○ By considering 3 types of planets: Dry planets (Type-1), hydrated planets (Type-2) and ocean planets (Type-3), Shah and colleagues answered three important questions in their study: (i) How much H2O, a planet can store in its interior? (ii) How different is the radius of such a hydrated object in comparison to its dry counterpart? (iii) How much does the radius change if the internal reservoir of H2O equivalent is moved into an isolated surface ocean?
○ They found that internal water storage capacities in the hydrated planets (Type 2, like our earth) equivalent to 0-6 wt% H2O corresponding to up to ≈ 800 km deep ocean layers. In the mass range between 0.1- 3, the effect of hydration on the total radius is found to be less than equal to 2.5% (≤ 2.5%). While, the radii of super-Earths can be considerably smaller if water is stored in the interior instead of an isolated surface ocean.
Shah and colleagues presented planetary structure models that account for the hydration of the Mg-silicate mantles and the iron cores. This allows for assessing the corresponding effects of water-interior interactions and ocean differentiation on the mass-radius relation and the partitioning of water between surface and internal reservoirs.
The discovery of low density exoplanets in the super-Earth mass regime suggests that ocean planets could be abundant in the galaxy. Understanding the chemical interactions between water and Mg-silicates or iron is essential for constraining the interiors of water-rich planets. 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, 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 planets that include reliable prescriptions for hydration of the cores and mantles. These models can be used to refine previous results for which hydration has been neglected and to guide future characterization of observed exoplanets.— said Shah, lead author of the study.
Shah and colleagues have developed numerical tools to solve for the structure of multi-layered planets with variable boundary conditions and compositions.
In order to estimate the effects of hydration and ocean separation on mass-radius (M-R) relations they considered three types of planets: Dry planets (Type-1), hydrated planets (Type-2) and ocean planets (Type-3). The surface oceans are assumed to consist of pure water. An overview of the different types is provided in Fig. 1 above. The Type-1 planets have fully OH and H depleted interiors and no surface oceans. For the Type-2 planets the same boundary conditions, that is bulk composition and surface conditions, as for the Type-1 planets have been employed, but the mantles & cores are hydrated. A comparison between Type-1 and Type-2 planets allows for the assessment of the effect of hydration on total planetary radii. To estimate the effect of ocean differentiation they computed the amount of H2O equivalent in the Type-2 planets and added it as an isolated surface ocean on top of a dry mantle for the Type-3 planets. Since they were interested in estimating maximum effects in this study, intermediate cases where the water is partially distributed between an internal reservoir and a surface ocean, were not considered.
In their paper, they addressed three main questions to illustrate the application of their model:
- What is the maximum amount of H2O equivalent that a terrestrial object of a given size, composition, and surface conditions can store in its interior?
- How different is the radius of such a hydrated object in comparison to its dry counterpart?
- How much does the radius change if the internal reservoir of H2O equivalent is moved into an isolated surface ocean?
They found that internal water storage capacities in the hydrated planets (Type 2, like our earth) equivalent to 0-6 wt% H2O corresponding to up to ≈ 800 km deep ocean layers. In the mass range, between 0.1- 3, the effect of hydration on the total radius is found to be less than equal to 2.5% (≤ 2.5%), whereas the effect of separation into an isolated surface ocean is ≤ 5 %. The radii of super-Earths can be considerably smaller if water is stored in the interior instead of an isolated surface ocean. Furthermore, they found that their results are very sensitive to the bulk composition and planet mass.
These results reveal the necessity of accounting for internal hydration for a more reliable characterization of the compositions and structures of exoplanets, in particular in the super-Earth regime. This will become even more essential with the increased precision of the mass-radius measurements that are expected in the near future.
This work has been carried out within the framework of the National Centre of Competence in Research Planets supported by the Swiss National Science Foundation. The authors acknowledge the financial support of the SNSF.
Reference: Oliver Shah, Yann Alibert, Ravit Helled, Klaus Mezger, “Internal water storage capacity of terrestrial planets and the effect of hydration on the M-R relation”, ArXiv, pp. 1-34, 2021. https://arxiv.org/abs/2012.06455v4
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