Numerical Simulations of Drainage Grooves in Response to Extensional Fracturing: Testing the Phobos Groove Formation Model

The long, shallow, parallel grooves that cut across the surface of the Martian moon Phobos remain enigmatic, with distinct implications for understanding the evolution of loose airless soils and the origin of Mars and its satellites. Phobos orbits deep inside the fluid Roche limit and is gradually spiraling in, creating global tidal strain, which leads to a model in which orbital decay is driving the formation of surface grooves on Phobos. Here we test this model by simulating the extension of a mildly cohesive regolith shell, driven by Phobos's tidal strain imposed by orbital migration. Modeling Phobos as a rubble-pile interior overlaid by a cohesive layer, we find that the tidal strain could create parallel fissures with regular spacing, depending on the latitude and longitude. Fracture opening triggers drainage of upper loose material into these deep-seated valleys, which we show could lead naturally to the formation of groove-like structures. We map the prominent linear depressions on Phobos and find that some at midlatitudes correlate with the model-predicted groove orientations. Our analysis supports a layered heterogeneous structure for Phobos with possible underlying failure-induced fractures, as the precursor of the eventual demise of the de-orbiting satellite. However, we do observe significant areas showing absences and anomalies that are not consistent with the tidal fracturing model. The in situ observations on Phobos by the Mars Moons eXploration mission will conclusively determine the origin of its enigmatic striations.

Automated summary

The parallel grooves on the surface of the Martian moon Phobos are still enigmatic and have important implications for understanding the origin of Mars and its satellites. By simulating the extension of a regolith shell driven by tidal strain, we confirm the model that the grooves on Phobos are formed by orbital decay. The grooves are likely formed by the drainage of loose material into deep-seated valleys caused by fracture opening.