The Martian 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of one — that was gravitationally captured by the Red Planet long ago, suggests a new reprint study based on previously unpublished photographs.
For years, researchers have pondered the origins of Phobos and its twin, Deimos. Some have theorized that the moons are former asteroids that it lured 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 replicate the pair’s near-circular orbits around Mars.
Another hypothesis suggests that a a giant influencelike the one who created ours month, was dug out by a duo from the Red Planet; but Phobos has a different chemical composition than Mars, which makes this scenario unlikely as well.
Finding out exactly how Phobos was born is one of the goals of the Japan Aerospace Exploration Agency The Martian Moons eXploration (MMX) mission.which should be launched in 2026. Sonia Fornasier, 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As she and other scientists analyzed the images to fine-tune the spacecraft’s planned trajectory, Fornasier came across unpublished photographs.
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Filmed by high resolution cameras on board Mars Express spacecraft, the European Space Agency’s (ESA) orbiter that has been studying Mars and its moons since 2003, these 300 or so images document Phobos’ characteristics beautifully. These include the 9-kilometer-wide (5.6-mile-wide) Crater Stickney, the largest landmark on Phobos.
Fornasier and her colleagues used the images to analyze the intensity of sunlight reflected by Phobos from different angles. This technique, called photometry, allowed them to determine how much light Phobos reflects when the sun is directly in front of, or at an angle away from, the sun.
The researchers discovered that Phobos’ surface does not reflect light evenly. Some areas, such as the northeast rim of the crater, were highly reflective. But the team’s analysis also found that overall, Phobos’ surface looked noticeably brighter than before Sun was directly overhead. This phenomenon, called opposition surge, is characteristic of many solar system objects without air. Also, the researchers found that Phobos’ surface is porous, like sand. This led the team to suggest that the moon’s surface might be covered in a thick layer of dust with ridged particles, whose shadows disappeared when directly illuminated.
Both of these properties are also features of Jupiter-family comets, comets whose orbits are gravitationally modified by Jupiter. These include the “rubber duck” Comet 67P, which ESA’s The Rosetta mission seen up close in 2016. In fact, Phobos’ photometric properties matched those of Comet 67P almost perfectly. Therefore, the team concluded that Phobos was probably a comet captured by Mars.
The study’s findings also have implications for Deimos. Fornasier noted that if Phobos was once a comet, Deimos could also be a comet. In fact, based on the study, her team suggests that the two moons may have once been joined as a single two-armed comet that was captured and eventually torn apart by Mars’ gravity. In other words, Mars’ twin moons may actually be two halves of one whole.
“If the satellites of Mars are indeed captured comets, this implies that comets can also capture tellurics [terrestrial] planet,” added Fornasier. She said that some moons of gas giants like Saturn probably originated from Kuiper belt, the donut-shaped region that encompasses the solar system and from which many comets originate. However, astronomers have not yet identified a “comet moon” for the terrestrial planets, making Phobos a potential first.
Nevertheless, the interpretation of comets also has problems. Some photometric parameters, such as the fraction of scattered light, do not correspond to those of comets. Either way, Fornasier said, dynamical simulations — which consider the motion of celestial bodies, including Mars and Phobos — will help the team determine the likelihood of such cometary capture. Ultimately, however, the MMX program, which will physically sample bits of Phobos, is probably the best hope for solving the murky origins of this mysterious moon.
The new study will soon be published in the journal Astronomy and Astrophysics, i available on the arXiv reprint server.