The Floating Coal in the Martian Sky

We’re calling Phobos, ‘the floating coal in the Martian Sky’,” explains Hideaki Miyamoto, from the University of Tokyo and leader of the mission landing and operation team.

Phobos orbits Mars at just 6,000km above the Martian surface, on an orbit that is close to being perfectly circular. Current observations of the inner moon indicate a low density rocky body which reflects far less light than Mars, giving it a coal black appearance.

Simulant of the regolith that may be found on the surface of Phobos.

A dark surface could indicate the presence of carbon-rich organics, such as those found in the carbonaceous chondrite population of asteroids. This might support the formation idea that Phobos and Deimos are captured asteroids, snagged by Mars’s gravity. However, Phobos’s dark spectra (reflected light) is anomalous even for this carbon-rich asteroid class. To unravel this mystery, we need the sample of the moon’s rocks returned by the Martian Moons eXploration mission.

Gathering a sample from Phobos will be on the biggest challenges of the Martian Moons eXploration mission. A major unknown is the structure of the rocky regolith the spacecraft will encounter as it moves towards a touchdown on the Martian moon surface.

Phobos and Deimos were observed by NASA’s Mars Pathfinder mission in the late 1990s from Mars’s surface. The data from the lander was able to estimate the moons’ surface temperature, but was not able to estimate the grain size of the soil nor its electric charge. Without this information, it is difficult to optimise the design of the spacecraft’s landing equipment.

Studying limestone at the Yoshizawa Limestone Industry Company quarry to help create the simulent of the surface of Phobos.

Considering this problem are an international team of researchers from Japan, USA, Europe and Russia. Based on models for the two formation ideas for the Martian moons (either captured asteroid or coalescing debris from an impact with Mars), the researchers are artificially simulating the expected soil structure. Named the “Phobos rock” experiment, this project is also being assisted by the Yoshikawa Limestone Industry Company.

Designing the spacecraft sampling instruments to handle the variety of expected soil types will enable the mission to get the best results when it touches down on the moon surface.