From John Davies, Lancaster, UK
Discussing various causes for the “linear virgae” on Saturn's moons, Adam Mann mentions grooves on Phobos produced by “rolling boulders” (23/30 December 2017, p 12). This notion was fired by early, low-definition pictures of Mars's moon. Gravity on Phobos's surface is nearly 2000 times less than on Earth. Any boulder hitting the surface hard enough to dent it and start a groove would bounce off again, if it didn't shatter.
More study has found two possible mechanisms for the features seen: on Phobos due to its small orbital radius around Mars, and made by bodies captured by Mars and broken into multiple pieces by the same tidal forces.
