Water Trapping on Tidally Locked Terrestrial Planets Requires Special Conditions

TL;DR

This study uses a comprehensive climate model to investigate conditions under which tidally locked planets could trap water on their night side, affecting planetary habitability.

Contribution

It demonstrates that water trapping on tidally locked planets depends on continent distribution, geothermal heat flux, and surface water amount, providing new insights into planetary habitability.

Findings

Night-side sea ice remains around 10 meters thick, preventing complete water trapping.

Large continents on the night side can form thick ice sheets, reducing but not eliminating surface water.

Planets with low geothermal heat flux and extensive continents are most susceptible to water trapping.

Abstract

Surface liquid water is essential for standard planetary habitability. Calculations of atmospheric circulation on tidally locked planets around M stars suggest that this peculiar orbital configuration lends itself to the trapping of large amounts of water in kilometers-thick ice on the night side, potentially removing all liquid water from the day side where photosynthesis is possible. We study this problem using a global climate model including coupled atmosphere, ocean, land, and sea-ice components as well as a continental ice sheet model driven by the climate model output. For a waterworld we find that surface winds transport sea ice toward the day side and the ocean carries heat toward the night side. As a result, night-side sea ice remains O(10 m) thick and night-side water trapping is insignificant. If a planet has large continents on its night side, they can grow ice sheets O(1000 m) thick if the geothermal heat flux is similar to Earth's or smaller. Planets with a water complement similar to Earth's would therefore experience a large decrease in sea level when plate tectonics drives their continents onto the night side, but would not experience complete day-side dessication. Only planets with a geothermal heat flux lower than Earth's, much of their surface covered by continents, and a surface water reservoir O(10 %) of Earth's would be susceptible to complete water trapping.