Mars' moon Phobos might be a captured comet, study suggests

Deimos and Phobos are natural satellites orbiting Mars. For years, scientists have tried to determine their origins but have not been able to confirm their past with 100 percent certainty. Some theories propose that the twin moons were captured by Mars' gravity due to their chemical composition, which is believed to be similar to that of rocks in the main asteroid belt between Mars and Jupiter.

Another theory suggests that a giant past impact could have ejected Phobos and Deimos from the Red Planet. However, this hypothesis is often rejected because of the differences in the composition of the two objects.

The Martian Moons eXploration (MMX) mission, led by Sonia Fornasier from Université Paris Cité and her team, aims to analyze Deimos and Phobos thoroughly. The researchers examined 300 unpublished images from a high-resolution camera mounted on the Mars Express spacecraft, focusing on the specific features of Phobos visible in the photographs.

Fornasier's team used the images to study the intensity of Phobos' reflected sunlight from different angles. This technique, known as photometry, allows scientists to determine exactly how much light Phobos reflects. They found that Phobos' surface does not reflect light evenly.

Some areas of Phobos were highly reflective, and its surface appeared significantly brighter at specific Sun angles. Researchers claim that this phenomenon is characteristic of airless objects in the Solar System.

Additionally, the photos revealed that Phobos' surface is porous, leading to suspicions that the object might be covered with a layer of dust with specifically arranged grooves, whose shadows disappear when illuminated.

The current findings are significant because the discovered properties of Mars' satellite are characteristic of comets from the Jupiter family. The team notes that Phobos might have been a comet captured by Mars. Although the discovery mainly concerns Phobos, Fornasier notes that if Phobos might have been a comet, it is possible that Deimos was as well.

Moreover, the team suggests that both objects might once have been a single comet that was torn apart by Mars' gravity. This implies that there is a probability that the twin moons are two halves of one comet. More precise studies will be conducted when the MMX program collects samples from fragments of Phobos and examines them.