Programme

The evolution of Venus is, at present, poorly known, despite it being our closest planetary neighbour. The relatively small number of impact craters implies that its surface is much younger than that of Mars and the Moon; their quasi-random distribution shows that Venus does not exhibit Earth-like plate tectonics. This has led to suggestions of global resurfacing – but how this happened is an open question. Whether resurfacing occurred catastrophically or in a steady manner has major implications for interior and climate evolution. Another key question is absent plate tectonics, how are interior processes linked to the surface deformation and volcanism. There is evidence of subduction in up to a dozen locations. Subduction is the first step in initiating plate tectonics, and raises the question of why Venus appears to lack plate tectonics.

Venus is particularly important to our understanding of habitability. Venus’ enhanced D/H ratio suggests that it has lost large amounts (possibly several terrestrial oceans) of water, but it is not clear whether it condensed (as happened on Earth) or whether this water was lost in the steam atmosphere phase; if it had a liquid water ocean phase, Venus may have been habitable for billions of years. There is no consensus on how much water there is in Venus’ interior, and how much of this water has been outgassed, a question which has important implications for Venus’ atmospheric water and in turn for its habitability through time. Exoplanet transit detection surveys have a bias to detecting exoplanets close to their parent stars: the growing number of such Venus-like exoplanets discoveries emphasizes the relevance of Venus in the search for habitable exoplanets.

Three new spacecraft missions to Venus have been selected in the past year: ESA’s EnVision orbiter, and NASA’ VERITAS orbiter and DAVINCI+ entry probe; further missions are in development in Russia, India, Japan and elsewhere. In this presentation we will review how the observations from these new missions may constrain the range of possible evolutionary scenarios.

The present-day Solar System manifests itself as a tranquil and constant environment, including - from our perspective - slowly orbiting planet(esimal)s and occasional flybys of more daring asteroids and comets. At its birth, however, planets formed within a turbulent, gaseous circumstellar disk, during which mass transport of dust and pebble-sized objects, the migration of the gas giants and the ensuing hurtle of asteroids and comets contributed to shaping the habitable terrestrial planet region. One of the most fundamental questions in cosmochemistry and astrophysics, is whether life is commonplace in the universe and if the initial conditions and physiochemical processes that shaped our Solar System can be expected to occur within exoplanetary systems. To assess this issue from a cosmochemical perspective, we need to first understand the nature and origin of planetary building blocks, including how and when the rocky worlds acquired their prebiotic inventory (i.e., volatiles and organics) that ultimately led to the emergence of life on Earth. In a classical view, a differentiated Earth, after the Moon-forming giant impact, suffered impacts of volatile-rich bodies (i.e., comets and water-rich asteroids) from the outer Solar System that supplied our planets hydrosphere, atmosphere and biosphere with abundant volatiles and organics. In more recent models, invoking pebble accretion of the rocky planets during the lifetime of the protoplanetary disk, the planets within the habitable zone acquired their prebiotic inventory much earlier, through accretion of icy pebbles from the outer Solar System. The required mass transport within this model can be tested and traced through the chemical and isotope investigation of chondritic components (i.e., dust and pebble-sized objects that were present during the disk lifetime). Here, I will discuss a multitude of cosmochemical data to outline the different views on mass transport in the disk, including chondrule-matrix complementarity and the chondrite dichotomy, as well as its implications for accreting the rocky worlds.

