Mapping Phobos Surface Migration for MMX

The Martian Moons eXploration (MMX) mission, scheduled for an October launch, now possesses a detailed surface atlas to guide its landing and sampling operations on Phobos. Isabel Herreros and Sébastien Charnoz developed a morphodynamic atlas that moves beyond traditional static topography to map how loose material, or regolith, migrates across the moon. This atlas provides essential context because Phobos exists in an environment where simple downhill slope calculations fail to predict where surface particles settle.

Forces Acting on a Low-Gravity World

On Earth, a rock at the top of a mountain rolls to the base because gravity pulls it straight down. On Phobos, the gravitational pull is so weak that it is easily overwhelmed by external factors. Phobos orbits in a synchronous rotation, keeping the same face toward Mars, and this configuration induces significant asymmetric tidal stress. These forces add to the centrifugal force from the moon's rotation and the Coriolis effect experienced by moving particles. Consequently, the net acceleration vector, which determines the downward direction for a grain of dust, shifts continuously depending on the location across the surface. This is not a simple back-and-forth oscillation. Because the tide acts as a bias in the local vector field, material is systematically nudged toward regions where the vector components sum to a local minimum, effectively acting like a continuous vibration that migrates grains toward depositional zones.

Historically, researchers relied on static digital terrain models (DTMs) to identify where material might be unstable. These models provided a snapshot of the geography, such as craters and ridges, but they could not indicate where dislodged material eventually ends up. The new atlas integrates these physical forces to predict movement. The researchers calculated these Regolith Migration Pathways (RMPs) using a custom numerical simulation of the net acceleration vector at thousands of points across the surface. The model computes the effective surface slope by taking the gradient of the gravitational potential, adjusted for the rotating frame, to define the RMP as the trajectory a particle follows if it begins to slide due to a surface perturbation.

Decoding the Regolith

The atlas reveals a network of pathways where loose material drifts from rough, cratered highlands down to smoother, spectrally neutral plains. By calculating these routes, the MMX team can distinguish between pristine ancient surface material and regolith that has been recently transported and mixed by the moon's complex orbital dynamics. When the lander touches down, knowing whether it is sampling an original surface deposit or an accumulation of drifted dust changes the scientific interpretation of the mission. If the lander picks up material from a depositional zone at the end of an RMP, the sample may contain a mixture of rocks from various parts of the moon. Conversely, high-relief zones, where movement is dislodging material, offer a more direct window into the composition of the local bedrock. The atlas allows mission planners to select sites that maximize the historical context of these samples, ensuring they collect material with a known geological history.

Imagine a grain of dust on the edge of a crater. On a large, stable moon, that grain stays put unless an impact or seismic event provides enough energy to overcome friction. On Phobos, the persistent tidal bias means the grain is in a state of precarious equilibrium. A tiny, constant perturbation from the rotating tidal field eventually provides the energy needed to cross the threshold of static friction. Once movement begins, the grain follows the net acceleration vector toward a gravitational minimum. This is a subtle, slow process, but over geological time it is the dominant factor in shaping the distribution of loose surface material. What remains unknown is the specific distribution of grain sizes and the exact cohesion of the dust, which dictates the threshold energy required for a particle to enter a migration pathway.