Martian moon's orbit suggests the Red Planet had a ring

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An artist's illustration of Mars with rings and part of the solar system's ringed planet club. A new study suggests that the Martian moon Deimos may have once been a ring.
An artist's illustration of Mars with rings and part of the solar system's ringed planet club. A new study suggests that the Martian moon Deimos may have once been a ring. (Image credit: Future)

A cycle of moon formation could explain the slightly tilted orbit of Mars' moon Deimos.

Mars has two moons circling the planet, called Phobos and Deimos. For many years, scientists supposed that both of these moons were captured asteroids, or space rocks. But new research shows the orbit of Deimos would not make that possible.

Deimos is very slightly tilted to the Martian equator, by only two degrees. Initially, the difference was so small that many scientists overlooked the matter.

Related: Moons of Mars: Amazing Photos of Phobos and Deimos

"The fact that Deimos' orbit is not exactly in plane with Mars' equator was considered unimportant, and nobody cared to try to explain it," study lead author Matija Cuk, a research scientist at the SETI Institute, said in a statement. "But once we had a big new idea and we looked at it with new eyes, Deimos' orbital tilt revealed its big secret."

The secret came from looking at the motions of Phobos, which orbits closer to the Martian surface and is slowly spiraling into the planet. Eventually, Phobos will drop so close to Mars that the gravity of the much larger planet will pull the moon into pieces — forming a ring.

Study co-authors David Minton, a professor at Purdue University, and Andrew Hesselbrock, who was his graduate student at the time of the research, suggest that Phobos' future is not a one-off event. Instead, after the moon is pulled apart, eventually the pieces will reform into another moon. This not only will happen to Phobos, but has happened already other times in the Martian past.

This breaking up and reforming of moons would in turn explain how Deimos' orbital tilt happened. 

Related: A moon of Mars may have once been rings (& might be again) 

Images captured by NASA's Mars Reconnaissance Orbiter reveal that the surface of Deimos is mostly smooth, marred only by recent impact craters.

Images captured by NASA's Mars Reconnaissance Orbiter reveal that the surface of Deimos is mostly smooth, marred only by recent impact craters. (Image credit: NASA/JPL/University of Arizona)

"This cyclic Martian moon theory has one crucial element that makes Deimos’ tilt possible: a newborn moon would move away from the ring and Mars ... in the opposite direction from the inward spiral Phobos is experiencing due to gravitational interactions with Mars," the SETI Institute said in the statement.

"An outward-migrating moon just outside the rings can encounter a so-called orbital resonance, in which Deimos' orbital period is three times that of the other moon," the institute added. "We can tell that only an outward-moving moon could have strongly affected Deimos, which means that Mars must have had a ring pushing the inner moon outward."

This theoretical outward-moving moon would have been huge, at 20 times more massive than Phobos. Phobos is theorized to be two generations younger than this moon, which broke up and reformed twice — the second time forming Phobos. Also, the age of Phobos favors the theory. Deimos is billions of years old, but Phobos is as young as 200 million years old — meaning it formed when dinosaurs roamed the Earth.

So far, no spacecraft has been able to get up close to either Martian moon to test geological theories, but that could change soon. The Japanese Aerospace Exploration Agency (JAXA) plans to send a mission to Phobos in 2024, called Martian Moons Exploration (MMX). If all goes to plan, MMX will pick up a sample from Phobos to return to Earth.

"I do theoretical calculations for a living, and they are good, but getting them tested against the real world now and then is even better," Cuk said in the statement.

The research was presented at the 236th meeting of the American Astronomical Society, held virtually until June 3. A paper based on the work has been accepted for publication in Astrophysical Journal Letters.

Follow Elizabeth Howell on Twitter @howellspace. Follow us on Twitter @Spacedotcom and on Facebook. 

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Elizabeth Howell
Elizabeth Howell
Staff Writer, Spaceflight

Elizabeth Howell (she/her), Ph.D., was a staff writer in the spaceflight channel between 2022 and 2024 specializing in Canadian space news. She was contributing writer for Space.com for 10 years from 2012 to 2024. Elizabeth's reporting includes multiple exclusives with the White House, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, "Why Am I Taller?" (ECW Press, 2022) is co-written with astronaut Dave Williams. 

3 Comments Comment from the forums
  • rod
    This link has the arxiv report too. https://ui.adsabs.harvard.edu/abs/2020arXiv200600645C/abstract The abstract states ""We numerically explore the possibility that the large orbital inclination of the martian satellite Deimos originated in an orbital resonance with an ancient inner satellite of Mars more massive than Phobos. We find that Deimos's inclination can be reliably generated by outward evolution of a martian satellite that is about 20 times more massive than Phobos through the 3:1 mean-motion resonance with Deimos at 3.3 Mars radii. This outward migration, in the opposite direction from tidal evolution within the synchronous radius, requires interaction with a past massive ring of Mars. Our results therefore strongly support the cyclic martian ring-satellite hypothesis of Hesselbrock and Minton (2017). Our findings, combined with the model of Hesselbrock and Minton (2017), suggest that the age of the surface of Deimos is about 3.5-4 Gyr, and require Phobos to be significantly younger."

    The surface age of Deimos is critical here and reconciling the age differences between Phobos and Deimos. I am glad to see *we numerically explore the possibility* in reports like this. The space.com article stated "Deimos is billions of years old, but Phobos is as young as 200 million years old — meaning it formed when dinosaurs roamed the Earth."
    Reply
  • rod
    FYI, one more note here on this newer model for Deimos and Phobos moons. https://phys.org/news/2020-06-martian-moon-orbit-hints-ancient.html More testing of the model is said to becoming later. That is a very good idea in science. "The discovery of the past orbital resonance all but clinches the cyclic ring-moon theory for Mars. It implies that for much of its history, Mars possessed a prominent ring. While Deimos is billions of years old, Ćuk and collaborators believe Phobos is young as astronomical objects go, forming maybe only 200 million years ago, just in time for the dinosaurs. These theories may be up for some serious testing in a few years, as Japanese space agency JAXA plans to send a spacecraft to Phobos in 2024, which would collect samples from the moon's surface and bring them back to Earth. Ćuk is hopeful that this will give us firm answers about the murky past of the Martian moons: "I do theoretical calculations for a living, and they are good, but getting them tested against the real world now and then is even better."
    Reply
  • Torbjorn Larsson
    Howell's article is not bad, but a more comprehensible one is here: https://gizmodo.com/more-evidence-that-mars-once-had-a-ring-and-will-again-1843888169 .

    As rod notes, a very testable theory. Besides the moon ages, there should be lots of ring debris at the martian equator - AFAIU Curiosity is roaming on top of that!
    Reply