The Origin of Sub-Neptune’s Atmosphere: The Story of Magma and Atmosphere

Title: The origins of the atmosphere of an exoplanet

Authors.: Edwin S. Kate, Bruce Fegley Jr., Laura Schaefer, Eric B. Ford

First Author Foundation: Department of Geophysical Sciences, University of Chicago, Chicago, Illinois, USA

condition: Posted in a B C

One of the most important topics in astronomy is finding potentially habitable planets. Many telescopes are working to unveil these planets, such as KeplerAnd the he-goat It was launched recently JWST. To check habitability, we must understand the planet’s surface environment. However, we cannot currently directly see the surface of the planet. The only way to explore the environment is through transmission spectrum of the planet, which can track the presence of water and other particles in the atmosphere of an exoplanet. In short, the transmission spectrum measures the contrast Crossing Depth at different wavelengths. With limited observations, various theories and models are needed to infer the surface and interior composition of the planet from atmospheric observations. Today’s paper looks at the interaction between the atmosphere and magma, with the goal of identifying the source of hydrogen and oxygen in the atmosphere. The evolution from a proto-seed to a habitable planet is a long walk. This work helps us understand one major step in it.

Earth-size rocky (or terrestrial) planets are a prime candidate for hosting life, simply because we better understand life on Earth. However, their small size relative to the host stars prevents us from determining their atmospheres, because the transit depth is small. Neptune, which has a radius of 1.6–3.2 Earth radii and a density of <4 g/cm^3, is easier to observe than Earth-sized planets due to its larger size. Our current knowledge of rocky sub-Neptunes assumes that it consists of a core, a rocky shell, a magma shell, and a center-to-surface atmosphere, as shown in Figure 1. Based on this assumption, atmospheric interactions with magma are expected to dominate the atmospheric mass and configure it. This work investigates the two most important reactions illustrated in Figure 1: atmospheric dissolution of magma and chemical reactions involving the atmosphere and magma, and explains how these influences shape the atmosphere.

FFigure 1. A simple illustration of the sub-Neptune structure assumed in this work. The figure also shows the two main processes of magma-atmosphere interaction, namely oxidation-reduction and volatile dissolution. Figure 1 in the paper.

The authors postulate only planets containing Fe, Mg, Si, O, and H. However, since the abundance of the elements remains unclear, they were taken as free parameters in the model. Iron can only react with water to form hydrogen and iron oxide. The balances of the hydrogen solution and water vapor are also calculated: gas can be absorbed by magma, changing the mass and content of the atmosphere. The magma-atmosphere interface temperature was fixed at 3000 K, and then the authors built a model to calculate the mass of molten rock, that is, the mass of magma. With the chemical properties extrapolated from the experimental data, it solves the equilibrium state of the above mentioned physical and chemical processes and obtains the mass of hydrogen in the atmosphere, the mass of hydrogen in the magma, the mass of water in the atmosphere, and the mass of water in the magma.

In this work, researchers study the state of equilibrium in two different scenarios. One when elemental hydrogen comes from nebula accumulationLike earth, magma contains iron oxide. In this scenario, iron oxide reacts with hydrogen and forms iron. As more hydrogen accumulates in the nebula, more hydrogen is generated. However, due to the high solubility in water, most of it is dissolved in magma. Hydrogen still dominates the atmosphere, resulting in a lower average molecular weight of the atmosphere. The other scenario is that elemental hydrogen is created by iron and water in the planet, and water comets and asteroids bring water to the planet as elemental water. As the primary water grows, it gradually becomes the main component in the atmosphere, and the average molecular weight is constantly increasing up to >10 times the mass of hydrogen.

These results show magma issues. For example, suppose we know the radius of a sub-Neptune of five Earth masses and the hydrogen all come from the accretion nebula, volatile The mass remains undetermined due to the unspecified fraction of iron oxide in the planet. The discrepancy is even more pronounced when compared to the results in which we neglect gas dissolution. It also indicates that increasing or decreasing the radius is more difficult than previously thought, due to the large amount of insulating gases dissolved in the magma. Fortunately, one can, equipped with an average molecular weight and observing the mass of the atmosphere, know the origin of the planet’s atmosphere, as shown in Figure 2.

Figure 2. The interaction of magma and the atmosphere results in a different atmosphere mass and average molecular weight. Region A corresponds to planet or planet migration, or the dissolution of Si in the metallic iron core. If a planet is observed in the B region, its atmosphere is likely to have originated from asteroids and comets. Planets in region C gain gas through nebula accretion. It is unlikely that the D-zone planets can be explained by the model in this work, while the origin of the atmosphere of the E-zone cannot be determined simply by these two factors. Figure 8 in the paper.

This work adopted a simplified model with many assumptions and limitations; For example, most of the chemical data they used have not been verified by laboratory and/or numerical experiments. However, it still indicates that the atmosphere should not be treated as an isolated layer. With the creation of more realistic models in the future, a more accurate and accurate interaction picture will be presented before our eyes. With this image, we can further investigate how a planet might have a habitable atmosphere and, finally, how a planet might be habitable.

Astrobite Edited by Jason Hinkle

Featured Image Credit: earth.com