Mars’ moon Phobos could be a comet — or at least part of it — that was captured by the Red Planet’s gravity long ago, according to a new preprint study based on previously unpublished images.
For years, researchers have puzzled over the origins of Phobos and its twin brother, Deimos. Some have theorized that the moons are former asteroids that were lured by them Mars‘gravity, because their chemical composition is similar to that of certain rocks in the main asteroid belt between Mars and Jupiter. However, computer models simulating this recording process have failed to replicate the pair’s nearly circular paths around Mars.
Another hypothesis suggests that a gigantic impactlike that which created our Moon, took the duo out of the Red Planet; but Phobos has a different chemical composition than Mars, making this scenario also unlikely.
Finding out exactly how Phobos formed is one of the goals of the Japan Aerospace Exploration Agency Mars Moons eXploration (MMX) missionscheduled for launch in 2026. Sonia Fornasier, professor of astronomy at Paris Cité University and lead author of the new study, is an instrument scientist for the MMX mission. As she and other scientists analyzed images to refine the spacecraft’s planned path, Fornasier came across unpublished photos.
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Taken by high-resolution cameras on board the Mars Express spacecraft, a European Space Agency (ESA) orbiter that has been studying Mars and its moons since 2003, these more than 300 images do an excellent job of documenting the features of Phobos. This includes the 9-kilometer-wide Stickney Crater, Phobos’ largest landmark.
Fornasier and her colleagues used the snapshots to analyze the intensity of sunlight reflected by Phobos from different angles. This technique, called photometry, allowed them to determine how much light Phobos reflected when the sun was directly in front of it or at an oblique angle.
The researchers found that Phobos’ surface did not reflect light evenly. Some regions, such as the northeastern rim of the crater, were highly reflective. But the team’s analysis also found that Phobos’ surface appeared noticeably brighter overall the sun was directly above the head. This phenomenon, called an opposition wave, is characteristic of many airless objects in the solar system. The researchers also discovered that the surface of Phobos was porous, like sand. This led the team to suggest that the moon’s surface may be covered by a thick layer of dust with striated particles, whose shadows disappear when illuminated directly.
Both features are also characteristics of comets of the Jupiter family, which are comets whose orbits are adjusted by Jupiter’s gravity. These include the “rubber ducky” Comet 67P, from ESA The Rosetta mission was studied closely in 2016. In fact, the photometric properties of Phobos matched those of comet 67P almost perfectly. The team thus concluded that Phobos may have been a comet captured by Mars.
The study’s findings also have implications for Deimos. Fornasier noted that if Phobos was once a comet, perhaps Deimos was one too. Based on the research, her team even suggests that the two moons were once connected as one bilobed comet, which became stuck and eventually torn apart by Mars’ gravity. In other words, Mars’ twin moons may actually be two halves of one whole.
‘If the Mars satellites are indeed captured comets, this implies that comets can also be captured by telluric satellites. [terrestrial] planets,” Fornasier added. She said some moons of gas giants like Saturn likely come from the Kuiper belt, the doughnut-shaped region that envelops the solar system and from which many comets emerge. However, astronomers have not previously identified a ‘cometary moon’ for terrestrial planets, making Phobos a potential first.
Nevertheless, comet interpretation also has problems. Some photometric parameters, such as the fraction of scattered light, do not match those of comets. In any case, Fornasier says, dynamic simulations – which take into account movements of celestial bodies including Mars and Phobos – will help the team determine the likelihood of such a cometary inclusion. But ultimately, the MMX program, which will physically sample chunks of Phobos, is probably the best hope for solving the murky origins of this mysterious moon.
The new study appears in the journal Astronomy and Astrophysics available on the preprint server arXiv.