For a planet like the Earth, water plays a fundamental role in the global climate system. Today, the Earth is in radiative equilibrium balance where the insolation it receives from the Sun is balanced by outgoing infrared emission from the planet’s surface. This gives the Earth an average surface temperature of 288 K or 15 degrees Centigrade. If the Earth were closer to the Sun, it will receive a greater amount of insolation from the Sun and this will heat up the planet, causing more water vapour to be released into the atmosphere. The increase in atmospheric water vapour brings about a stronger greenhouse effect. This causes the Earth to give off more infrared radiation to balance the larger insolation it is getting from the Sun. As a result, if the Earth were nearer to the Sun, it will settle into a new radiative equilibrium balance with a somewhat higher mean surface temperature.
Nevertheless, there is a minimum distance a planet like the Earth can be from its parent star before the amount of insolation received by the planet becomes so great that radiative equilibrium balance can never be achieved. Any closer than this minimum distance, the planet undergoes what is known as a runaway greenhouse effect. As more water vapour is released into the planet’s atmosphere, the greenhouse effect becomes stronger. This causes the planet to become hotter and release yet more water vapour and so on. Eventually, all water on the planet’s surface is vaporised and the surface temperature everywhere on the planet settles at a scorching temperature of ~1400 K, rendering the planet uninhabitable for life. Due to the onset of runaway greenhouse effect, the minimum distance a habitable Earth-like planet can be from its parent star denotes the inner edge of the habitable zone around the star. Any nearer, a runaway greenhouse effect occurs and makes the planet too hot to be habitable.
Although a runaway greenhouse effect seems like a certainty for an Earth-like planet that is too close to its parent star, it may not always be the case since it assumes that there is a sufficient reservoir of water everywhere on the planet’s surface. In fact, a planet whose surface is predominately land with only small areas of surface water can escape a runaway greenhouse effect entirely. A planet like this is referred to as a land planet. In contrast, the Earth is commonly referred to as an ocean planet. On a land planet, the rate at which water is delivered to the atmosphere is too slow to trigger a runaway greenhouse effect. Water that evaporates from the hot regions simply get transported and deposited in permanent cold traps on the planet. The atmosphere of the land planet settles into a collapsed state with most of its water inventory being captured in permanent cold traps. Potential permanent cold traps include the planet’s night side or the planet’s poles. It is important to note that the night side of a land planet can only serve as a cold trap if the planet is tidally-locked where one hemisphere of the planet constantly faces its parent star while the other hemisphere is in perpetual night.
Figure 1: Artist’s impression of a desert planet with polar ice caps.
Figure 2: Artist’s impression of a desert planet.
The cold trapping rate depends very much on the thickness of the planet’s atmosphere. A thicker atmosphere will lead to less efficient cold trapping as the permanent cold traps are warmer. The outcome is that a land planet with an atmosphere that is not too thick can settle into a collapsed state by accumulating most of its water inventory in permanent cold traps at a rate that is sufficiently high to keep the planet from entering a runaway greenhouse state. At these permanent cold traps, the low temperatures allow water to accumulate as ice to form ice sheets. For a land planet that is not tidally-locked and whose axial tilt is not too large, ice sheets can exist over the poles. Similarly, a land planet that is tidally-locked can have ice sheets on its permanent nightside hemisphere. The low temperatures at the permanent cold traps suggest that all accumulated water is likely to be in the form of solid ice. Nevertheless, it is worth considering if there are any processes that can sustain bodies of liquid water for extended periods of time since liquid water is an essential prerequisite for habitability.
As water accumulates as ice on an ice sheet, the ice sheet thickens and gravity drives the flow of ice within the ice sheet. This has the effect of transporting ice away from the cold traps, towards warmer regions where the ice can potentially melt to produce liquid water. On Earth, ice sheets that are just a couple of kilometres thick can transport ice over distances of several hundred kilometres. As a result, one can imagine a land planet with an ice sheet over one of its poles or over its nightside hemisphere. Gravity driven ice flows bring the edge of the ice sheet towards regions where the temperature is warm enough for melting to occur. This leads to liquid water being present along the edge of the ice sheet, akin to pools of melt water at the end of a glacier. Here, water evaporates and precipitates back onto the ice sheet. In addition to liquid water at the edge of the ice sheet, bodies of liquid water can also exist within the ice sheet itself if the ice sheet happens to be over regions of sufficiently high geothermal heat flux.
Gliese 581c is a super-Earth orbiting the red dwarf star Gliese 581. This exoplanet has at least 5.6 times the mass of Earth and it orbits at the inner edge of the habitable zone around its parent star. The close proximity to its parent star means that Gliese 581c is likely to be tidally-locked, with a permanent nightside and a permanent dayside. Assuming that Gliese 581c is a land planet with a small inventory of water and its atmosphere is in a collapsed state where water is captured in permanent cold traps, a simulation was preformed by Leconte et al. (2013) to determine its properties. If Gliese 581c has a surface atmospheric pressure of 200 mbar, the temperature on the permanent nightside hemisphere can be as low as -100 degrees Centigrade. In contrast, the temperature on the planet’s permanent dayside can get higher than 200 degrees Centigrade around the substellar point. Most of the dayside hemisphere of Gliese 581c is simply a blistering hot desert.
Figure 3: Schematic diagram of Gliese 581c showing a dry dayside hemisphere and an ice sheet over the nightside hemisphere. Credit: Leconte et al. (2013)
A land planet with a limited water inventory can escape a runaway greenhouse state and settle into a collapsed state with most of its water being accumulated in ice sheets over its coldest regions. This is an interesting scenario because such a planet can still remain somewhat habitable even though it is closer to its parent star than the inner edge of the star’s classical habitable zone. Thermal emission spectrum and reflection spectrum from a planet can be used to differentiate whether the planet is in a runaway greenhouse state or if the planet is in a collapsed state with most of its water locked in ice sheets. If the planet is in a runaway greenhouse state, its thermal emission spectrum will shift toward shorter wavelengths due to hotter temperatures and its reflection spectrum will feature prominent water absorption bands due to the large abundance of water vapour in the atmosphere. If the planet is in a collapsed state where water is captured in permanent cold traps, its thermal emission spectrum will shift toward longer wavelengths due to cooler temperatures and its reflection spectrum will be more uniform due to the low abundance of water vapour in the atmosphere.
Figure 4: Synthetic reflection spectrum (top) and thermal emission spectrum (bottom) of Gliese 581c assuming the planet has a 200 mbar atmosphere. The dash lines show the runaway greenhouse state while the solid lines show the collapsed state. Credit: Leconte et al. (2013)
Leconte et al. (2013), “3D climate modeling of close-in land planets: Circulation patterns, climate moist bistability and habitability”, arXiv:1303.7079 [astro-ph.EP]