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Surface refreshing of Martian moon Phobos by orbital eccentricity-driven grain motion


The surface of the Martian moon Phobos exhibits two distinct geologic units, red and blue, characterized by their spectral slopes. The provenance of these units is uncertain yet crucial to understanding the origin of the Martian moon and its interaction with the space environment. Here we present a combination of dynamical analyses and numerical simulations of particle dynamics to show that periodic variations in dynamic slopes, driven by orbital eccentricity, can cause surface grain motion. For regions with steep slopes that vary substantially over one Phobos orbit, the surface is excavated at a faster rate than the space weathering timescale. Our model predicts that this new mechanism is most effective in regions that coincide with blue units. Therefore, space weathering is the likely driver of the dichotomy on the moon’s surface, reddening blue units that represent pristine endogenic material.

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Fig. 1: Changes in the local surface slopes over one Phobos orbit.
Fig. 2: Local simulations of grain motion on the surface.
Fig. 3: High-slope regions around Stickney crater that undergo large variations coincide with blue surface units.
Fig. 4: The blue regions across Phobos undergo eccentricity-driven regolith motion.
Fig. 5: Required volume fluxes necessary to uncover subsurface material before a certain weathering timescale.

Code availability

The code used to generate the datasets is available from the corresponding author on reasonable request.

Data availability

The datasets generated and analysed during the current study are available from the corresponding author on reasonable request.


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R.-L.B acknowledges support from JAXA’s Aerospace Project Research Associate Program. N.B. conducted this work as a JSPS International Research Fellow. S.T.C. was supported by the JAXA International Top Young Fellowship Program. The authors also thank P. Michel for constructive feedback on the results and implications of this work. Grain dynamics simulations were calculated on the YORP cluster run by the Center for Theory and Computation at the Department of Astronomy at the University of Maryland. For data visualization, the authors made use of the freeware, multi-platform, ray-tracing package, Persistence of Vision Raytracer.

Author information




R.-L.B. conceptualized the study, designed and performed the local simulations of granular dynamics, and led the research. N.B. initiated the project through a study of the three-body elliptical problem on Phobos, performed the gravitational dynamics calculations, and contributed to the analyses. S.T.C. provided geophysical and geomorphological expertise, remote-sensing analysis, and constructed the model for regolith development on Phobos. Y.K. provided guidance and advice on the formulation and scope of the research. M.F. provided guidance and discourse on the implications of the results. Y.K. and M.F. provided expertise in small-body exploration and contextualized the research in the frame of the MMX mission. All authors contributed to the interpretation of the results and preparation of the manuscript.

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Correspondence to Ronald-Louis Ballouz.

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Ballouz, RL., Baresi, N., Crites, S.T. et al. Surface refreshing of Martian moon Phobos by orbital eccentricity-driven grain motion. Nat. Geosci. 12, 229–234 (2019).

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