Mars’ moon Phobos may actually be a comet — or at least part of a comet — that was captured by the red planet long ago, according to a new preprint study based on previously unpublished images.
For many years, researchers have been puzzling over the origins of Phobos and its twin, Deimos. Some have assumed that the moons were former asteroids that were attracted to them MarsGravity, because its 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 not been able to replicate the pair’s near-circular paths around Mars.
Another hypothesis suggests that A Giant effectLike the one who created us moon, uprooting 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 Mars Moon Exploration Mission (MMX).It is scheduled to be launched in 2026. Sonia Fornacer, professor of astronomy at Paris City University and lead author of the new study, is an instrument scientist for the MMX mission. While she and other scientists were analyzing the images to fine-tune the spacecraft’s planned trajectory, Fornacier came across unpublished images.
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It was captured by high-resolution cameras on board the ship Mars Express spacecraft, a European Space Agency (ESA) orbiter that has studied Mars and its moons since 2003, and these 300 images beautifully document Phobos’ features. This includes the 9 km (5.6 mi) wide Stickney Crater, Phobos’ largest feature.
Fornacer and her colleagues used the snapshots to analyze the intensity of sunlight reflected from 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 it or at an opposite angle.
The researchers discovered that the surface of Phobos did not reflect light uniformly. Some areas, such as the northeastern rim of the crater, were highly reflective. But the team’s analysis also showed that overall, the surface of Phobos appeared noticeably brighter the sun It was directly overhead. This phenomenon, called opposition boom, is a characteristic feature of many airless Solar System objects. The researchers also found that Phobos’ surface was porous, like sand. This has led the team to suggest that the moon’s surface may be covered in a thick layer of dust with grooved particles, whose shadows disappear when illuminated directly.
These two properties are also characteristic of Jupiter-family comets, comets whose orbits are modified by Jupiter’s gravity. These comets include the European Space Agency’s Comet 67P The Rosetta mission was studied more closely in 2016. In fact, Phobos’ optical 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 have implications for Demos as well. Fornacer noted that if Phobos was guilty, then perhaps Deimos was guilty as well. In fact, based on the study, her team suggests that the two moons may have once been bound together as a single bilobed comet that was eventually trapped and torn apart by Mars’ gravity. In other words, Mars’ twin moons may actually be two halves of a single whole.
“If Martian satellites were indeed captured by comets, this means that comets may also be captured by telluric [terrestrial] Fornacer added that some of the moons of giant gas planets such as Saturn likely originated from planets Kuiper beltIt is the donut-shaped region covering the solar system, and from which many comets originate. However, astronomers have not identified a ‘comet moon’ of terrestrial planets before now, making Phobos the likely first.
However, the comet interpretation also faces problems. Some photometric parameters, such as the fraction of scattered light, do not match those of comets. However, Fornacier said, dynamical simulations — which take into account the movements of celestial bodies, including Mars and Phobos — will help the team determine the likelihood of such a comet trap. Ultimately, the MMX program, which will take physical samples of Phobos, may be the best hope for resolving the mysterious origins of this enigmatic moon.
The new study will be published soon in the Journal of Astronomy and Astrophysics Available on the arXiv preprint server.
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