World Science Scholars
3.2 Life at the Surface
summary
The three possible surfaces of super-Earths

• The first type are water planets in which water constitutes a significant percentage of the planetary mass.
• These planets would have extremely deep oceans, on the order of 100km, that would cause so much pressure at their depths that water would solidify into exotic and dense forms of ice. This ice would form a barrier between the rocky crust and the ocean.
• This is different from the second type, which are still water planets but with much shallower oceans. This is critical because these planets would not have ice at their ocean floors, allowing direct contact between rock and liquid water.
• Earth falls into the third category of rocky planets with incomplete ocean coverage, allowing the rocky crust to be partially exposed to the atmosphere.
• The critical question is: how common is this third type of planet? It requires a very finely-tuned percentage of water–even 0.2% of the planetary mass as water will place a planet in the second category.

The deep water cycle

• There is a process, well-known on the earth, which might make the third type of super-Earth more common than previously thought.
• The deep water cycle is a process by which water from the surface and atmosphere is cycled through the crust into the mantle and then back out again. Specifically, water-laden crust from the ocean floor is slowly pulled down into the mantle, bringing copious amounts of water with it.
• Negative feedback is a hallmark of the deep water cycle. If too much water is drained from the surface, more is pushed back up from the mantle into the atmosphere and oceans, and vice versa.
• Some modeling studies have suggested that the deep water cycle applies to large super-Earths, raising hope that planets with significantly more water than Earth might still have partially-dry, rocky surfaces if much of their water ends up trapped below the crust.

Determining atmospheric features

• The surface itself is only half of the story–atmosphere also plays an enormous role in the chemistry of life. This raises another critical question: what controls the atmosphere of these super-Earths?
• The atmospheric development of Earth is already well understood. A major component of Earth’s atmospheric composition is the carbon cycle.
• Like the deep water cycle, the carbon cycle is the process by which carbon (primarily in the form of $CO_2$) transitions between reservoirs in the atmosphere, oceans, crust, and mantle. An important step in the cycle is the interaction between $CO_2$ dissolved in water and the rocks of the crust.

The carbon cycle on water planets

• On Earth, the carbon cycle is dependent on rocks exposed to the atmosphere or to water, so there could be problems achieving a carbon cycle on water planets.
• It turns out that some carbon will be able to break through the high-pressure ices on the ocean floors of water planets. The ice actually convects, albeit extremely slowly, and over time CO2 will reach the surface.
• However, this $CO_2$ would not reach the surface unscathed. The high pressures of the ocean will encase it in a solid lattice of water molecules known as a clathrate. Therefore clathrates will play a critical role in determining which gases reach the surface.
• Because the atmospheric dynamics of water planets could be so fundamentally different from those of Earth-like rocky planets, measurements of atmospheric gas abundance could allow us to easily classify exoplanets as either water planets or rocky planets.

The Kepler Mission has dramatically increased the pool of Super-Earths that we can search.

• With only three basic measurements of an exoplanet – radius, mass, and atmospheric composition – astronomers can now begin to distinguish rocky, partially-dry super-Earths from water planets.
• Thanks to the Kepler mission, astronomers now estimate that there are 5 billion super-Earths available for astronomical exploration. While this is not necessarily a large number within our very large galaxy, it provides a starting point for future searchers.
• Kepler has also shown us that there are super-Earths much closer to Earth than previously believed. This allows for more focused future missions, like the Transiting Exoplanet Survey Satellite (TESS).

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