  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.
