We now know why Phobos, the moon of Mars, has stretch marks

Detail of the surface of Phobos, the largest moon of Mars

Detail of the longitudinal marks taken by the Viking I Probe.
Image: NASA / Dr. Edwin V. Bell, II (NSSDC/Raytheon ITSS)

We know that Phobos, the largest of the moons of Mars, is doomed. Its end will come sometime in the next 50 to 100 million years. Until then, the satellite holds many mysteries, and one of them is the strange belt of longitudinal marks similar to scratches that adorn its surface. A team of researchers has solved at least that question.

Phobos is an irregularly shaped moon with a diameter of about 22km. It always rotates offering the same face to Mars because it presents what is called tidal lock (its rotation is synchronized with its orbital period). Its orbit is also unusually close to the Red Planet, just 6,000km from its surface. Tidal forces, coupled with its proximity to Mars, are causing Phobos to progressively lose speed. The moon is, so to speak, in a slow but irreversible downward spiral towards Mars, which at some point will cause its disintegration. At that point, the fragments of the Moon will either fall to the Martian surface or end up form a ring around the planet not unlike that of Saturn.

Until very recently it was believed that the longitudinal markings that adorn this doomed moon were the result of some kind of unusual impact with another object, specifically with some kind of asteroid that rolled across its surface. However it is not so. A team of astronomers has dedicated itself to studying the effects of the gravitational forces that Mars exerts on Phobos by entering data from both bodies into a simulator. The result is that the longitudinal lines of the moon are… fracture cracks. These are huge dust-filled canyons that widen and open wider and wider as Mars pulls Phobos towards itself.

“Open fractures cause material to drain from the surface and saturate newly formed canyons,” the authors explain in a study published in The Planetary Science Journal. “That leads to the formation of the smooth incision-like stretch marks we see today.”

Training will be one of the objectives of the Martian Moons Exploration (MMX), a mission launched by the Japanese Space Agency. The MMX will not only explore Phobos and Deimos from orbit, but plans to land a 25-kilo rover manufactured by the German Aerospace Center. The MMX Rover will explore Phobos and take samples from the Moons for study. Among the objectives of the mission is to find out if Phobos and Deimos are really asteroids captured by Martian gravity, fragments of a missing major moonor parts of the planet itself released as a result of some impact. The takeoff of the mission is scheduled for 2024 [The Planetary Science Journal vía IFL Science]

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