Hidden Water in Magma Ocean Exoplanets

We demonstrate that the deep volatile storage capacity of magma oceans has significant implications for the bulk composition, interior, and climate state inferred from exoplanet mass and radius data. Experimental petrology provides the fundamental properties of the ability of water and melt to mix. So far, these data have been largely neglected for exoplanet mass–radius modeling. Here we present an advanced interior model for water-rich rocky exoplanets. The new model allows us to test the effects of rock melting and the redistribution of water between magma ocean and atmosphere on calculated planet radii. Models with and without rock melting and water partitioning lead to deviations in planet radius of up to 16% for a fixed bulk composition and planet mass. This is within the current accuracy limits for individual systems and statistically testable on a population level. Unrecognized mantle melting and volatile redistribution in retrievals may thus underestimate the inferred planetary bulk water content by up to 1 order of magnitude

Nucleation and growth of iron pebbles explains the formation of iron-rich planets akin to Mercury

The pathway to forming the iron-rich planet Mercury remains mysterious. Mercury's core makes up 70% of the planetary mass, which implies a significant enrichment of iron relative to silicates, while its mantle is strongly depleted in oxidized iron. The high core mass fraction is traditionally ascribed to evaporative loss of silicates, e.g. following a giant impact, but the high abundance of moderately volatile elements in the mantle of Mercury is inconsistent with reaching temperatures much above 1,000 K during its formation. Here we explore the nucleation of solid particles from a gas of solar composition that cools down in the hot inner regions of the protoplanetary disc. The high surface tension of iron causes iron particles to nucleate homogeneously (i.e., not on a more refractory substrate) under very high supersaturation. The low nucleation rates lead to depositional growth of large iron pebbles on a sparse population of nucleated iron nano-particles. Silicates in the form of iron-free MgSiO$_3$ nucleate at similar temperatures but obtain smaller sizes due to the much higher number of nucleated particles. This results in a chemical separation of large iron particles from silicate particles with ten times lower Stokes numbers. We propose that such conditions lead to the formation of iron-rich planetesimals by the streaming instability. In this view, Mercury formed by accretion of iron-rich planetesimals with a sub-solar abundance of highly reduced silicate material. Our results imply that the iron-rich planets known to orbit the Sun and other stars are not required to have experienced mantle-stripping impacts. Instead their formation could be a direct consequence of temperature fluctuations in protoplanetary discs and chemical separation of distinct crystal species through the ensuing nucleation process.Comment: Accepted for publication in Astronomy & Astrophysic

Outgassing on stagnant-lid super-Earths

We explore volcanic 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 ME (Earth masses). We derive scaling laws to quantify first- and second-order influences of these parameters on volcanic outgassing after 4.5 Gyrs of evolution. Given commonly observed astrophysical data of super-Earths, we identify a range of possible interior structures and compositions by employing Bayesian inference modelling. [..] The identified interiors are subsequently used as input for two-dimensional (2-D) convection models to study partial melting, depletion, and outgassing rates of CO2. 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 ME, depending on thermal state) and an upper mass where outgassing becomes very inefficient (~5--7 \ME, 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. The majority of outgassing occurs before 4.5 Gyrs, especially for planets below 3 ME. We conclude that for stagnant-lid planets, (1) compositional and structural properties have secondary influence on outgassing compared to planet mass and thermal state, and (2) confirm that there is a mass range for which outgassing is most efficient and an upper mass limit, above which no significant outgassing can occur. Our predicted trend of CO2-atmospheric masses can be observationally tested for exoplanets. These findings and our provided scaling laws are an important step in order to provide interpretative means for upcoming missions such as, e.g., JWST and E-ELT, that aim at characterizing exoplanet atmospheres.Comment: Accepted for publication in A&A, 19 Figures, 20 page

Linking the evolution of terrestrial interiors and an early outgassed atmosphere to astrophysical observations

