The presence of liquid water on a planet’s surface is a
prerequisite for habitability. Planets circling within the habitable zone of
red dwarf stars are believed to be tidally-locked. This is because red dwarf
stars are much cooler than stars like the Sun and a planet must be situated
much closer in to receive a similar amount of warmth Earth gets from the Sun.
As a result, strong tidal interaction between the planet and its host star quickly
drives the planet into a tidally-locked configuration. A tidally-locked planet
always presents the same hemisphere towards its host star, resulting in a
permanent day side and a permanent night side.
On the planet’s permanent night side, large amounts of water
can become trapped in kilometres-thick ice sheets. This mechanism is known as
water trapping and it can potentially cause the planet’s day side to be depleted
of water. The consequence is that the planet becomes less habitable or not
habitable at all since it is only on the planet’s day side where photosynthesis
is possible. A study by Yang et al. (2014) suggests that water trapping is
unlikely to remove all the water from the day side of a tidally-locked planet.
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For a planet that is mostly covered by ocean, surface winds
transport sea ice toward the day side and ocean currents transport heat toward
the night side. As a result, sea ice on the planet’s night side remains thin
and water trapping is insignificant. Water trapping starts to become significant
on a planet whose water content and continental coverage is similar to Earth’s.
Ice sheets with thickness ~1,000m can form on continents located on the
planet’s cold night side. The trapping of so much water on the night side creates
a large decrease in the planet’s sea level.
Furthermore, if plate tectonics happen to move all
continents to the night side, water trapping would become more severe.
Nonetheless, if the planet’s geothermal heat flux is similar to Earth’s, the
thickness of the ice sheet on the planet’s night side would be limited and
complete removal of water from the planet’s dayside is unlikely. For water
trapping to remove all water from a planet’s dayside, a combination of special
conditions must be met. These conditions include the planet having a geothermal
heat flux lower than Earth’s, most of its surface is covered by continents and
its surface water content is only ~10 percent of Earth’s.
Reference:
Yang et al. (2014), “Water Trapping on Tidally Locked
Terrestrial Planets Requires Special Conditions”, arXiv:1411.0540 [astro-ph.EP]