Scientists have been predicting the demise of the Martian moon Phobos and its rebirth as a planetary ring for a while now. However, a new study suggests that this isn't the first time it's headed for destruction – nor is it likely to be the last.

Compared to our moon and other spherical satellites like it, Phobos and its little brother Deimos are tiny potato-shaped oddities that have caused a fair bit of head scratching among scientists. Heavily cratered and among the least reflective objects in the Solar System, their origins remain a mystery.

Scientists have posited that they are composed of rubble comprising Type I or II carbonaceous chondrites, i.e. the same material found in asteroids and dwarf planets. While multiple hypotheses abound as to their formation, one in particular suggests that they were created when the impact that led to the formation of Mars' North Polar Basin or Borealis Basin sent debris hurtling into space.

While some scientists have suggested that this led directly to the formation of Phobos, Purdue University scientists David Minton and Andrew Hesselbrock are proposing an alternative theory: that the debris first formed a ring before clumping together to form a moon. In other words, Phobos spends its time alternating between being a planetary ring and a moon. Both Minton and Hesselbrock believe that the process was kickstarted around 4.3 billion years ago with the formation of the North Polar Basin, which covers about 40 percent of the planet in its northern hemisphere.

"That large impact would have blasted enough material off the surface of Mars to form a ring," says Hesselbrock, a doctoral student in physics and astronomy.

According to the authors' model, the rubble forming the ring eventually drifted away from the planet and began to clump, ultimately forming a moon. Due to the fact that Phobus' orbit brings it closer to Mars every year, the process eventually repeats itself and the moon disintegrates when it passes the Roche limit, the minimum distance it can approach the planet without being torn apart by the latter's tidal forces.

With each cycle, the new satellite that forms is five times smaller than its predecessor and the authors believe that Phobus has probably gone through this process three to seven times. At the moment, it is in the latest stage of this cycle and it's expected to break apart, become a set of rings again and send debris raining down on the planet when it reaches the Roche limit again in 70 million years. This model would also explain Mars' topographical quirks.

"You could have had kilometer-thick piles of moon sediment raining down on Mars in the early parts of the planet's history, and there are enigmatic sedimentary deposits on Mars with no explanation as to how they got there," says Minton, an assistant professor of Earth, atmospheric and planetary sciences at Purdue. "And now it's possible to study that material."

Explaining why he doesn't think that Phobos was formed as a direct result of the impact on Mars 4.3 billion years ago, Minton doesn't think it would have been possible for the moon to survive in one piece for such a long period of time. To do so, it would have had to form far from Mars and cross through the resonance of Deimos. Owing to the gravitational influence they would have had on each other, Deimos' orbit would have been affected. But nothing of the sort has happened.

"Not much has happened to Deimos' orbit since it formed," Minton says. "Phobos passing through these resonances would have changed that."

To further test their model, Minton and Hesselbrock plan to study either the dynamics of the first set of rings or the debris left behind on Mars by previous moons.

This study was funded by NASA and published in Nature Geoscience.

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