What To Know
- The surface of Phobos is also covered with a layer of loose regolitha mixture of rock and dust very different from the more solid surfaces of many other moons.
- Porosity refers to the proportion of voids or holes in the surface material, which may indicate a structure composed of loosely aggregated dust grains, similar to the surface of comets.
- The phase integral measures the total amount of light reflected by an object at different phases of illumination, while the geometric albedo is a measure of the reflectivity of an object’s surface when the Sun is directly overhead.
Researchers have recently discovered surprising clues about Mars’ moon Phobos that suggest it may not be what we thought. The discovery comes from an analysis of never-before-seen images taken by the European Space Agency’s (ESA) Mars Express spacecraft.
Two moons that intrigue
Phobos and Deimos, the two moons of Marchare distinguished by several unique characteristics. On the one hand, they are extremely small. Phobos, the larger of the two, is only about 22 kilometers in diameter, while Deimos is about twelve kilometers. Additionally, they have surfaces riddled with craters that are the result of countless meteorite impacts over the eons. Phobos is particularly intriguing because its orbit in decline which brings it inexorably closer to a collision with Mars in about 30 to 50 million years. Furthermore, unlike many other moons in the solar system, it has kilometer-long grooves whose origin is still debated. Some theories suggest that they are the result of impacts or tidal forces exerted by Mars. The surface of Phobos is also covered with a layer of loose regolitha mixture of rock and dust very different from the more solid surfaces of many other moons. It may be the result of millions of years of meteorite impacts that ground the moon’s surface into fine dust. These features—small size, cratered surface, grooves, and loose regolith—are unlike those seen on most other moons in the solar system. For example, the large moons of Jupiter and Saturn, such as Ganymede and Titan, are much larger and have atmospheres or geologically active surfaces. Similarly, our Moon, although cratered, has a relatively solid surface and shows signs of past volcanic activity.
Phobos. Credits: NASA/JPL-Caltech/University of Arizona
A different origin?
The origin of Phobos and Deimos is thus debated. Two main theories exist: one proposes that they formed from debris resulting from a giant collision with Marsthe other that it is aboutcaptured asteroids. Each of these theories has its limitations: the asteroid hypothesis does not fully account for their nearly circular orbit, while the debris hypothesis does not fully match their composition. New observations could shed light on this mystery. Researchers have recently analyzed new photos of Phobos taken by the Mars Express probe. This analysis revealed photometric characteristics surprising evidence suggesting that Phobos may have a different origin than previously thought. In detail, a photometric analysis consists of measuring and analyzing the light reflected by a celestial object. By examining the images of Phobos, the researchers discovered that Phobos shares several properties with the comet 67P/Churyumov-Gerasimenko. More precisely, it presents a red spectrum typical of certain types of surfaces rich in organic materials and complex compounds. In addition, Phobos has a high surface porosity. Porosity refers to the proportion of voids or holes in the surface material, which may indicate a structure composed of loosely aggregated dust grains, similar to the surface of comets. The researchers also observed opposition effect values similar to those of comet 67P. The opposition effect is a phenomenon where an object appears significantly brighter when it is directly opposite the Sun compared to an observer. This increase in brightness occurs due to the reduction of shadows and the coherent scattering of light on a rough or porous surface.
Comparison with other dark bodies in the solar system
To further their analysis, the researchers compared the photometric properties of Phobos with those of other dark bodies in the solar system. To do this, they examined the phase integral and geometric albedo of Phobos. The phase integral measures the total amount of light reflected by an object at different phases of illumination, while the geometric albedo is a measure of the reflectivity of an object’s surface when the Sun is directly overhead. These comparisons revealed Striking similarities between Phobos and Jupiter-family comets. These comets typically come from the Kuiper Belt, a vast reservoir of icy bodies beyond the orbit of Neptune. They have dark surfaces rich in organic material. Researchers have also found parallels with Phoebea moon of Saturn. Phoebe is believed to be a captured Kuiper Belt object. Like Phobos, it has a dark surface and similar photometric characteristics.
Future prospects with the MMX mission
These findings suggest that Phobos may not simply be a moon formed from debris from Martian collisions or a captured asteroid, as previously thought. The similarities to comets open up the possibility that this object actually originated from the Kuiper Belt. This hypothesis is revolutionary because it would change our understanding of the history and evolution of Martian moons. If Phobos is indeed a captured comet, it could provide valuable clues about the dynamic processes that shape the orbits and compositions of celestial bodies in the solar system. The mission Martian Moons eXploration The Japan Aerospace Exploration Agency’s (JAXA) MMX mission, scheduled for 2026, could provide the data needed to test this hypothesis. The mission aims to return samples of Phobos to Earth after three years of observations. By analyzing these samples, scientists could gain crucial information about Phobos’ composition and determine whether the Martian moon is actually a comet in disguise. The study is accepted for publication in Astronomy and Astrophysics.