Recent planet formation theories and isotopic studies have suggested that the embryos of Earth (building blocks) accreted very early within the initial 2 Ma of the formation of solar system [1-2]. The early accretion implies the role played by the short-lived radionuclide (SLR) 26Al in the large-scale heating of embryo’s interiors [3-4]. Further, the new isotopic measurements have recommended that the Enstatite chondrites contain enough hydrogen to deliver sufficient water to Earth [5]. It implies to the formation of an impact-induced steam atmosphere on the surface of embryos during accretion [6-7]. Based on these new finding, we performed numerical simulations to study the early thermal evolution and core-mantle segregation of Earth’s embryos (0.2ME-0.6ME) by considering heat of SLR 26Al and blanketing effect of the impact-generated steam atmosphere during accretion [8]. In addition to this, we also incorporated the heat energies of SLR 60Fe along with long-lived radionuclides 40K, 235U, 238U and 232U. The numerical simulations were performed by considering the duration of accretion and initial water content of the accreting planetesimals as a free parameter. The bulk composition of the embryos was considered to be Enstatite type [5,9]. The initial water content (Xwp) of the accreting planetesimals was assumed to be in the range of 0.1 – 0.54% by weight [5]. The pressure dependent liquidus and solidus temperature of iron and silicate were calculated at each spatial point inside embryos [3-4]. The Stoke’s law was used to calculate the descend velocity of metallic blobs to form iron core at the center. The results of this study show the formation of the magma ocean of several depths at surface of growing embryos because of significant blanketing by the impact-generated steam atmosphere. Further, the core-mantle segregation in the interior was complete within the initial ~5 Ma of the formation of the solar system if the embryos accreted in the initial ~1.3-1.5 Ma after the formation of CAIs. The massive 0.4ME-0.6ME embryos owing to high pressure in the interior required early differentiation (after the melting of iron or 10% melting of silicates) for complete core-mantle differentiation within the initial 5 Ma. These results seem to be consistent with the results of new finding for the rapid accretion and differentiation of main accretion phase of Earth within the initial ~5 Ma of the solar system [9].

References: [1] Levison, H.F., et al. 2015. Proc. Natl. Acad. Sci. U. S. A. 112, 14180–14185. [2] Williams, C.D., Mukhopadhyay, S., 2019. Nature 565, p.78. [3] Bhatia, G.K., Sahijpal, S., 2016. Meteor. Planet. Sci. 51, 138-154. [4] Bhatia, G.K., Sahijpal, S., 2017a. Meteor. Planet. Sci. 52, 295-319. [5] Piani, L., et al., 2020. Science 369, 1110-1113. [6] Abe, Y., Matsui, T., 1985. J. Geophys. Res: Solid Earth 90, C545-C559. [7] Abe, Y., Matsui, T., 1986. Proc. 17th Lunar Planet. Sci. Conf. Part 1, J. Geophys. Res. (Suppl.) 91, E291-E302. [8] Bhatia, G.K., 2021. Planet. Space Sci. (Accepted, https://doi.org/10.1016/j.pss.2021.105335). [9] Schiller, M. et al., 2020. Sci. Adv. 6(7), p.eaay7604.

Chondritic meteorites are thought to be representative of the material that formed the Earth. However, the Earth is depleted in volatile elements in a manner unlike that in any chondrite, and yet these elements retain chondritic isotope ratios. Here we use N-body simulations to show that the Earth did not form from chondrites, but rather by stochastic accretion of many precursor bodies whose variable compositions reflect the temperatures at which they formed. Earth’s composition is reproduced when initial temperatures of planetesimal- to embryo-sized bodies are set by disk accretion rates of (1.08±0.17)×10-7 solar masses/yr, although they may be perturbed by 26Al heating on bodies formed at different times. Our model implies that a heliocentric gradient in composition was present in the protoplanetary disc and that planetesimals formed rapidly within ~1 Myr, in accord with radiometric volatile depletion ages of the Earth.

A growing number of likely rocky planets have been discovered beyond our solar system, thanks to recent advances in observational techniques. The formation and evolution of the rocky planets in our solar system have long been discussed. As for rocky exoplanets, however, we would have to consider the issue from a broader perspective by taking into account physical processes and conditions that the solar-system rocky planets may not have experienced. For example, rocky exoplanets include objects larger than the Earth and Mars, namely, super-Earths. Also, they include not only high-density but also low-density ones, indicating that the latter retains a hydrogen-rich primordial atmosphere. Although planets grow in hydrogen-dominated circumstellar discs, the accumulation of disc gas (or nebula gas) on rocky planets has not been well discussed because of the absence of evidence that the Earth had such a primordial atmosphere of nebular origin. In addition, recent ultra-violet observations detected a few low-mass exoplanets that have their atmospheres flow out. While such thermal escape of planetary atmospheres is not seen in the present-day solar system, rocky exoplanets so far detected and recently targeted are known to receive stellar irradiation strong enough that their atmospheres are vigorously escaping. In this lecture, I will overview the current understanding of rocky planets that form and evolve in such environments different from what produced the rocky planets in our solar system, including our latest research results.

