Internal water storage capacity of core-dominated planets and the effect of hydration on the M-R relation
Abstract
Water is a key component in many solar system objects. The discovery of low-density exoplanets in the super-Earth mass regime furthermore suggests that ocean-planets could be abundant in the galaxy. To constrain the interiors of water-rich planets, understanding the chemical interactions between water and Mg-silicates is essential. Hydration effects have, however, been mostly neglected by the astrophysics community so far. As such effects are unlikely to have major impacts on theoretical mass-radius relations in the context of exoplanets this is justified as long as the measurement uncertainties are large. However, upcoming missions, such as the PLATO mission (scheduled launch 2026), are envisaged to reach a precision of up to ~ 3% and ~ 10% for radii and masses, respectively. As a result, we may soon enter an area in exoplanetary research where various physical and chemical effects such as hydration can no longer be ignored. Our goal is to construct interior models for core-dominated planets which include reliable prescriptions for mantle hydration. These models can be used to refine previous results for which hydration has been neglected and to guide future characterization of observed exoplanets. We have developed numerical tools to solve for the structure of multi-layered planets with variable boundary conditions and compositions. We couple these tools with equations of state of both dry and hydrated minerals. The hydration behaviour is obtained by constructing water saturation models based on available data from both experiments and first-principle calculations. Here we compare two cases to estimate the effect of hydration on mass-radius relations for simple bulk compositions: Hydrated mantles vs. dry mantles and hydrated mantles vs. dry mantles + ocean with the same amount of water being added as a differentiated surface layer. We find water storage capacities in the hydrated planets of 0 - 4.5 wt% corresponding to up to ~ 210 km deep ocean layers for the ocean planets. The effect of hydration on the total radius, denoted as δR1, is found to be < 1.5% whereas the effect of differentiation into an isolated surface ocean, δR2 , is generally larger (< 2.5 %). Furthermore, we find that the water content is very sensitive to the bulk composition. As a result, future studies accounting for more complex bulk compositions are desirable.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMP076.0005S
- Keywords:
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- 0738 Ice;
- CRYOSPHERE;
- 4299 General or miscellaneous;
- OCEANOGRAPHY: GENERAL;
- 6207 Comparative planetology;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 6297 Instruments and techniques;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS