[P-AE22] Exoplanets

[PAE22-09] Water Storage Capacity of Rocky Exoplanet Interiors

*Christine Houser¹, Cayman T Unterborn², Jun Tsuchiya³ (1.Earth-Life Science Institute, Tokyo Institute of Technology, 2.Arizona State University, 3.Geodynamics Research Center, Ehime University)

Keywords:mantle transition zone, water worlds, mineral physics

A tectonically active planet has the capacity to transport and store water deep in its interior. The Earth transports water from the surface into the upper mantle by the subduction of cold oceanic lithosphere at subduction zones. On Earth we observe that most of the water returns to the surface, but some water may leak deeper into the planet. The minerals wadsleyite and ringwoodite are high pressure forms of olivine that comprise a majority of the mineralogy between approximately 400-700 km of Earth’s mantle (ie. the mantle transition zone). While the upper and lower mantle minerals can host almost no water in their structure, the transition zone minerals can host hydrogen as defects in their crystal structure in the range of 1-3 wt.% water. Thus, the transition zone minerals have the greatest potential for long-term water storage in a rocky mantle. In addition, there are minor phases that can host hydrogen in their crystal structure under colder temperature and higher aluminum conditions. Here we show how the storage capacity of water in a planet's interior changes as the planet grows from Mars size to two Earth masses and how water at the surface modifies this internal storage capacity. Assuming Earth composition, as planet mass increases, the transition zone minerals shift to shallower depth creating a thinner shell that can store water. Since this shell is moving to larger radius as planet mass increases, we find the volume increases rapidly as a planet grows from Mars to Earth size and then plateaus for Earth to 2xEarth-mass planets. As oceans of water accumulate at the planet’s surface, the interior pressure and temperature profile changes due to the change in the density profile which also shifts the transition zone minerals to shallower depth. We find that a dynamic planet can quickly saturate its transition zone if tectonics transports water into the deep interior. Once these minerals are saturated, any additional water supply will lead to flux melting, such as that which drives arc volcanism in Earth's upper mantle. Thus, tectonically active, water-rich rocky planets may likely be able to sustain volcanism although the depth and nature of the dehydration reaction changes with time.