Chemical volatile gas speciation and atmospheric evolution on terrestrial planets

Subsequently to the magma ocean state, secondary atmospheres build up via early volcanic degassing of planetary interiors. The terrestrial planets Venus, Earth and Mars are believed to have originated from similar source material but reveal different present-day atmospheric compositions, pressures and temperatures. To investigate how such diverse atmospheres emerge, we have developed a model that allows us to compute the molar fractions of gases involved in volcanic degassing in the C-H-O-N-S system by using the mass balance method and Gibbs energy minimization. The model takes into account pressure, temperature, oxygen fugacity, melt composition and the effect of volatile species on each other. We find that all the parameters mentioned above have a significant effect on the molar composition of the released gases and that hence varying parameters like planet mass, atmospheric pressure, tectonic regime, volatile budget or the abundance of heat sources can lead to significantly different atmospheric compositions and degassing rates.

Parameterisations of interior properties of rocky planets

The observations of Earth-size exoplanets are mostly limited to information on their masses and radii. In this work, we investigate the structure of a rocky planet shortly after formation and later stages of thermal evolution assuming the planet is differentiated into a metal core and a rocky mantle (consisting of Earth-like minerals but variable iron number). We derive possible initial temperature profiles after the accretion and magma ocean solidification. We then develop parameterisations for the thermodynamic properties inside the core depending on planet mass, composition and thermal state. We provide the community with robust scaling laws for the interior structure, temperature profiles and core- and mantle-averaged thermodynamic properties for planets composed of Earth’s main minerals but with variable compositions of iron and silicates. The scaling laws allow to investigate variations in thermodynamic properties for different interior thermal states in a multitude of application such as deriving mass-radius scaling laws or estimating magnetic field evolution and core crystallisation for rocky exoplanets.

Global volatile cycles on early Earth

The understanding of volatile cycles on Earth is essential as they have a huge influence on mantle and surface processes. Outgassing from the interior strongly affects the chemical composition of the atmosphere and the oceans. Volatiles in the mantle influence the rheology, trigger melting and lead to ore deposit formation. At the surface, weathering and recycling acts as a huge carbon sink and has a profound effect on the global climate. The aim of this study is the simulation of these volatile cycles on early Earth. We focus mainly on greenhouse gas forming H-C-N as well as on Xe, Ar and Ne as trace elements. For this approach, we enhance a thermo-chemical model of the lithosphere and deeper mantle. The cycles operate from the interior to the surface and back via outgassing, condensation and crustal recycling. One of the main questions of this project is through which mechanisms volatiles could be recycled into the mantle before the on-set of plate tectonics as it operates today (e.g., by sagduction, delamination, erosion). (picture: Gaillard et al. in review; © Vulpius).

Formation and evolution of crust on early Earth

After the magma ocean phase 4.5 Gyr ago, the radioactive elements K, Th, and U became the most important heat producing elements on Earth. Since their presence highly affect the heat budget of Earth and other terrestrial planets over large time spans, we have to trace their redistribution behavior in the mantle and crust as precisely as possible. In this study, we model partition coefficients depending on pressure, temperature, and melt composition based on Blundy and Wood (2003) by further developing their partitioning model and parameterize it to higher pressure ranges. The resulting code will be implemented into the global mantle convection code CHIC, which will help to determine the melting behavior and primitive crust formation rates of Earth and Mars. Together with an added mineralogical database we will explore the compositional mantle development. In addition, re-melting and differentiation processes of basaltic crust to more felsic material will mark the beginning of continental crust formation. With this, the project aims to create a statistical overview of crustal formation and recycling mechanisms, which will help to improve our understanding of global crustal development processes.

Volatile release from intrusive magma systems

In this subproject we calculate the volatile release of magma bodies emplaced at different depths within the lithosphere. The contribution of outgassing to the build-up and composition of the atmosphere on early Earth is crucial, not least for the emergence and evolution of life, since it is one of the major volatile sources. While studies of extrusive outgassing exist in the literature, intrusive degassing is mainly neglected.

Due to cooling, nominally mafic minerals crystallize out and the remaining melt is enriched with incompatible elements and molecules, including volatiles such as H₂O and CO₂. They accumulate in the melt until the particular saturation level is exceeded and a volatile phase forms. The volatiles released from the magma body are buoyant and may ascent due to already existing cracks and fissures or may even create new cracks. We compute the amount of H₂O and CO₂ degassing during crystallization for different pressures. Therefore, we take into account the solubility, the initial volatile content, the pressure and temperature, the specific partition coefficient and the oxygen fugacity. Finally, we benchmark our results with the geological record and compare them with the already existing simulated data for extrusive outgassing to determine the possible impact of intrusive degassing. (picture: © Vulpius).