A terrestrial planet is molten during formation and may remain so if subject to intense insolation or tidal forces. Observations continue to favour the detection and characterisation of hot planets, potentially with large outgassed atmospheres. We aim to determine the radius of hot Earth-like planets with large outgassed atmospheres and explore differences between molten and solid silicate planets and their influence on the mass-radius relationship and transmission and emission spectra. An interior-atmosphere model, combined with static structure calculations, tracks the evolving radius of a rocky mantle that is outgassing CO$_2$ and H$_2$O. Synthetic emission and transmission spectra are generated for CO$_2$ and H$_2$O dominated atmospheres. Atmospheres dominated by CO$_2$ suppress the outgassing of H$_2$O to a greater extent than previously realised, as previous studies have applied an erroneous relationship between volatile mass and partial pressure. We therefore predict more H$_2$O can be retained by the interior during the later stages of magma ocean crystallisation. Furthermore, formation of a lid at the surface can tie outgassing of H$_2$O to the efficiency of heat transport through the lid, rather than the atmosphere's radiative timescale. Contraction of the mantle as it solidifies gives $\sim5\%$ radius decrease, which can partly be offset by addition of a relatively light species to the atmosphere. We conclude that a molten silicate mantle can increase the radius of a terrestrial planet by around $5\%$ compared to its solid counterpart, or equivalently account for a $13\%$ decrease in bulk density. An outgassing atmosphere can perturb the total radius according to its speciation. Atmospheres of terrestrial planets around M-stars that are dominated by CO$_2$ or H$_2$O can be distinguished by observing facilities with extended wavelength coverage (e.g., JWST).Comment: 19 pages, published in A&A, abstract shortene

Can we constrain interior structure of rocky exoplanets from mass and radius measurements?

We present an inversion method based on Bayesian analysis to constrain the interior structure of terrestrial exoplanets, in the form of chemical composition of the mantle and core size. Specifically, we identify what parts of the interior structure of terrestrial exoplanets can be determined from observations of mass, radius, and stellar elemental abundances. We perform a full probabilistic inverse analysis to formally account for observational and model uncertainties and obtain confidence regions of interior structure models. This enables us to characterize how model variability depends on data and associated uncertainties. We test our method on terrestrial solar system planets and find that our model predictions are consistent with independent estimates. Furthermore, we apply our method to synthetic exoplanets up to 10 Earth masses and up to 1.7 Earth radii as well as to exoplanet Kepler-36b. Importantly, the inversion strategy proposed here provides a framework for understanding the level of precision required to characterize the interior of exoplanets. Our main conclusions are: (1) observations of mass and radius are sufficient to constrain core size; (2) stellar elemental abundances (Fe, Si, Mg) are key constraints to reduce degeneracy in interior structure models and to constrain mantle composition; (3) the inherent degeneracy in determining interior structure from mass and radius observations does not only depend on measurement accuracies but also on the actual size and density of the exoplanet. We argue that precise observations of stellar elemental abundances are central in order to place constraints on planetary bulk composition and to reduce model degeneracy. [...]Comment: 19 pages, 18 figures, accepted in Astronomy & Astrophysics (no changes to previous version

Konstruktion und Charakterisierung einer Mutante des GPI-Biosyntheseweges von Plasmodium falciparum

Gylkosylphosphatidylinositole sind eine Klasse von Glykolipiden, die bei allen Eukaryonten vorkommen. Die Biosynthese der GPI-Anker erfolgt durch schrittweise Übertragung der einzelnen Zuckerkomponenten auf Phosphatidylinositol. Anschließend wird der GPI-Anker auf das C-terminale Ende eines neu synthetisierten Proteins übertragen, welches in der Membran verankert werden soll. Diese Arbeit befasst sich mit der Dolichol-Phosphat-Mannose-Synthase von Plasmodium falciparum. Der durch die Dol-P-Man-Synthase katalysierte Syntheseschritt besteht in der Übertragung von GDP-Man auf Dol-P. Dol-P-Man fungiert als Mannosedonor in der GPI-Biosynthese und N-Glykosylierung und O-Mannosylierung von Proteinen. Die bekannten Dol-P-Man-Synthasen verschiedener Organismen weisen große Unterschiede untereinander auf. Die Sequenzanalyse des Proteins von P. falciparum, der Nachweis seiner hydrophilen Eigenschaft durch Transfektionsstudien und die Übersicht über durchgeführte Komplementationsversuche lassen die Hypothese zu, dass das DPM1-Protein von P. falciparum eine Sonderstellung unter den bereits charakterisierten Proteinen einnimmt und somit als mögliches Antimalaria-Target fungieren könnte

