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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The code used to generate the datasets is available from the corresponding author on reasonable request.
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.
The authors declare no competing interests.
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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). https://doi.org/10.1038/s41561-019-0323-9
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