The presence of highly siderophile elements in Earth’s mantle indicates that a small percentage of Earth’s mass was delivered after the last giant impact in a stage of `late accretion.' There is ongoing debate about the nature of late-accreted material and the sizes of late-accreted bodies. We examine the provenances of `leftover' planetesimals present in the inner disk in the late stages of accretion simulations. Commonly, some planetesimal-sized bodies with similar provenances to the Earth-like planets are left at the end of the main stage of growth. The most chemically-similar planetesimals are typically fragments of proto-planets ejected millions of years earlier. If these bodies are later accreted by the planet, they will represent late-accreted mass that naturally matches Earth's composition. The planetesimal-sized bodies that exist during the giant impact phase can have large core mass fractions, with core provenances similar to the proto-Earth. These bodies are an important potential source for highly siderophile elements. The range of core fractions in leftover planetesimals complicates simple inferences as to the mass and origin of late accretion based on the highly siderophile elements in the mantle.

We have developed the currently most comprehensive and self-consistent approach to realistically simulate the formation of habitable planets. Our approach begins with simulating the collisional growth of planetesimals and continues with resolving giant impacts and the full formation of terrestrial planets. It takes into account the dynamical friction due to the debris and planetesimal disks, migration of planetesimals and embryos, and the perturbation as well as possible migration of giant planets. As the most important step toward a fully comprehensive and realistic model, our approach incorporates SPH simulations into N-body integrations in real time allowing, for the first time, collisions to be simulated accurately as they occur. Results point to several important findings. For instance, in the context of our solar system, almost all simulations produced an Earth-analog. They also demonstrated that the similarities between the size and mass of Earth and Venus are a natural outcome of the formation process, and Mars-sized planets appear in systems where the mass distribution in the planetesimal disk is not uniform. When studying the effects of giant planets, results showed that secular resonances are the main reason that our solar system does not have Super-Earths. They are also the reason that terrestrial planets form interior to 2.1 AU. Simulations also show that the capture into resonance of migrating giant planets does not play a significant role on the formation of habitable planets, and while giant planets may affect the inventory of planet-forming material and water-carrying objects, especially when they migrate, they play no role in the mechanics of the formation of habitable planets and the transfer/transport of water to them. Formation and water delivery is merely due to the mutual interactions of planetary embryos, a process that occurs even when no giant planet exists. We will present the results of our study and discuss their applications to extrasolar habitable planets. 

There is growing interest in the abundance of exoplanets with a water content comparable to that of the Earth. Theoretical predictions so far assumes that water-rich planetesimals beyond the snow line are the source of water. It is also predicted that planets in the habitable zone around M dwarfs have bimodal distribution of water contents. On the other hand, since planets are generally formed in protoplanetary disks, they acquire disk gas to form primordial atmospheres. In this study, we focused on the production of water by the reaction of hydrogen in the primordial atmosphere with oxides in the magma ocean as another process of water capture. When this reaction occurs efficiently, it is known that even a sub-Earth-mass planet can obtain more water than the Earth ocean mass (Kimura & Ikoma 2020). However, when planetary growth, migration, giant impacts, and dissipation of protoplanetary disk gas occur simultaneously, it is still unknown to what extent this water production process affects the final planetary water abundance distribution. Therefore, we developed a planetary population synthesis model including the effects of water production in the primordial atmosphere and predicted the water content distribution of exoplanets theoretically. The results show that, in contrast to the results of previous studies, the terrestrial planets in the habitable zone of M dwarfs have a diverse water content. In particular, the water production process in the primordial atmosphere is found to have a great impact on the occurrence of Earth-like aqua planets. This result suggests that planets with appropriate water content for a temperate climate will be normally found around M dwarfs.

In the core accretion paradigm of planet formation, gas giants form a massive atmosphere in a run-away gas accretion phase once their progenitors exceed a threshold mass: the critical core mass. On the one hand, the majority of observed exo-planets, being smaller and rock/ice-dominated, never crossed this line. On the other hand, these exo-planets have accreted substantial amounts of gas from the circumstellar disk during their embedded formation epoch.

