Saturday, November 15, 2014

Trapping Water on the Night Side of a Planet

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.

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.

Yang et al. (2014), “Water Trapping on Tidally Locked Terrestrial Planets Requires Special Conditions”, arXiv:1411.0540 [astro-ph.EP]