Water-Trapped Worlds

TL;DR

Tidally-locked planets around M-dwarfs can trap most of their water as ice on the nightside, potentially leaving the habitable dayside dry, which influences their habitability and surface conditions.

Contribution

This paper introduces a model showing how water can be trapped on the nightside of tidally-locked planets, reducing surface water on the habitable side.

Findings

Planets with less than a quarter Earth's oceans can trap most surface water as ice.

Trapped water on the nightside can leave the dayside dry and potentially habitable.

The distribution of residual liquid water depends on geophysical factors like CO2 regulation.

Abstract

Although tidally-locked habitable planets orbiting nearby M-dwarf stars are among the best astronomical targets to search for extrasolar life, they may also be deficient in volatiles and water. Climate models for this class of planets show atmospheric transport of water from the dayside to the nightside, where it is precipitated as snow and trapped as ice. Since ice only slowly flows back to the dayside upon accumulation, the resulting hydrological cycle can trap a large amount of water in the form of nightside ice. Using ice sheet dynamical and thermodynamical constraints, I illustrate how planets with less than about a quarter the Earth's oceans could trap most of their surface water on the nightside. This would leave their dayside, where habitable conditions are met, potentially dry. The amount and distribution of residual liquid water on the dayside depend on a variety of geophysical factors, including the efficiency of rock weathering at regulating atmospheric CO2 as dayside ocean basins dry-up. Water-trapped worlds with dry daysides may offer similar advantages as land planets for habitability, by contrast with worlds where more abundant water freely flows around the globe.