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A composite photo of Mars' twin moon Phobos and Deimos suggests that the pair may be two halves of an ancient comet trapped in the Red Planet a long time ago.

Mystery of the Twin Satellites

For many years, astronomers have puzzled over the origins of the twin moons Phobos and Deimos. Some proposed that they are former asteroids that were attracted by Mars' gravity since their chemical composition is similar to that of space rocks in the main asteroid belt between Mars and Jupiter. However, computer models used to simulate this capture process cannot replicate the nea-circular paths of the twin moons around Mars.

Another theory assumes that a giant impact, like the one that formed our moon, dug the pair out of Mars. However, this scenario is also unlikely because Phobos has a chemical composition different from that of the Red Planet.

To find out exactly how Phobos was born as one of the moons of Mars, Japan Aerospace Exploration Agency (JAXA) is set to launch Martian Moons eXploration (MMX) mission in 2026. One of the key scientists in the MMX mission is Sonia Fornasier, an astronomer professor at the Paris Cité University.

Unveiling the Secrets of Phobos

Fornasier also leads a team of researchers who analyze the photometric properties of Phobos using Mars Express mission observations. They analyzed resolved photos of the mysterious moon captured between 2004 and 2022 by optical tools on board the Mars Express probe.

Launched by the European Space Agency (ESA) two decades ago, the orbiter has explored Mars and its moons using high-resolution HRSC and SRC cameras. About 300 photos exquisitely document some of the strange geographic features of Phobos, including the Stickney crater, the moon's largest landmark that measures 5.6 miles (9 kilometers) wide.

Fornasier and colleagues used the images to study the intensity of sunlight reflected by Phobos from various angles. Known as photometry, this technique enabled them to determine the amount of light reflected by the moon when the Sun was right in front or at an offset angle. The findings of the study were discussed in the paper "Phobos photometric properties from Mars Express HRSC observations."

It was discovered that the surface of Phobos did not reflect light from the Sun uniformly. Some regions were highly reflective, like the northeastern rim of the Stickney crater. The analysis of the researchers also revealed that overall, the surface of Phobos appeared noticeably brighter when the Sun was directly overhead. This phenomenon is known as an opposition surge, and it is characteristic of many airless objects in the Solar System.

The scientists also discovered that the surface of Phobos was porous like sand. This indicates that the surface of the mysterious moon may be covered by a thick dust layer with grooved particles whose shadows disappear when directly illuminated by the Sun.

Both of these properties are also observed in Jupiter-family comets or those with orbits that are gravitationally attracted by the gas giant. These include Comet 67P which was studied up close by ESA in 2016. The photometric properties of Phobos almost perfectly matched those of Comet 67P, so the experts suggest that Phobos may be a comet, or at least part of one, which was gravitationally captured by Mars.