The ridges on Phobos’s surface tell the story of its inner rupture
Grooving in response to Phobos’ tidal orbital decay. (a) Linear depressions traversing the surface of Phobos (ESA/DLR/FU Berlin). (b) Our simulated Phobos is a weak rubble pile covered by an outer regolith consisting of a cohesive layer (blue) covered by a shallow loose granular layer (white), as highlighted in the inset. The blue arrows indicate the tidal forces exerted by Mars, and the red patches represent the 23 local areas that we have simulated. Our simulations force these patches to deform to mimic Phobos’s reshaping as it spirals inward. (c) As cells are stretched and compressed (top), fractures occur, as indicated by abruptly accelerated regolith particles. (d) A parallel pattern of grooves and accompanying subsurface fractures develop in a regular space. The orientation of the failure is generally perpendicular to the direction of the local principal tensile stress. The morphology and pattern of these extensional depressions are consistent with some linear grooves on Phobos. The data shown is for the patch located at 60° N and 0° E with the cohesive force cp = 36 kPa. Credit: The Planetary Science Journal (2022). DOI: 10.3847/PSJ/ac8c33
Phobos, the innermost moon of Mars at 22 km in diameter, is a wonderful body. Unlike its little brother Deimos, Phobos has developed a striking pattern of parallel linear features that run across its surface. These grooves are a distinctive global feature of Phobos, not present on Deimos. How they formed has puzzled planetary geologists for more than forty years, ever since they were first photographed in geological detail by NASA’s Viking missions.
In a new paper published in The Journal of Planetary Science, researchers from Tsinghua University, the University of Arizona, Johns Hopkins University, and Beihang University have taken an important step toward solving this puzzle. The new study proposes that these grooves are surface expressions of hidden canyons within Phobos, which are early signs that the moon is falling apart due to increased tidal forces from Mars.
In addition to its strange linear markings, another special feature of Phobos is its orbit, so close to Mars (only 6,000 km) that the tides cause it to spiral closer to about 2 meters every 100 years. Mars is pulling it down. The rapid pace of this evolution (it is predicted to collide with Mars in about forty million years) has inspired researchers to propose that the grooves are striations, torn apart by Martian gravity.
But until now, it has been impossible to show that such a tectonic surface mechanism could work. The problem with the striation idea is that it requires a somewhat stronger outer shell that fractures when Phobos’s shape changes beneath it. Phobos has a near-surface porosity of at least 40%, so it seems impossible to sustain networks of large cracks in a pile of fluffy dust, even in gravity less than 1/1000 that of Earth.
Using the most detailed supercomputer simulations of the problem to date, Bin’s team explored the idea that the loose dust rests on a somewhat cohesive subshell, a material that is also weak but strong enough to sustain deep fissures. The loose dust then drains into those crevices.
“This is the first time that millions of particles have been used to explicitly model the stretching and compression of granular regolith undergoing tidal evolution,” says Bin Cheng of Tsinghua University, who led the new study. “Therefore, we can directly check the model against observations of grooves on the surface of Phobos.” The new models give a strong agreement with the observations that have been obtained so far. If correct, then by going back in time, they can tell us about the early history of Mars. Extended forward, they can predict how Phobos will evolve as it spirals.
Bin and his team represented the top 150m of Phobos’s surface as two rectangular stacks consisting of 3 million grains, with the top 50m very loose and the deeper grains slightly cohesive. “Kind of like a sandwich cookie,” says Bin. They put these rectangular piles in various places on Phobos, representing the potato-shaped moon as an ellipsoid. From this, they calculated the biaxial stress each patch would experience as Phobos’s interior deformed beneath them as the tide rose.
The resulting structures were found to bear a striking resemblance, in size, spacing, and orientation, to many of the grooves observed in Phobos’s mid-latitudes, including their parallel patterns and even their pitted-to-scalloped-to-linear morphologies.
Not all grooves are predicted to form in this way, but for those that do, simulations provide a clear view of the process. The tidal stress, as it increases, opens narrow, parallel fissures in the substrate. This triggers the drainage of weaker material in the upper layer into deeper fissures, leading to the formation and evolution of complex groove morphologies that can evolve further, somewhat analogous to cracks forming in a melting glacier. deforms, except here they form in dry, dusty regolith, in microgravity. , for tens of millions of years.