A new surface map of Phobos, Mars' larger moon, has been created by two European researchers and is expected to assist Japan's space agency, JAXA, in its upcoming Martian Moons eXploration (MMX) mission. The atlas, titled "The dynamical surface of Phobos: A morphodynamic atlas," was published in Earth and Planetary Science Letters and provides detailed information about how material moves across Phobos' surface. This understanding could help scientists better interpret samples collected during the MMX mission, which is scheduled to launch in October 2024 and reach Phobos by 2027, with a planned return of samples to Earth by 2031. Phobos has a mysterious origin—it could be a captured asteroid, a rubble pile, or a moon formed from debris after a massive collision. Its surface features include grooves, landslides, impact craters, and the large Stickney Crater. To create the atlas, the researchers used a digital terrain model (DTM) of Phobos, mapping out acceleration vectors and identifying areas where these forces create organized patterns on the moon’s surface. The movement of material on Phobos is influenced not only by its topography but also by orbital and rotational forces, such as tidal and centrifugal effects. The researchers modeled how regolith—loose surface material—moves under the combined influence of gravity, tides, rotation, and friction. They identified large areas where material moves coherently and mapped out specific routes, called Regolith Migration Pathways (RMPs), where regolith tends to accumulate. These pathways are found in smooth, spectrally neutral regions, suggesting long-term accumulation of material. The study suggests that many of Phobos' surface features and differences in color or composition can be explained by the slow movement of regolith along these pathways. RMPs are found running along major craters, with movement mostly confined to the interiors of large craters and the eastern slope of Stickney Crater. The researchers also considered a 14-degree friction angle, which might reflect past regolith movement. These pathways highlight areas where surface reshaping, crater filling, and changes in color and texture have occurred over time. The MMX mission will collect two samples from different locations on Phobos: one near the sub-Mars point and one near the anti-Mars point. These regions have different regolith histories. The sub-Mars site has material from Stickney Crater's slope, which has been exposed to space weathering for a long time. The anti-Mars site, in contrast, may have regolith that is still moving, potentially preserving information about Phobos’ original or more pristine surface. The researchers note that their model is simplified and does not account for all factors, such as Phobos’ slight wobbling in orbit or the detailed mechanical properties of its surface material. Nonetheless, their work provides a valuable framework for interpreting future samples in the context of Phobos’ surface processes and geological history.