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Mystery Of How Mars’s Doomed Moon Got Its Stripes May Have Been Solved

So long, Phobos, and thanks for all the fissures.

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Katy Evans

Managing Editor

clockNov 18 2022, 15:29 UTC
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Image credit: NASA/JPL-Caltech/University of Arizona
A few theories have been forward for what caused the strange stripes on Phobos, snapped here by NASA's Mars Reconnaissance Orbiter in 2008. Image credit: NASA/JPL-Caltech/University of Arizona

There are many mysterious things about Mars’s doomed moon Phobos. We don’t know where it came from, what’s inside it, what the strange straight lines on its surface are, and it’s possibly in the process of being ripped apart by Mars. In fact, according to a new paper, that’s what’s causing Phobos’s weird tiger stripes.

We know it’s going to die one day, probably in around 50 million years – the question is, how? Astronomers have discovered that the unusual parallel grooves covering the surface, thought to probably be the results of some ancient impact, are actually canyons full of dust that are growing wider as the extreme gravitational forces between moon and planet tear it apart.

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Both of Mars’s moons are ominously named after the twin sons of Mars, the Roman god of war: Phobos, the larger of the two, for the god of fear and panic; and Deimos, the smaller, for his brother, the god of dread and terror. The reason Phobos is known as Mars’s “doomed “ moon, however, is that the satellite is caught in a death spiral around Mars, slowly falling towards the Red Planet at 1.8 centimeters (0.7 inches) every year (which is faster than Venice is sinking) until they eventually collide.

Both of Mars’s moons are tidally locked to the planet, meaning they always present the same face toward Mars. Tidal forces that pull on objects in space are due to their gravitational interactions and can have observable effects, like the effect the gravitational pull of the Moon has on Earth’s water in the form of tides.

Visible effects on the solid surface of objects in space, however, are usually pretty small. That’s not to say it doesn’t occur – the icy shell of Saturn’s moon Enceladus is covered in stress fractures from the tidal pull of its giant planet.

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Orbiting Mars in just 7 hours and 39 minutes, Phobos is pretty close to Mars, making it plausible the tidal forces are causing the surface fracturing of the small moon. This idea has even been put forward before. Orbiting so near puts Phobos closer and closer to the Roche limit, the place where gravitational forces will pull any solid orbiting object apart. This occurs because the gravity on the planet-facing side is so much larger than on the other side it can't stay in one piece. Eventually, Phobos will be destroyed, forming a smaller version of Saturn's rings around Mars.

Alternative hypotheses have been put forward too. In 2018 it was suggested bouncing boulders carved the grooves on Phobos's surface. 

In the new study, the team performed hundreds of simulations using a 3D mathematical model of a Phobos-like object, looking into the stretching and squeezing that occurs from tidal forces within a Phobos-like orbit. They found that by modeling Phobos as a "rubble-pile interior overlaid with a cohesive layer" many of the simulations created evenly spaced parallel fissures like the ones seen on the Martian moon. 

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“Fracture opening triggers drainage of upper loose material into these deep-seated valleys, which we show could lead naturally to the formation of groove-like structures,” they wrote.   

Not all regions of Phobos matched the simulations – the grooves around its equator weren't explained by their predictions, for example. However, the findings do suggest that some (or even most) of the stripes are being caused by tidal fracturing as the moon continues on its doomed path to Mars collision. 

All this makes for an intriguing target for the Japanese Space Agency's 2024 Martian Moons eXploration (MMX) mission to land spacecraft on both Phobos and Deimos, returning samples in 2029. We might even be able to witness the beginning of the end of Phobos.

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 The study is published in The Planetary Science Journal


spaceSpace and PhysicsspaceAstronomy
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