We investigate the hydrodynamical and thermodynamical properties of proto-planetary atmospheres by direct numerical modeling of their formation epoch. Our studies cover one-dimensional (1D) spherically symmetric, two-dimensional (2D) axially symmetric, and three-dimensional (3D) hydrodynamical simulations with and without radiation transport. We check the feasibility of different numerical grid geometries (Cartesian vs. spherical), perform convergence studies, and scan the physical parameter space with respect to planet mass and optical depth of the surrounding. 

In terms of hydrodynamic evolution, no clear boundary demarcates bound atmospheric gas from disk material in a 3D scenario in contrast to 1D and 2D computations. Atmospheres denote open systems where gas enters and leaves the Bondi /Hill sphere in both directions. In terms of thermodynamics, we compare the gravitational contraction of the forming atmospheres with its radiative cooling and hydrodynamical advection of energy, as well as the interplay of these processes. The coaction of radiative cooling of atmospheric gas and advection of atmospheric-disk gas prevents the proto-planets to undergo run-away gas accretion. Hence, this recycling process provides a natural explanation for the preponderance of super-Earth like planets.

To advertise the general purpose approach of our code development, I will also briefly overview the most recent results obtained in a variety of other astrophysical research fields from proto-planets on supersonic, eccentric orbits (Mai et al. 2020), to accretion and multiplicity in massive star formation (Oliva & Kuiper 2020), up to the formation of the progenitors of the first supermassive black holes in the early universe (Hirano et al. 2017, Science).

References: Ormel, Kuiper, Shi (2015); Ormel, Shi, Kuiper (2015); Cimerman, Kuiper, Ormel (2017); Moldenhauer, Kuiper, Kley, Ormel (2021)

The most promising potential habitable worlds outside the solar system might be planets that are very different from Earth. Cold super-Earths which retain their primordial, H-He dominated atmosphere could have surfaces that are warm enough to host liquid water. This would be due to the collision induced absorption (CIA) of infra-red light by hydrogen, which increases with pressure. Here we investigate the duration of this potential exotic habitability by simulating planets of different core masses, envelope masses and semi-major axes. Evolution models for the host-stars luminosity and the planet's intrinsic heat and radius are incorporated, as well as an atmosphere evaporation model. We find that terrestrial and super-Earth planets with masses of 1 to 10 Earth mass can maintain temperate surface conditions for more than 9 Gyr at radial distances larger than 2 AU. This suggests that a large number of planets in the galaxy could be habitable, and that the concept of planetary habitability should be more inclusive. 

Atmospheres with a significant mass fraction of condensable gases may be common among terrestrial or sub-Neptune exoplanets. A wide variety of important planetary climate problems involve understanding of dynamical properties of condensible-rich atmospheres. Recent theoretical advances have shown that non-dilute dynamics, either in large or local scales, differs in fundamental ways from that in the dilute conditions. We further examine the properties of small-scale moist convection in non-dilute atmospheres using numerical simulations. Here we start with an extreme case of pure steam atmospheres using an idealized, fully compressible, non-hydrostatic model, with parameterized instantaneous condensation, rainout, and evaporation. e show that the atmosphere is comprised of two characteristic regions, an upper condensing region dominated by gravity waves and a lower noncondensing region characterized by convective overturning cells. Velocities in the condensing region are much smaller than those in the lower noncondensing region, and the horizontal temperature variation is small overall. Condensation in the thermal photosphere is largely driven by radiative cooling and tends to be statistically homogeneous. Some condensation also happens deeper, near the boundary of the condensing region, due to triggering by gravity waves and convective penetrations and exhibit random patchiness. This qualitative structure is insensitive to varying parameters in the model, but quantitative details may differ. Our results confirm theoretical expectations that atmospheres close to the pure-steam limit do not have organized deep convective plumes in the condensing region and suggest that the generalized convective parameterization scheme discussed Ding & Pierrehumbert (2016) should be sufficient for global models of atmospheres near the pure-steam limit. Lastly, if available, I will present latest results for atmospheres that allow certain fraction of non-condensable gas.

