Long grooves on Phobos are likely stress fractures caused by tidal forces. This image was acquired by the HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter on March 23, 2008. (NASA/JPL-Caltech/University of Arizona)
Phobos, Mars’ largest moon — although at just 16 miles wide still quite small — is slowly but steadily being torn apart by the gravitational pull of Mars… and it bears the scars to prove it. Long parallel grooves seen wrapping around the surface of Phobos are most likely stress fractures, visible evidence of the tidal forces that will one day cause the moon to break apart entirely. And when it does it will create a temporary ring of rubble and dust around Mars making our rusty red neighbor resemble a miniature Saturn.
At least for anywhere from one to a hundred million years, but hey—even Saturn’s majestic rings are thought to be just a fleeting feature on a cosmic time scale.
Mars and Phobos imaged by Hubble on May 12, 2016. Credits: NASA, ESA, and Z. Levay (STScI), Acknowledgment: J. Bell (ASU) and M. Wolff (Space Science Institute).
From a 2019 article by Joel Davis at Astronomy.com:
Astronomers have long known that Phobos, the larger and nearer ...