Mars’ moon Phobos may actually be a comet—or at least part of it—that was gravitationally captured by the Red Planet long ago, suggests a new preprint study based on previously unpublished photographs.
Scientists have puzzled over the origin of Phobos and its twin Deimos for years. Some believe that the moons are former asteroids that they attracted Mars‘ gravity 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 were unable to replicate the pair’s near-circular paths around Mars.
Another hypothesis suggests that a huge impactlike the one that created ours Moon, a duo from the Red Planet; but Phobos has a different chemical composition than Mars, so this scenario is also unlikely.
Finding out exactly how Phobos was born is one of the goals of the Japan Aerospace Exploration Agency The Martian Moons eXploration (MMX) mission.whose launch is planned for 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. While she and other scientists were analyzing the images to fine-tune the spacecraft’s planned path, Fornasier came across the unpublished photographs.
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Captured by high definition cameras on board Mars Express spacecraft, the European Space Agency (ESA) orbiter that has been studying Mars and its moons since 2003, these 300-odd images exquisitely document the features of Phobos. This includes the 9-kilometer-wide (5.6-mile) Stickney Crater, the largest landmark on Phobos.
Fornasier and her colleagues used the images to analyze the intensity of Phobos’ sunlight reflected from different angles. This technique, called photometry, allowed them to determine how much light Phobos reflected when the sun was directly in front of or below the offset angle.
Scientists have discovered that the surface of Phobos does not reflect light evenly. Some areas, such as the northeast rim of the crater, were highly reflective. However, the team’s analysis also showed that, overall, the surface of Phobos appeared noticeably brighter sun was directly overhead. This phenomenon, called opposition accretion, is characteristic of many solar system objects without air. Scientists also discovered that the surface of Phobos was porous, like sand. This led the team to believe that the moon’s surface may 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 comets in the Jupiter family, which are comets whose orbits are gravitationally modified by Jupiter. These include ESA’s Comet 67P “rubber duck”. The Rosetta mission studied up close in 2016. In fact, the photometric properties of Phobos matched Comet 67P almost perfectly. So the team concluded that Phobos was possibly a comet captured by Mars.
The study’s findings have implications for Deimos as well. Fornasier noted that if Phobos was once a comet, Deimos may have been as well. In fact, based on the study, her team suggests that the two moons may have once been joined together as a single two-lobed comet that was trapped and eventually torn apart by Mars’ gravity. In other words, Mars twins may actually be two halves of one whole.
“If the Martian satellites are indeed captured comets, this means that comets can also be captured by telluric [terrestrial] Planets,” Fornasier added. She said some of the moons of gas giants like Saturn likely came from Kuiper belt, the donut-shaped region that surrounds the solar system and from which many comets originate. However, astronomers have yet to identify a “comet moon” for the terrestrial planets, so Phobos is 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 physically sample chunks of Phobos, is probably the best hope for solving the obscure origins of this mysterious moon.
The new study is available in the journal Astronomy and Astrophysics and available on the arXiv preprint server.