The long, shallow grooves that cut across the surface of Phobos represent one of the most monumental surface structures in our Solar System. These grooves, typically 100–200 meters wide and 10–30 meters deep, went over the long history since their origin, which thus stand as records of past and on-going geological processes on this tiny moon. Finding the formation mechanism of the grooves, therefore, will help us to understand the properties and evolution of loose airless soils and the origin of Mars and its satellites.
Morphological analysis shows many of Phobos' linear features are aligned with the satellite’s intermediate axis (the tangent to its orbit) and maintain a mostly parallel orientation, within a few degrees, over tens of kilometers. Several mechanisms have been proposed to explain the formation of the parallel groove system, but the origin remains controversial. Surface structural failure is thought to have happened on Phobos during its orbital decay toward Mars. However, the formation of tensile fissures was not modeled explicitly, and it is still a puzzle that if their geometry and orientation are consistent with Phobos’ grooves.
In this study, we model explicitly in 3D the stretching and squeezing of granular regolith using a Soft-Sphere Discrete Element Model code DEMBody. The granular bed is assumed to consist of a cohesive subsurface layer and a loose upper-layer. We show that the tidal strain opens fissures within the cohesive substrate and then triggers drainage flow of upper loose material into the underlying open fissures, forming geological striations depending on location that are a good match to observations. Regimes of granular failure modes are systematically analyzed to deduce the critical cohesion strength required for the formation of grooves with parallel pattern. The high degree of similarity between model-predicted structures and those observed on Phobos indicates that the tidal-grooving scenario, which requires mildly cohesive regolith, can explain at least some linear striations on Phobos.