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The demise of Phobos and development of a Martian ring system

Nature Geoscience volume 8, pages 913–917 (2015)Cite this article

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Abstract

All of the gas giants in our Solar System host ring systems, in contrast to the inner planets. One proposed mechanism of planetary ring formation is disruption or mass shedding of moons. The orbit of Phobos, the larger of Mars’s two moonlets, is gradually spiralling inwards towards Mars and the moon is experiencing increasing tidal stresses. Eventually, Phobos will either break apart to form a ring or it will crash into Mars. We evaluate these outcomes based on geologic, spectral and theoretical constraints, in conjunction with a geotechnical model that helps us determine the strength of Phobos. Our analysis suggests that much of Phobos is composed of weak, heavily damaged materials. We suggest that—with continued inward migration of the moon—the weakest material will disperse tidally in 20 to 40 million years to form a Martian ring. We predict that this ring will persist for 106 to 108 years and will initially have a comparable mass density to that of Saturn’s rings. Any large fragment of Phobos that is strong enough to escape tidal breakup will eventually collide with Mars in an oblique, low-velocity impact. Our analysis of the evolution of Phobos underscores the potential orbital and topographic consequences of the growth and self-destruction of other inwardly migrating moons, including those that met their demise early in our Solar System’s history.

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Figure 1: The orbit of Phobos evolves inwards with time6,7.
Figure 2: Tidal stresses compete with the strength of Phobos.
Figure 3: Ultimately Phobos will either break apart or crash into Mars.
Figure 4: The expected lifetime of a Martian ring is at least 106–108 Myr.

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Acknowledgements

B.A.B. thanks the Open Earth Systems project funded by National Science Foundation grant EAR-1135382. K. Ferrier, B. Marsh, H. Lenferink, B. Buffett, R. Citron and M. Manga provided valuable feedback on earlier versions of this work. The authors gratefully acknowledge the support of M. Manga.

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  1. Department of Earth and Planetary Science, University of California, Berkeley, Berkeley, California 94720, USA

    Benjamin A. Black & Tushar Mittal

  2. Department of Earth and Atmospheric Science, The City College of New York, City University of New York, 160 Convent Avenue, New York City, New York 10031, USA

    Benjamin A. Black

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B.A.B. and T.M. jointly planned the research, performed the calculations, and wrote and revised the manuscript.

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Black, B., Mittal, T. The demise of Phobos and development of a Martian ring system. Nature Geosci 8, 913–917 (2015). https://doi.org/10.1038/ngeo2583

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