Astronomy is on the precipice of characterizing the atmospheric composition and climate of rocky planets with JWST and high-resolution spectrographs on the ELTs. A clear lesson from previous characterization of sub-Neptune exoplanets is that aerosol coverage will complicate the interpretation of astronomical observations. This necessitates a predictive understanding of the expected aerosol distribution on rocky exoplanets with three-dimensional general circulation models (GCMs). In this talk, I will present results from GCM studies of the climate of a range of temperate rocky exoplanets orbiting M dwarf stars to determine both how condensate clouds impact observable properties and how the day-night forcing pattern of tidally locked exoplanets affects the formation and evolution of large-scale storms. I will first present how climate and cloud variability may affect the detection of the disequilibrium biosignature pair of carbon dioxide and methane in the atmosphere of TRAPPIST-1e with JWST. I will then show results from high-resolution simulations of tidally locked rocky exoplanets with varying rotation rate that test previous predictions for how tropical cyclogenesis favorability depends on planetary properties. Lastly, I will demonstrate the need for further high-resolution global models of temperate rocky planets in order to constrain their net cloud radiative feedback and potential for habitable surface conditions. 

The atmospheres of rocky, tidally-locked exoplanets may exhibit distinctly different circulation regimes depending on the rotation rate, composition, and opacity. Additionally, such planets within the habitable zone may be rendered uninhabitable due to runaway condensation of greenhouse gases, such as water or CO2 on their nightside surfaces (atmospheric collapse). Here, we use the THOR GCM to explore the atmospheres of M dwarf planets and to understand the factors governing circulation and atmospheric collapse. The GCM has been recently updated with a conservative boundary layer module to capture turbulence near the planet surface and a new wavelength-dependent radiative transfer module based on the HELIOS code. We model planets orbiting M0, M4, and M9 stars, with atmospheres of CO2, methane, and ammonia, as a means of disentangling the effects of opacity and mean molecular weight. Our CO2 atmospheres are driven by heating at the surface and the circulation depends heavily on the rotation rate and host star type. As the rotation rate increases, the circulation transitions from a “divergent-overturning” pattern to a rotationally-influenced, Matsuno-Gill structure. These atmospheres are also susceptible to atmospheric collapse, which becomes more pronounced at faster rotation rates. Methane and ammonia atmospheres are dominated by short-wave absorption at high altitudes. This results in a distinctly different circulation pattern that is disconnected from the surface and relatively insensitive to rotation rate. Methane atmospheres are unique in that the surface becomes much colder than the atmosphere. Such “weird” atmospheres may or may not be feasible from a formation perspective, but these simulations illuminate some previously under-explored phenomena in M dwarf planet atmospheres. 

Using a 3D general circulation model, we demonstrate that a confirmed rocky exoplanet and a primary observational target, TRAPPIST-1e presents an interesting case of climate bistability. We find that the atmospheric circulation on TRAPPIST-1e can exist in two distinct regimes for a 1 bar nitrogen-dominated atmosphere. One is characterized by a single strong equatorial prograde jet and a large day-night temperature difference; the other is characterized by a pair of mid-latitude prograde jets and a relatively small day-night contrast. The circulation regime appears to be highly sensitive to the model setup, including initial and surface boundary conditions, as well as physical parameterizations of convection and cloud radiative effects. We focus on the emergence of the atmospheric circulation during the early stages of simulations and show that the regime bistability is explained by a delicate balance between the zonally asymmetric heating, mean overturning circulation, and mid-latitude baroclinic instability. The relative strength of these processes places the GCM simulations on different branches of the evolution of atmospheric dynamics. The resulting steady states of the two regimes have consistent differences in the amount of water content and clouds, affecting the water absorption bands as well as the continuum level in the transmission spectrum, although they are too small to be detected with current technology. This regime bistability motivates more 3D model intercomparisons across a wide parameter space, which can provide better observational constraints for TRAPPIST-1e and similar planets.

