Tidal disruptions of rubble piles: The case of Phobos

TL;DR

This study investigates the tidal disruption of rubble-pile satellites like Phobos, using analytic and numerical methods to determine when and how they break apart as they migrate inward towards Mars.

Contribution

It provides new estimates for Phobos's tidal disruption distance, challenging previous assumptions about its material strength and highlighting the importance of internal structure in disruption processes.

Findings

Phobos will be tidally disrupted beyond 2 Mars radii if it has low cohesive strength.

Surface stripping can occur without internal failure in low-strength satellites.

Previous strength estimates for Phobos may be overestimated, affecting disruption predictions.

Abstract

Many small satellites in the Solar System have sub-synchronous orbits, meaning their orbits are decaying due to tidal dissipation. Unless they have substantial material strength, they will eventually tidally disrupt before reaching their planet's surface. We studied the fate of rubble-pile satellites as they migrate inward, with a particular focus on the case of Phobos. We used a combination of analytic estimates and numerical simulations to determine the failure mode and tidal disruption distance of a Phobos-like satellite, as a function of its shape and cohesive strength. Both analytically and numerically, we identify a regime for satellites with low cohesive strengths whereby their surface can be tidally stripped without undergoing internal failure. Our numerical simulations demonstrate that Phobos will be destroyed beyond 2 Mars radii if it has a bulk strength similar to those estimated for small bodies recently visited by spacecraft. Based on our results and some additional arguments, we suggest that previous studies on the fate of Phobos have overestimated its strength, and therefore underestimated its tidal disruption distance. We also speculate that if Phobos undergoes some tidal stripping, its ultimate fate may be determined by runaway collisional erosion rather than a pure tidal disruption. However, the ultimate tidal disruption distance for Phobos will depend on its unknown internal structure and bulk material properties, which will be constrained by JAXA's Martian Moons eXploration (MMX) mission and its IDEFIX rover. These results have implications for theories about the origin and evolution of the Martian moons and for tidal disruptions of other small, irregularly shaped satellites.