri said Phobos and Deimos “are the remainders of this lost moon,” which they argue was in a near-synchronous orbit (i.e. an orbital period that matches the rotational rate of the planet) around Mars.
To run these simulations, however, the team had to figure out how Mars and the two moons interacted over the eons, including the tidal forces at play and the resulting dissipation of energy.
Thanks to NASA’s InSight probe, and its ability to monitor seismic activity on Mars, scientists have an improved understanding of what’s happening beneath the Martian surface. The same cannot be said, however, for the Martian moons, but scientists do have photos and measurements gathered by remote sensing. Phobos and Deimos are probably like Swiss cheese, filled with lots of cavities, a portion of which may contain water ice.
Equipped with their updated variables, the scientists ran the models, showing the birth of the two moons occurred at some point between 1 billion and 2.7 billion years ago. The rather large discrepancy has to do with uncertainties about the porousness of the two moons. Better data could clear this up, and the good news is that the Japanese space agency is planning a mission to Phobos, called Martian Moons Exploration, in which a probe will return surface samples at some point later this decade.
While the study offers an intriguing look at the moons’ history, Matija Ćuk, a research scientist at the SETI Institute, wasn’t convinced by the findings.
“The authors do have a detailed model for tides within Mars, but really stretch the physics when it comes to tides within the moons,” he said in an email. “More importantly, their scenario makes no sense as they project Phobos’s and Deimos’s orbits to overlap billions of years ago at [a very] high relative velocity—much more than what is expected by the breakup of a combined body.”
Ćuk said the idea of a single Martian moon in a near-synchronous orbit some 2 billion to 3 billion years ago is “probably not plausible,” as the moon would rapidly move away from such a position, and ultimately “does not help resolve any of the questions people have about the origin of Phobos and Deimos,” he said.
To which he added: “I am surprised this paper got accepted in a high-profile journal like Nature Astronomy.”
Ćuk, along with colleague David Minton from Purdue University, have their own ideas about Phobos and Deimos. In research published last year, the scientists provided further proof that Phobos is caught in a cycle of death and rebirth that periodically and temporarily produces rings around the Red Planet. These rings eventually spawn brand new moons, in what is an alternative explanation for how Mars got its moons.