The strange mystery of the tiger stripes of Phobos could finally be solved

The Martian moon Phobos is not long for this universe.

According to astronomers’ calculations, the potato-shaped satellite is slowly but inexorably approaching its host planet. Eventually, in about 100 million years, the gravitational interaction between the two bodies will tear Phobos apart, giving the red planet a temporary dusty ring.

According to a new study, these gravitational interactions may already be having an observable effect. At least some of the mysterious shallow parallel grooves that cover that of the moon the entire surface could be the result of a fracture as his eye socket slowly decays and tidal forces pull harder and harder on his bones.

“Our analysis supports a heterogeneous layered structure for Phobos with possible underlying failure-induced fractures, as a precursor to the eventual disappearance of the deorbiting satellite,” write a team of astronomers led by Bin Cheng of the ‘Tsinghua University in China and the University of Arizona.

The tidal forces that pull on bodies in a system are the result of their gravitational interaction, stretching their structures along an axis that separates them.

Usually any significant effect this distortion might have on a solid surface is quite small. Where tidal forces can be easily observed in the movements of our planet’s liquid oceans, the visible effects on landmasses are less obvious.

This does not mean that tidal forces between other solid bodies cannot have more obvious consequences. Stretching caused by tidal forces can in some cases cause stress fracturing. We saw it in Saturn’s moon Enceladus, whose icy shell has deep fractures parallel to its south pole caused by tidal stress.

With an orbit of just 7 hours and 39 minutes, Phobos is quite close to Mars, closing in at a rate of about 1.8 centimeters per year. At this close proximity, it is absolutely possible that tidal forces could induce surface fracturing across its 27 kilometer (16.8 mile) wide body. The idea that the stripes of Phobos are the result of such an interaction has also been previously considered and deemed plausible.

However, it is unclear whether the current configuration and interaction of Phobos and Mars could produce the observed stripes, and other explanations are also in the works. For example, a 2018 study found that the scratches could be the result of rolling rocks.

Cheng and his colleagues therefore conducted 3D mathematical modeling by explicitly examining the tidal stretching and squeezing of a layered body similar to Phobos, with a loose, rubbery exterior resting on a cohesive layer beneath.

The researchers performed hundreds of simulations using their model. In a significant number of these simulations, tidal forces caused the cohesive layer to separate and fracture into parallel grooves, causing the upper loose regolith to drain into the fractures below. The result is a striped and ridged surface very similar to regions seen on Phobos.

Not all areas of Phobos were consistent with the model, the team found. In particular, the grooves around that of the moon the equator did not correspond to forecasts. But the results show that at least some of the scratches could be caused by fracturing like the moon spirals towards death by the disembowelment of the tides. This would mean we are witnessing the beginning of the end for Phobos.

These findings could therefore have implications for the study of other moons that experience significant orbital decay, such as Neptune’s moon Triton. The drained rubble could also expose pristine material on Phobos, could make the grooves a very interesting study region for the upcoming Mars lunar mission by the Japanese Space Agency.

This mission should provide conclusive evidence of the origin of these mysterious scratches – but tidal disruption certainly seems like an intriguing possibility.

“By modeling Phobos as a rubble pile interior covered by a cohesive layer, we find that tidal strain could create parallel fissures with regular spacing,” the researchers write in their paper.

“Our analysis suggests that some of the rilles lining the surface of Phobos are likely early signs of the deorbiting satellite’s eventual demise.”

Mashed potatoes, anyone?

The research has been published in The Journal of Planetary Science.