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Transition from eyeball to snowball driven by sea-ice drift on tidally locked terrestrial planets


Tidally locked terrestrial planets around low-mass stars are the prime targets for future atmospheric characterizations of potentially habitable systems1, especially the three nearby ones—Proxima b (ref. 2), TRAPPIST-1e (ref. 3) and LHS 1140b (ref. 4). Previous studies suggest that if these planets had surface oceans they would be in an eyeball-like climate state5,6,7,8,9,10: ice free in the vicinity of the substellar point and ice covered in the remaining regions. However, an important component of the climate system—sea-ice dynamics—has not been fully considered in previous studies. A fundamental question is whether an open ocean is stable against a globally ice-covered snowball state. Here we show that sea-ice drift cools the ocean’s surface when the ice flows towards the warmer substellar region and melts through absorbing heat from the ocean and the overlying air. As a result, the open ocean shrinks and can even disappear when atmospheric greenhouse gases are not much more abundant than on Earth, turning the planet into a snowball state. This occurs for both synchronous rotation and spin–orbit resonances (such as 3:2). These results suggest that sea-ice drift strongly reduces the open-ocean area and can significantly impact the habitability of tidally locked planets.

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Fig. 1: Sea-ice concentrations in synchronous rotation orbits.
Fig. 2: Physical mechanisms for the effect of sea-ice drift.
Fig. 3: Effects of continents on the sea-ice concentrations.
Fig. 4: Snapshots of ice concentrations in 3:2 resonance orbits.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author on reasonable request.

Code availability

The source codes of the model CCSM3 can be downloaded from, and changes of the model are available from the corresponding author.


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We are grateful to F. Ding, T. J. Fauchez, Y. Liu and J. Lin for fruitful discussions, to Y. Hu and Y. Ashkenazy for their great help in improving the manuscript and to Y. Wang for his help in modifying source codes of the model. J.Y. acknowledges support from the National Natural Science Foundation of China (NSFC) grants 41861124002, 41675071, 41606060 and 41761144072.

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J.Y. formulated the problem, designed the experiments, analysed the data, drew the figures, and wrote the manuscript. J.Y. and W.J. performed the numerical experiments. All authors contributed to data analyses.

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Correspondence to Jun Yang.

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Yang, J., Ji, W. & Zeng, Y. Transition from eyeball to snowball driven by sea-ice drift on tidally locked terrestrial planets. Nat Astron 4, 58–66 (2020).

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