A study has found, based on previously unpublished images, that Mars' moon Phobos may actually be a comet, or at least part of a comet, that was possessed by the Red Planet long ago.
The name "Phobos" in ancient Greek means "fear." For many years, scientists have been puzzled about the origins of Phobos and its twin, Deimos. Some have assumed that they are former asteroids pulled by Mars' gravity, because their chemical composition is similar to that of certain rocks in the main asteroid belt between Mars and Jupiter. However, computer models simulating this dragging process have not been able to replicate the two moons' near-circular paths around Mars.
Another hypothesis suggests that a giant collision, like the one that created our moon, led to the duo being ejected from the red planet, but "Phobos", which looks like a potato, has a different chemical composition from Mars, which also makes this scenario unlikely.
Sonia Fornacier, a professor of astronomy at Paris City University and the lead author of the new study, was an instrument scientist on the Japan Aerospace Exploration Agency's Mars Moon Exploration (MMX) mission, which is scheduled to launch in 2026 and aims to find out exactly how Phobos was born. With other scientists, she was analyzing the images to adjust the planned trajectory of the spacecraft, when she found the unpublished images.
These strange images, numbering 300, were taken by high-resolution cameras aboard the Mars Express spacecraft of the European Space Agency (ESA), which has been studying Mars and its moons since 2003, and wonderfully documents the features of “Phobos.” This includes the 9 km (5.6 mi) wide Stickney Crater, Phobos' largest feature.
Fornacer and her colleagues used the footage 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 team 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 the surface of Phobos appeared noticeably brighter when the Sun was directly overhead.
This phenomenon, called an opposition surge, is a characteristic of many airless Solar System objects. Scientists also found that the surface of Phobos 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 characteristics are also characteristic of Jupiter family comets, which are comets whose orbits are modified by Jupiter's gravity. These comets include comet 67P.
In fact, Phobos's optical properties matched those of comet 67P almost perfectly. Therefore, the team concluded that Phobos was probably a comet captured by Mars.
Fornacer pointed out 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.
However, the comet interpretation also faces some problems, as some optical parameters, such as the fraction of scattered light, do not match those of comets. Ultimately, the Mars Moon Exploration Mission (MMX), which will sample Phobos, will be the best hope for resolving the origins of this mysterious moon.