Mars’ moon Phobos may actually be a comet — or at least part of one — that was gravitationally captured by the Red Planet long ago, suggests a new preprint study based on previously unpublished photos.
For years, researchers have puzzled over the origins of Phobos and its twin, Deimos. Some theorize that the moons are former asteroids attracted to them Marsgravity because their chemical composition is similar to that of some rocks in the main asteroid belt between Mars and Jupiter. However, computer models simulating this capture process have failed to reproduce the pair’s near-circular paths around Mars.
Another hypothesis suggests that a gigantic impactlike that which created ours moon, dug up the duo from the Red Planet; but Phobos has a different chemical composition than Mars, making this scenario unlikely as well.
Finding out exactly how Phobos was born is one of the goals of the Japan Aerospace Exploration Agency Martian Moons eXploration (MMX) mission.scheduled to launch in 2026. Sonya Fornazier, professor of astronomy at Paris Cité University and lead author of the new study, is the instrument scientist for the MMX mission. As she and other scientists analyzed images to fine-tune the spacecraft’s planned path, Fornazier came across unpublished photographs.
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Captured by high definition cameras on board Mars Express Spaceship, a European Space Agency (ESA) orbiter that has been studying Mars and its moons since 2003, these 300 images exquisitely document the features of Phobos. This includes the 9-kilometer (5.6-mile-wide) Stickney Crater, the largest landmark on Phobos.
Fornasier and her colleagues used the snapshots to analyze the intensity of sunlight reflected off Phobos from different angles. This technique, called photometry, allows them to determine how much light Phobos reflects when the sun is directly in front or at an offset angle.
The researchers discovered that the surface of Phobos does not reflect light evenly. Some regions, such as the northeast rim of the crater, were highly reflective. But the team’s analysis also showed that, overall, the surface of Phobos appears noticeably brighter when the sun was directly overhead. This phenomenon, called opposition, is characteristic of many airless objects in the solar system. In addition, the researchers found that the surface of Phobos is porous, like sand. This led the team to hypothesize that the moon’s surface may be covered in a thick layer of dust with ridged particles whose shadows disappear when directly illuminated.
Both properties are also characteristics of Jupiter-family comets, which are comets whose orbits are gravitationally modified by Jupiter. These include the ESA’s “rubber duckling” Comet 67P The Rosetta mission was closely scrutinized in 2016. In fact, the photometric properties of Phobos match those of Comet 67P almost perfectly. So, the team concluded that Phobos was probably a comet captured by Mars.
The results of the study also have an impact on Deimos. Fornazier noted that if Phobos was once a comet, Deimos might also have been a comet. In fact, based on the study, her team hypothesizes that the two moons may have once been bound together as a single bicuspid comet that was captured and eventually torn apart by Mars’ gravity. In other words, the twin moons of Mars may actually be two halves of a whole.
“If the Martian moons are indeed captured comets, this means that comets can also be captured by tellurics [terrestrial] planets,” Fornazier added. She said that some moons of gas giants like Saturn probably came from The Kuiper Belt, the doughnut-shaped region that encompasses the Solar System and from which many comets originate. However, astronomers have not identified a “cometary moon” for terrestrial planets until now, making Phobos a potential first.
However, the interpretation of the comet also has problems. Some photometric parameters, such as the fraction of scattered light, do not match those of comets. In any case, Fornasier said, the dynamical simulations — which take into account the motions of celestial objects including Mars and Phobos — will help the team determine the likelihood of such a comet capture. Ultimately, though, the MMX program, which will take physical samples from Phobos, is probably the best hope for solving the dark origins of this mysterious moon.
The new research is forthcoming in the journal Astronomy and Astrophysics and available on the arXiv preprint server.