The redox state of the mantle is the main factor influencing volcanic outgassing

We coupled a numerical mantle convection model, a petrologic melting model, and a gas speciation model to simulate the evolution of CO₂, CO, H₂O, and H₂ outgassing for rocky planets. We find that the oxygen fugacity of the mantle is by far the most important control on the outgassed mass and abundance of these species, followed by the mantle thermal state and the initial mantle water content. The speciation between H₂ and H₂O shows a gradient in redox, while the speciation between CO and CO₂ shows a gradient in partitioning during melting.

Outgassing on stagnant-lid super-Earths

We explore volcanic CO₂-outgassing on purely rocky, stagnant-lid exoplanets of different interior structures, compositions, thermal states, and age. We focus on planets in the mass range of 1-8 Earth masses. We derive scaling laws to quantify first- and second-order influences of these parameters on volcanic outgassing after 4.5 Gyr of evolution. Given commonly observed astrophysical data of super-Earths, we identify a range of possible interior structures and compositions by employing Bayesian inference modeling. In total, we model depletion and outgassing for an extensive set of more than 2300 different super-Earth cases. We find that there is a mass range for which outgassing is most efficient (2-3 Earth masses, depending on thermal state) and an upper mass where outgassing becomes very inefficient (5-7 Earth masses, depending on thermal state). In summary, depletion and outgassing are mainly influenced by planet mass and thermal state. Interior structure and composition only moderately affect outgassing rates. The majority of outgassing occurs before 4.5 Gyr, especially for planets below 3 Earth masses. These findings and our provided scaling laws are an important step in order to provide interpretative means for upcoming missions such as JWST and E-ELT, that aim at characterizing exoplanet atmospheres.

Magma oceans and enhanced volcanism on TRAPPIST-1 planets due to induction heating

Low-mass M stars are plentiful in the Universe and often host small, rocky planets detectable with current instrumentation. These stars host magnetic fields, some of which have been observed to exceed a few hundred gauss. Recently, seven small planets have been discovered orbiting the ultra-cool M dwarf TRAPPIST-1, which has an observed magnetic field of 600 G. We suggest electromagnetic induction heating as an energy source inside these planets. If the stellar rotation and magnetic dipole axes are inclined with respect to each other, induction heating can melt the upper mantle and enormously increase volcanic activity, sometimes producing a magma ocean below the planetary surface. We show that induction heating leads the four innermost TRAPPIST-1 planets, one of which is in the habitable zone, either to evolve towards a molten mantle planet, or to experience increased outgassing and volcanic activity, while the three outermost planets remain mostly unaffected.

Volcanism and outgassing of stagnant-lid planets: Implications for the habitable zone

Rocky exoplanets are typically classified as potentially habitable planets, if liquid water exists at the surface. The latter depends on several factors like the abundance of water but also on the amount of available solar energy and greenhouse gases in the atmosphere for a sufficiently long time for life to evolve. The range of distances to the star, where surface water might exist, is called the habitable zone. Here we study the effect of the planet interior of stagnant-lid planets on the formation of a secondary atmosphere through outgassing that would be needed to preserve surface water. We find that volcanic activity and associated outgassing in one-plate planets is strongly reduced after the magma ocean outgassing phase for Earth-like mantle compositions, if their mass and/or core-mass fraction exceeds a critical value. As a consequence, the effective outer boundary of the habitable zone is then closer to the host star than suggested by the classical habitable zone definition, setting an important restriction to the possible surface habitability of massive rocky exoplanets, assuming that they did not keep a substantial amount of their primary atmosphere and that they are not in the plate tectonics regime.

Water-rich planets: how habitable is a water layer deeper than on Earth?

Water is necessary for the origin and survival of life as we know it. We study possible constraints for the habitability of deep water layers and introduce a new habitability classification relevant for water-rich planets (from Mars-size to super-Earth-size planets). A new ocean model has been developed that is coupled to a thermal evolution model of the mantle and core. We find that heat flowing out of the silicate mantle can melt an ice layer from below (in some cases episodically), depending mainly on the thickness of the ocean-ice shell, the mass of the planet, the sur- face temperature and the interior parameters (e.g. radioactive mantle heat sources). We conclude that water-rich planets with a deep ocean, a large planet mass, a high average density or a low surface temperature are likely less habitable than planets with an Earth-like ocean.