The discovery and characterization of Earth-sized planets that are in, or near, a tidally-locked state are of crucial importance to understanding terrestrial planet evolution, and for which Venus is a clear analog. Exoplanetary science lies at the threshold of characterizing hundreds of terrestrial planetary atmospheres, thereby providing a statistical sample far greater than the limited inventory of terrestrial planetary atmospheres within the Solar System. However, the model-based approach for characterizing exoplanet atmospheres relies on Solar System data, resulting in our limited inventory being both foundational and critical atmospheric laboratories. Present terrestrial exoplanet demographics are heavily biased toward short-period planets, many of which are expected to be tidally locked, and also potentially runaway greenhouse candidates, similar to Venus. Here we describe the rise in the terrestrial exoplanet population and the study of tidal locking on climate simulations. These exoplanet studies are placed within the context of Venus, a local example of an Earth-sized, asynchronous rotator that is near the tidal locking limit. We describe the recent lessons learned regarding the dynamics of the Venusian atmosphere and how those lessons pertain to the evolution of our sibling planet. We discuss the implications of these lessons for exoplanet atmospheres and their detection with observations using JWST and other future facilities. We outline the need for a full characterization of the Venusian climate in order to achieve a full and robust interpretation of terrestrial planetary atmospheres.

We conducted the first study on Ga-Al systematics in Archaean and Palaeoproterozoic banded iron formations (BIFs). Adjacent Fe oxide, metachert and mixed-type bands were analysed comparatively with solution-based SF-ICP-MS and ICP-MS/MS and laser-ablation SF-ICP-MS on nano-particulate pressed powder tablets and polished sections. Furthermore, we conducted a matrix-matched two-component mixing experiment with the BIF reference material IF-G and pure synthetic quartz sand. Results of the three comparative analytical procedures and the two-component mixing experiment assure a high quality of our analytical data even in the trace metal-poorest (meta)chert samples. Furthermore, the results suggest that finely dispersed Fe oxide particles dominate the Ga and Al content in BIF (meta)chert bands. Regardless of the samples' mineralogy, the Ga/Al ratios of BIFs range between 2×10^-4 and 1×10^-3. A compilation of Ga/Al ratios in the investigated samples throughout time shows that during Precambrian global marine Ga/Al ratios were most likely rather constant. The BIF Ga/Al ratios are above those of potential detritus but below those of modern seawater. Two conclusions are conceivable: (i) Precambrian seawaters had lower Ga/Al ratios than modern seawater, possibly due to the reduced importance of organisms and organic compounds during weathering, riverine and estuarine processes. (ii) Ga and Al were fractionated during BIF formation, and BIFs did not preserve the original seawater Ga/Al ratios.

Characterizing the interior structure of exoplanets is an essential part in understanding the diversity of observed exoplanets, their formation processes and their evolution. As the interior of an exoplanet is inaccessible to observations, an inverse problem must be solved, where numerical structure models need to conform to observed parameters such as mass and radius. Since the relative proportions of iron, silicates, water ice, and volatile elements are not known, this is a highly degenerate problem whose solution often relies on computationally-expensive and time-consuming inference methods such as Markov Chain Monte Carlo.

We present here ExoMDN, a new machine-learning-based approach to the interior characterization of observed exoplanets using Mixture Density Networks that improves upon our previous work (Baumeister et al., ApJ, 2020). This improved model, trained on a large database of 5.6 million synthetic interior structures, can make a complete probabilistic inference about possible planetary interior structures within a fraction of a second, without the need for extensive modeling of each exoplanet's interior. We can demonstrate how the model, trained on different sets of (potentially) observable parameters including the received irradiation at the planet’s orbit and the fluid Love number, can help to further constrain the interior of a large number of exoplanets. In particular, we can show how precisely these parameters need to be measured to well constrain the interior.

Inside the upper mantle, incompatible trace elements are redistributed from solid mantle rocks into partial melt. The melt that accumulated the trace elements and that is less dense than the surrounding material rises towards the surface and as a result enriches the crust and depletes the upper mantle. In the case of heat producing elements, this process can affect the thermal evolution and crust production of a planet, whereas in the case of volatiles, the outgassing and atmosphere evolution can be influenced. With mineral/melt partition coefficients, we can quantify the amount of redistributed elements. Due to a lack of high-pressure models and experimental results, partition coefficients were generally taken as constant in mantel evolution models, however, they dependent heavily on pressure, temperature and composition.

In this study, we inserted a P,T,X-dependent clinopyroxene/melt partition coefficient model that is applicable for higher pressure [1] into a mantle evolution code and investigated the effects. Due to their implications for the thermal and atmosphere evolution, we focused both on heat producing elements (Uranium, Thorium, Potassium) and volatiles (H₂O). As a result, we found that the planet size influences partitioning behavior due to differences in depth and temperature inside the melt zones in the upper mantle. With these results, we can infer the impact on various planetary processes, such as the outgassing of water, crust production, and thermal evolution.

[1] Schmidt, J.M. and Noack, L. (2021): Clinopyroxene/Melt Partitioning: Models for Higher Upper Mantle Pressures Applied to Sodium and Potassium, SysMea, 13(3&4), 125-136.

For rocky planets it is typically assumed that they are able to differentiate into a silicate mantle and a metal core, due to the fact that metals are denser than silicates and should sink towards the gravitational center, i.e. the core of the planet.

However, more massive planets experience a higher pressure and compressibility in their interior, which can strongly impact the differentiation of the planet, potentially leading to inefficient core formation and even to coreless planets (Elkins-Tanton and Seager, 2008; Lichtenberg, 2021). By using a mantle convection code, we show that even over geological timescales, and depending on the size and distribution of iron droplets forming during the magma ocean crystallisation or later on due to phase transition disproportionation in the silicate mantle, the iron may indeed never be able to sink to the centre of the planet to form a metal core. Since the ability to form a (large) core should decrease with increasing planetary mass, this study suggests that the mantle of super-Earths may be more iron-rich and therefore more reducing than for Earth, which would be reflected in their atmospheric composition and could potentially be confirmed by future observations of exoplanetary atmospheres.

References:
Elkins-Tanton and Seager, 2008, Coreless Terrestrial Exoplanets, ApJ 688, 628-635
Lichtenberg, 2021, Redox Hysteresis of Super-Earth Exoplanets from Magma Ocean Circulation, ApJL 914:L4