Periodic Reporting for period 1 - DIVERSE (DIVerse Exoplanet Redox State Estimations)
Período documentado: 2023-06-01 hasta 2025-11-30
New observational capabilities with the JWST and ARIEL space telescopes will strongly advance our ability to characterize exoplanetary atmospheres. While the community focusses mainly on biosignatures, in DIVERSE we will search for signatures of geophysical factors that influence habitability, specifically the diversity of planetary redox states. The redox state is of major importance for habitability, since reducing conditions favour prebiotic chemistry for life as we know it.
Atmospheres of rocky planets are typically divided into two distinct classes – H2/He-dominated (reduced) atmospheres of primordial origin, or secondary (more oxidized)
atmospheres of volcanic origin. In the Solar System, observations are limited to old, evolved atmospheres that became oxidized over time and do not allow to directly constrain the planets’ interior redox states. Furthermore, detection of reduced species such as CO or CH4 does not unambiguously link back to the interior redox state.
In contrast, if we were able to detect H2-dominated atmospheres lacking He, the most likely explanation would be strongly reduced degassing from the magma ocean or subsequent volcanism. Distinguishing these planets from those with primordial atmospheres would truly allow to constrain the planetary redox state and indicate how it depends on observables such as stellar composition or planetary mass. Estimates on the distribution and observability of planets with secondary outgassed H2 atmospheres are yet missing but became recently possible.
DIVERSE will build strong predictive, theoretical models, linking the interior evolution including core formation with atmospheric abundance and erosion models including the observability potential, to determine the diverse evolution pathways of reducing atmospheres of primary, secondary or hybrid origin. We will thus address whether (and for which planet types) the atmosphere could indeed serve as a window into the interior.
Atmospheres of rocky planets are typically divided into two distinct classes – H2/He-dominated (reduced) atmospheres of primordial origin, or secondary (more oxidized)
atmospheres of volcanic origin. In the Solar System, observations are limited to old, evolved atmospheres that became oxidized over time and do not allow to directly constrain the planets’ interior redox states. Furthermore, detection of reduced species such as CO or CH4 does not unambiguously link back to the interior redox state.
In contrast, if we were able to detect H2-dominated atmospheres lacking He, the most likely explanation would be strongly reduced degassing from the magma ocean or subsequent volcanism. Distinguishing these planets from those with primordial atmospheres would truly allow to constrain the planetary redox state and indicate how it depends on observables such as stellar composition or planetary mass. Estimates on the distribution and observability of planets with secondary outgassed H2 atmospheres are yet missing but became recently possible.
DIVERSE will build strong predictive, theoretical models, linking the interior evolution including core formation with atmospheric abundance and erosion models including the observability potential, to determine the diverse evolution pathways of reducing atmospheres of primary, secondary or hybrid origin. We will thus address whether (and for which planet types) the atmosphere could indeed serve as a window into the interior.
Our main work during the first two years of the DIVERSE project focussed on the development of groundbreaking new models as needed to address the main questions of the project. The DIVERSE model suite now covers thermal evolution during the accretion of low-mass planets (WP1), first preliminary model developments on the formation of a metal core (WP1 and WP2) while accounting for compositional variations affecting mantle mineralogy and core size (WP2), variations in the redox state during outgassing of reduced gases to the atmosphere (WP2 and WP3) and evolution of the resulting atmosphere incl. losses to space (WP3). A few first publications have been published or submitted highlighting these novel model developments and revealing already the first results obtained within the DIVERSE project, several publications on our main results are now in preparation and will be published in the remaining three years of the project.
Our novel 1D grid-based interior thermal model for a magma ocean planet uses a stepwise accretion modeling of the planet using pebble accretion rate to account for mass growth at each time step. The core formation is then modelled by assuming that iron droplets efficiently sink through the entire mantle to form a core, which will be adapted in the remaining time of the project to account for improved estimates on the sinking behaviour under realistic pressure-conditions. To better understand the differentiation of the rocky shell into solid mantle and magma ocean, we implemented a composition-dependent melting temperature calculation by emplying the extended Lindemann-Stacey melting law to estimate melting temperatures.
We also concentrated on the atmospheric evolution over time incl. the incorporation of stellar luminosity evolutionary tracks to model atmospheric escape. In a new model we modelled the redistribution of CHON(S) volatiles between the deep planetary interior, the crystallizing magma ocean, and the atmosphere. Building upon these estimations for initial magma ocean atmospheres, we have begun exploring the long-term evolution of these atmospheres. Our models include a number of relevant processes, such as atmospheric escape processes, carbon weathering, and a simple water condensation scheme. First results show that the magma ocean atmosphere plays a crucial role in the long-term habitability of the planet. Our models furthermore confirm that on Earth-like planets, Jeans escape primarily affects hydrogen. However, under strong escape fluxes, hydrodynamic escape can also drag along heavier atmospheric species, thus significantly altering the composition of the atmosphere for strong escape fluxes. This is particularly important in the early planet evolution, or for planets with substantial volcanic H2 fluxes as studied in the DIVERSE project.
We developed a coupled model that calculates the chemistry-dependent degassing from the interior (applicable to both magma ocean bodies and volcanism for solid mantles) and atmospheric buildup. We can thus track how the evolving atmosphere influences further degassing over billions of years due to atmospheric chemical evolution (assuming for now chemical equilibrium in the lower atmosphere together with photodissociation in the upper atmosphere) and solubility of CHONS volatiles in the melt. This model provides a key link between planetary interiors and atmospheres and can predict atmospheric composition and pressure for planets with varying initial redox states. Several follow-up publications utilizing this new tool (within our group and within the external scientific community) are currently in preparation.
We also concentrated on the atmospheric evolution over time incl. the incorporation of stellar luminosity evolutionary tracks to model atmospheric escape. In a new model we modelled the redistribution of CHON(S) volatiles between the deep planetary interior, the crystallizing magma ocean, and the atmosphere. Building upon these estimations for initial magma ocean atmospheres, we have begun exploring the long-term evolution of these atmospheres. Our models include a number of relevant processes, such as atmospheric escape processes, carbon weathering, and a simple water condensation scheme. First results show that the magma ocean atmosphere plays a crucial role in the long-term habitability of the planet. Our models furthermore confirm that on Earth-like planets, Jeans escape primarily affects hydrogen. However, under strong escape fluxes, hydrodynamic escape can also drag along heavier atmospheric species, thus significantly altering the composition of the atmosphere for strong escape fluxes. This is particularly important in the early planet evolution, or for planets with substantial volcanic H2 fluxes as studied in the DIVERSE project.
We developed a coupled model that calculates the chemistry-dependent degassing from the interior (applicable to both magma ocean bodies and volcanism for solid mantles) and atmospheric buildup. We can thus track how the evolving atmosphere influences further degassing over billions of years due to atmospheric chemical evolution (assuming for now chemical equilibrium in the lower atmosphere together with photodissociation in the upper atmosphere) and solubility of CHONS volatiles in the melt. This model provides a key link between planetary interiors and atmospheres and can predict atmospheric composition and pressure for planets with varying initial redox states. Several follow-up publications utilizing this new tool (within our group and within the external scientific community) are currently in preparation.