Groove formation on Phobos: Testing the Stickney ejecta emplacement model for a subset of the groove population

Numerous theories have been proposed for the formation of grooves on Phobos, and no single explanation is likely to account fully for the wide variety of observed groove morphologies and orientations. One set of grooves is geographically associated with the impact crater Stickney. We test the hypothesis that these grooves were formed by clasts that were ejected from the Stickney crater interior at velocities such that they were able to slide, roll, and/or bounce to distances comparable to observed groove lengths (of the order of one-quarter of the circumference of Phobos), partly crushing the regolith and partly pushing it aside as they moved. We show that this mechanism is physically possible and is consistent with the sizes, shapes, lengths, linearity, and distribution of Stickney-related grooves for plausible values of the material properties of both the regolith and the ejecta clasts. Because the escape velocity from Phobos varies by more than a factor of two over the surface of the satellite, it is possible for ejecta clasts to leave the surface again after generating grooves. We make predictions for the surface characteristics and distributions of such grooves and their deposits on the basis of this model, and then compare them with remotely sensed observations of Phobos׳ grooves. We find that many of their characteristics can be accounted for by a model in which grooves are formed by rolling and bouncing boulders ejected from Stickney. As a further test of this hypothesis, we examine a wide range of lunar boulder tracks, and find that they have considerable similarities to grooves on Phobos in terms of morphology, structure, and relationships with underlying topography. We therefore find that the emplacement of very low-velocity ejecta associated with the Stickney cratering event is a candidate mechanism for the formation of grooves on Phobos. This model and these predictions can be further tested by analysis of high-resolution image data from current and upcoming missions to this and other small airless bodies.