New dynamical atlas will inform MMX sample strategy
Two European researchers have published a morphodynamic atlas of Phobos designed to support the Japan Aerospace Exploration Agency's (JAXA) Martian Moons eXploration (MMX) sample-return mission, which is set to launch in October 2026 and reach Phobos in 2027. The research letter, titled "The dynamical surface of Phobos: A morphodynamic atlas," appears in Earth and Planetary Science Letters and is authored by Isabel Herreros of the Spanish Astrobiology Center (CAB, Madrid) and Sébastien Charnoz of the Institut de Physique du Globe de Paris.
The atlas combines models of the local acceleration field on Phobos with surface morphology to produce maps of "dynamical slope" and to identify preferred routes of regolith transport, which the authors call Regolith Migration Pathways (RMPs). These tools aim to provide context that can help interpret whatever material MMX collects and to guide where and how the spacecraft should attempt to sample.
Why a dynamical approach is necessary on Phobos
Phobos is a small, irregular body roughly 11 km across. In such a low-gravity environment, Herreros and Charnoz note that the motion of loose material cannot be inferred from topographic slope alone. Instead, surface-material motion is controlled by the combined effects of Phobos' self-gravity, time-dependent Martian tides, and inertial forces such as centrifugal acceleration. The atlas maps those combined accelerations and translates them into a local dynamical slope (shown in the paper as colour-coded panels) that better predicts where regolith will move over time.
The authors explicitly write that "in such a low-gravity regime, the displacement of loose material, like regolith, cannot be inferred from topographic slope alone, making a dynamical approach essential for interpreting Phobos’ surface morphology and for supporting the Martian Moons eXploration (MMX) mission led by JAXA." The work highlights dynamic features across the moon, labelling prominent zones (a through i in their figures) and projecting surface trajectories to show where and how material preferentially migrates.
Regolith Migration Pathways (RMPs) and sampling implications
The atlas identifies RMPs as "preferred routes of regolith transport under the combined effects of self-gravity, centrifugal, and tidal accelerations." The study includes global projections of surface trajectories with black lines highlighting RMPs. Those pathways suggest that certain regions will preferentially accumulate or lose fine material over time. For a sample-return mission like MMX, which aims to land, collect, and return material from Phobos to Earth by 2031 if successful, knowing where regolith concentrates or is actively transported can influence sampling-site selection and the scientific interpretation of returned grains.
The atlas therefore provides two practical benefits: first, it can help mission planners prioritise surface locations that are more likely to yield accessible regolith; second, it offers geological context to place returned samples within a dynamical framework rather than a strictly topographic one. That context is critical because, as the authors emphasise, a few grams of regolith can carry a great deal of information about the moon's origin and evolution.
Open questions about Phobos' origin
Herreros and Charnoz review the broader uncertainties about Phobos' history that motivate careful contextual mapping. Phobos might be a rubble-pile asteroid similar to certain small bodies, potentially captured into Martian orbit; alternatively, it could be a disk-derived moon that coalesced from debris thrown into orbit after a giant impact on Mars; a hybrid origin is also possible. Those competing hypotheses make the provenance of any returned samples a central scientific question, and the atlas aims to improve the ability to interpret sample provenance by revealing the dynamical behaviour of surface material.
Timing and mission context
JAXA's MMX mission is scheduled to launch in October 2026 and to arrive at Phobos in 2027, with plans to return samples to Earth in 2031 if the mission succeeds. The mission represents a first attempt to return material from the Martian system (from its moon rather than Mars itself), and it builds on JAXA's prior sample-return experience. The atlas by Herreros and Charnoz arrives in the lead-up to MMX's launch and will be available to mission scientists and planners as they refine sampling operations and select candidate sites.
Broader significance and future work
Producing a dynamical atlas for a low-gravity, tidally influenced moon like Phobos is an example of how computational and observational planetary science combine to reduce risk and increase science return for sample-return missions. Beyond immediate mission planning, the atlas provides a framework that other teams can extend as MMX returns imaging and in-situ data from Phobos: observed surface changes, measurements of regolith grain size distributions, or local gravity anomalies could be compared to the atlas predictions to refine models of regolith transport.
As Herreros and Charnoz demonstrate, interpreting millimetre- to gram-scale samples from small bodies benefits strongly from having a dynamical, not just morphological, context. With MMX's launch set for October 2026 and a planned Phobos arrival in 2027, the atlas is timely input for a mission whose returned samples may bear on fundamental questions about the formation history of Mars' moons and the broader evolution of the inner Solar System.
