A photograph of Phobos and its tracks. Image Credit: NASA.

Scientists Explain Mystery “Tracks” on Mars’ Moon Phobos

Mysterious "tracks" decorate the surface of Mars' moon Phobos. Now, scientists have explained what causes them.


Mars is a strange and mysterious planet. But its moons are no different. Mars’ moons are some of the most interesting in the solar system. They look more like asteroids trapped by Mars’ gravity than natural satellites of the planet. To better understand them, scientists have recently studied Phobos and the strange surface shapes that look like massive highways crossing it. Phobos’ mysterious grooves are thought to be the surface expressions of its hidden canyons. Additionally, scientists say this indicates it is falling apart under the increasing gravitational pull of Mars. This is what a new article published in The Planetary Science Journal proposes by researchers from the universities of Tsinghua, Arizona, Johns Hopkins, and Beihang.

A small moon

Phobos, which has a diameter of just 22 kilometers, is also unique for its unusual linear markings and orbit. Due to its close orbit to Mars (only 6,000 km), Phobos spirals closer to about 2 meters every 100 years due to the tides. Mars is essentially pulling down the tiny moon. The researchers say the grooves are essentially geological striations torn by Martian gravity. It is predicted that Phobos will collide with Mars in about forty million years. So far, however, no tectonic surface mechanism has been demonstrated to work. This striation idea has the disadvantage that the outer shell needs to be somewhat stronger, so it doesn’t break when Phobos’s shape changes beneath it. With a near-surface porosity of at least 40%, Phobos seems unable to sustain large crack networks in a pile of fluffy dust, even with gravity 1/1,000 that of Earth.


The evolution of Phobos

Scientists propose that loose dust rests on a more cohesive sublayer after performing simulations on the most sophisticated supercomputers available. This material is weak but strong enough to maintain deep fissures. These crevices then drain loose dust. The new study is the first to explicitly model the stretching and compression of granular regolith during tidal evolution using millions of particles,” explains Bin Cheng, a researcher at Tsinghua University. As a result, we can directly compare the model with observations of grooves on Phobos’ surface.” Observations obtained so far are consistent with the new models. In that case, we can learn about Mars’ early history by going back in time. Phobos’ evolution as it spirals can be predicted by extending this forward.

Deforming subsurface

In the study, Bin and his team modeled Phobos’ top 150 meters into two rectangular stacks that contained three million grains, with the top 50 meters having a very loose texture and the deeper grains having a more cohesive texture. Using this information, the researchers calculated the biaxial stress each patch would encounter as Phobos’s interior deformed beneath it. Many of the resulting structures have parallel patterns, as well as pitted-to-scalloped-to-linear morphologies that are observed in Phobos’s mid-latitudes. This is not the case for all grooves, however. However, for those that do, simulations are a great way to see how the process is carried out. Substrate fissures appear as a result of tidal stress increasing.


The weaker material in the upper layer is drained into deeper fissures because of this process. This way, complex ridge morphologies are formed and evolve over millions of years, similar to cracks forming in melting glaciers. However, these cracks are formed in dry, dusty regolith in microgravity. MMX, Japan’s planned Mars Moon Exploration mission, slated to launch in the mid-decade, will provide critical insights into this puzzling moon. It is expected that Phobos will deorbit in 20 to 40 million years after the tides separate it completely. The destruction of Phobos is expected to create a ring around the red planet. This would make Mars the brightest planet in Earth’s sky. Researchers predict that this disappearance has already begun, and the shallow grooves and canyons beneath the surface are the first signs.


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Written by Ivan Petricevic

I've been writing passionately about ancient civilizations, history, alien life, and various other subjects for more than eight years. You may have seen me appear on Discovery Channel's What On Earth series, History Channel's Ancient Aliens, and Gaia's Ancient Civilizations among others.

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