No. 16: Summer 1981
The Martian satellite Phobos is etched by curious grooves. Initially, the grooves were thought to be fracture lines formed by the impact that blasted out Stickney, the huge crater seen on Phobos. However, studies of the grooves revealed at least three families of grooves of different ages, with members of each family located on parallel planes cutting right through the body of the satellite.
Two recent papers have proposed radically different explanations. A. Horvath and E. Illes wonder whether Phobos might not be a layered structure, having once been part of a larger stratified body. J.B. Murray thinks the families of grooves might have been scraped out by disciplined formations of meteorites that were launched into space by Martian volcanos.
(Horvath, A., and Illes, E.; "On the Possibility of the Layered Structure of Phobos," Eos, 62:203, 1981. Also: Murray, J.B.; "Grooved Terrains on Planetary Satellites," Eos, 62:202, 1981.)
Comment. It is not easy to conceive of such well-drilled formations of meteorites. Neither is it easy to imagine a large, stratified body that might have given rise to Phobos.