Interior Characterization in Multiplanetary Systems: TRAPPIST-1

Interior characterization traditionally relies on individual planetary properties, ignoring correlations between different planets of the same system. For multi-planetary systems, planetary data are generally correlated. This is because, the differential masses and radii are better constrained than absolute planetary masses and radii. We explore such correlations and data specific to the multiplanetary-system of TRAPPIST-1 and study their value for our understanding of planet interiors. Furthermore, we demonstrate that the rocky interior of planets in a multi-planetary system can be preferentially probed by studying the most dense planet representing a rocky interior analogue. Our methodology includes a Bayesian inference analysis that uses a Markov chain Monte Carlo scheme. Our interior estimates account for the anticipated variability in the compositions and layer thicknesses of core, mantle, water oceans and ice layers, and a gas envelope. Our results show that (1) interior estimates significantly depend on available abundance proxies and (2) that the importance of inter-dependent planetary data for interior characterization is comparable to changes in data precision by 30 %. For the interiors of TRAPPIST-1 planets, we find that possible water mass fractions generally range from 0-25 %. The lack of a clear trend of water budgets with orbital period or planet mass challenges possible formation scenarios. While our estimates change relatively little with data precision, they critically depend on data accuracy. If planetary masses varied within ±24 %, interiors would be consistent with uniform (~7 %) or an increasing water mass fractions with orbital period (~2-12 %)

Interior Characterization in Multiplanetary Systems: TRAPPIST-1

Interior characterization traditionally relies on individual planetary properties, ignoring correlations between different planets of the same system. For multiplanetary systems, planetary data are generally correlated. This is because the differential masses and radii are better constrained than absolute planetary masses and radii. We explore such correlations and data specific to the multiplanetary system of TRAPPIST-1 and study their value for our understanding of planet interiors. Furthermore, we demonstrate that the rocky interior of planets in a multiplanetary system can be preferentially probed by studying the densest planet representing a rocky interior analog. Our methodology includes a Bayesian inference analysis that uses a Markov chain Monte Carlo scheme. Our interior estimates account for the anticipated variability in the compositions and layer thicknesses of core, mantle, water oceans, and ice layers, as well as a gas envelope. Our results show that (1) interior estimates significantly depend on available abundance proxies and (2) the importance of interdependent planetary data for interior characterization is comparable to changes in data precision by 30%. For the interiors of TRAPPIST-1 planets, we find that possible water mass fractions generally range from 0% to 25%. The lack of a clear trend of water budgets with orbital period or planet mass challenges possible formation scenarios. While our estimates change relatively little with data precision, they critically depend on data accuracy. If planetary masses varied within ±24%, interiors would be consistent with uniform (~7%) or an increasing water mass fractions with orbital period (~2%–12%)

Lithologic Controls on Silicate Weathering Regimes of Temperate Planets

Weathering of silicate rocks at a planetary surface can draw down CO2 from the atmosphere for eventual burial and long-term storage in the planetary interior. This process is thought to provide essential negative feedback to the carbonate-silicate cycle (carbon cycle) to maintain clement climates on Earth and potentially similar temperate exoplanets. We implement thermodynamics to determine weathering rates as a function of surface lithology (rock type). These rates provide upper limits that allow the maximum rate of weathering in regulating climate to be estimated. This modeling shows that the weathering of mineral assemblages in a given rock, rather than individual minerals, is crucial to determine weathering rates at planetary surfaces. By implementing a fluid-transport-controlled approach, we further mimic chemical kinetics and thermodynamics to determine weathering rates for three types of rocks inspired by the lithologies of Earthʼs continental and oceanic crust, and its upper mantle. We find that thermodynamic weathering rates of a continental crust-like lithology are about one to two orders of magnitude lower than those of a lithology characteristic of the oceanic crust. We show that when the CO2 partial pressure decreases or surface temperature increases, thermodynamics rather than kinetics exerts a strong control on weathering. The kinetically and thermodynamically limited regimes of weathering depend on lithology, whereas the supply-limited weathering is independent of lithology. Our results imply that the temperature sensitivity of thermodynamically limited silicate weathering may instigate a positive feedback to the carbon cycle, in which the weathering rate decreases as the surface temperature increases