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Edited by Harry L. Swinney, University of Texas at Austin, Austin, TX, and approved December 27, 2016 (received for review July 23, 2016)
Significance
The recent approach to comet 67P/Churyumov–Gerasimenko by the spacecraft Rosetta has revealed the presence of astonishing dune-like patterns. How can the radial outgassing, caused by heating when passing close to the sun, produce a vapor flow along the surface of the comet dense enough to transport grains? Drawing on the physical mechanisms at work for the formation of dunes on Earth and planetary bodies, we quantitatively explain the emergence and size of these bedforms, which are due to thermal winds. This work involves the understanding of the comet surface processes, especially regarding grain cohesion and grain–fluid interaction. It thus provides more keys to address the timely open question on the growth of planetesimals above the meter scale to form planets.
Abstract
Explaining the unexpected presence of dune-like patterns at the surface of the comet 67P/Churyumov–Gerasimenko requires conceptual and quantitative advances in the understanding of surface and outgassing processes. We show here that vapor flow emitted by the comet around its perihelion spreads laterally in a surface layer, due to the strong pressure difference between zones illuminated by sunlight and those in shadow. For such thermal winds to be dense enough to transport grains—10 times greater than previous estimates—outgassing must take place through a surface porous granular layer, and that layer must be composed of grains whose roughness lowers cohesion consistently with contact mechanics. The linear stability analysis of the problem, entirely tested against laboratory experiments, quantitatively predicts the emergence of bedforms in the observed wavelength range and their propagation at the scale of a comet revolution. Although generated by a rarefied atmosphere, they are paradoxically analogous to ripples emerging on granular beds submitted to viscous shear flows. This quantitative agreement shows that our understanding of the coupling between hydrodynamics and sediment transport is able to account for bedform emergence in extreme conditions and provides a reliable tool to predict the erosion and accretion processes controlling the evolution of small solar system bodies.
Footnotes
- ↵1To whom correspondence should be addressed. Email: philippe.claudin{at}espci.fr.
Author contributions: P.J., B.A., and P.C. designed research, performed research, contributed new reagents/analytic tools, analyzed data, and wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1612176114/-/DCSupplemental.
Giant ripples on 67P sculpted by thermal wind
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- Abstract
- Outgassing and Comet’s Atmosphere
- Threshold for Grain Motion and Cohesion
- Emergent Wavelength
- Sediment Transport and Bedforms
- Concluding Remarks
- Materials and Methods
- Geometry and Gravity of the Comet
- Thermo-Hydrodynamics of the Comet’s Atmosphere
- Sediment Transport
- Linear Stability Analysis
- Acknowledgments
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