Missing images suggest that Mars' mysterious moon Phobos may be a comet in disguise

Mars' moon Phobos may actually be a comet, or at least part of a comet, long captured by the red planet due to its gravity, according to a new study. A new study based on unpublished images.

For years, researchers have wondered about the origins of Phobos and its twin, Deimos. Some believe that the moons are ancient asteroids attracted by Mars' gravity, because their chemical composition is similar to that of certain rocks in the main asteroid belt between Mars and Jupiter. but, Computer models simulating this capture process have not been able to replicate the semi-circular paths For the couple around Mars, the specialized website Space.com also published data from a new study that will be published in the Journal of Astronomy and Astrophysics.

Another hypothesis suggests that a giant impact, like the one that created our Moon, tore the duo away from the Red Planet; But Phobos has a different chemical composition than Mars, making this scenario unlikely as well. Discovering exactly how Phobos came to be is one of the goals of the Mars Moons Exploration (MMX) mission. From the Japan Aerospace Exploration Agency, it is scheduled to launch in 2026. Sonia Fornacier, professor of astronomy at Paris City University and lead author of the new study. , is a world of tools for the MMX mission. While she and other scientists were analyzing the images to fine-tune the spacecraft's planned trajectory, Fornacier came across images that had never been seen before.

These images were captured by high-resolution cameras aboard the Mars Express spacecraft, a European Space Agency (ESA) orbiter that has studied Mars and its moons since 2003. More than 300 images document the properties of Phobos. This includes the 9-kilometre-wide Stickney Crater, the largest feature on the island of Phobos.

Fornacer and his 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 reflects when the sun is directly in front of it or at a distant angle.

The researchers found 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 when the Sun was directly overhead. This phenomenon, called opposition thrust, is a characteristic of many airless objects in the solar system. In addition, the researchers discovered that Phobos' surface was porous, like sand. This led the team to suggest that the moon's surface could be covered in a thick layer of dust with grooved particles, the shadows of which disappear when illuminated directly.

Both properties are also characteristic of Jupiter-family comets, comets whose orbits are influenced by Jupiter's gravity. These include “rubber duck” comet 67P, which ESA's Rosetta mission closely studied in 2016. In fact, Phobos's optical properties matched those of comet 67P almost perfectly. So, The team concluded that Phobos was probably a comet captured by Mars.

The study's findings also have implications for Demos. Fornacer noted that if Phobos was guilty, then perhaps Deimos was guilty as well. In fact, based on the study, his team suggests that the two moons were once connected as a single bilobed comet that became trapped and eventually 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, it means that comets could also be captured by telluric planets.” [terrestres]Fornacer added. He said that some of the moons of giant gas planets such as Saturn may have originated in the Kuiper Belt, the donut-shaped region that envelops the solar system and from which many comets originate. However, astronomers have not identified a “comet moon” of the planet. Terrestrial planets so far and which It makes Phobos bearable first.

However, the comet interpretation also has 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 the comet being trapped. Ultimately, however, the MMX program, which It will take physical samples of Phobos fragmentsPerhaps the best hope for solving the mysterious origins of this mysterious moon.

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