Among the thousands of extrasolar planets discovered by space and Earth-based observations, small rocky planets, with radii lower than 1.5 times the Earth’s radius, are the most exciting candidates to host life. Earth-like objects focus our attention to seek new habitable worlds. Deciphering their atmospheres is one the challenge of the James Webb Space Telescope upcoming measurements. Planetary atmospheres contribute to the formation of prebiotic building blocks: the atmosphere not only preserves organic chemistry at the surface, but also possibly provides organic molecules to the surface as observed in abundance on Titan through the interaction of methane with solar UV photons. This atmospheric organic source could be even more important for Trappist-1 planetary system as the ultracool dwarf TRAPPIST-1 has an Extreme-UV (EUV) emission much stronger than the sun. Furthermore EUV photochemistry is strongly related to ion chemistry in planetary upper atmospheres. In our group, we have developed an experimental platform (ATMOSIM) dedicated to the simulation of these processes. We have investigated the ion chemistry occurring in a large range of possible atmospheric chemical composition (CO2, N2 and H2 based atmospheres). I will present how the knowledge of ion chemistry informs us on the chemical potential for habitability and provides interesting observable chemical probes especially in the case of cloudy atmospheres.

An overlooked class of atmospheres dominated by carbon dioxide (CO2) as well as methane (CH4) has been proposed (Woitke et al. 2020) under chemical equilibrium. The coexistence of CO2 and CH4 allowed by thermodynamics leads to false positives for taking them as biosignatures (e.g. Krissansen-Totton et al. 2019). Here, we investigate the CO2-CH4-H2O coexisting atmospheres with a self-consistent thermal structure and find they are generally not stable against photochemical processes.

Determining the habitability of an exoplanet and interpreting atmospheric spectra requires an understanding of its atmospheric physics and chemistry. We use a 3-D Coupled Climate-Chemistry Model, consisting of the Met Office Unified Model and the UK Chemistry and Aerosols framework, to study the emergence of lightning and its chemical impact on tidally-locked Earth-like exoplanets, for a setup of Proxima Centauri b. chemical network includes the Chapman ozone reactions and hydrogen oxide (HOx=H+OH+HO2) and nitrogen oxide (NOx=NO+NO2) catalytic cycles. We find that photochemistry driven by stellar radiation supports a global stratospheric ozone layer, peaking between 20 and 50 km. We parameterise lightning flashes as a function of cloud-top height, following Earth sciences, and the resulting production of nitric oxide (NO) from the thermal dissociation of N2 and O2. Rapid dayside convection over and around the substellar point results in lightning flash rates of up to four flashes km-2 yr-1. These flashes induce a dayside atmosphere rich in NOx below altitudes of 20 km. Changes in dayside ozone are determined mainly by UV irradiance and the HOx catalytic cycle. Dayside-nightside thermal gradients result in strong winds that subsequently advect NOx towards the nightside, where the absence of photochemistry allows NOx chemistry to involve reservoir species. In this talk, I will discuss the distribution of lightning flashes and the subsequent spatial variability in lightning-induced chemistry around the planet. Furthermore, I will emphasize the need for accurate stellar UV spectra to study the photochemistry of exoplanet atmospheres.

Determining the habitability of an exoplanet and interpreting atmospheric spectra requires an understanding of its atmospheric physics and chemistry. We use a 3-D Coupled Climate-Chemistry Model, consisting of the Met Office Unified Model and the UK Chemistry and Aerosols framework, to study the emergence of lightning and its chemical impact on tidally-locked Earth-like exoplanets, for a setup of Proxima Centauri b. chemical network includes the Chapman ozone reactions and hydrogen oxide (HOx=H+OH+HO2) and nitrogen oxide (NOx=NO+NO2) catalytic cycles. We find that photochemistry driven by stellar radiation supports a global stratospheric ozone layer, peaking between 20 and 50 km. We parameterise lightning flashes as a function of cloud-top height, following Earth sciences, and the resulting production of nitric oxide (NO) from the thermal dissociation of N2 and O2. Rapid dayside convection over and around the substellar point results in lightning flash rates of up to four flashes km-2 yr-1. These flashes induce a dayside atmosphere rich in NOx below altitudes of 20 km. Changes in dayside ozone are determined mainly by UV irradiance and the HOx catalytic cycle. Dayside-nightside thermal gradients result in strong winds that subsequently advect NOx towards the nightside, where the absence of photochemistry allows NOx chemistry to involve reservoir species. In this talk, I will discuss the distribution of lightning flashes and the subsequent spatial variability in lightning-induced chemistry around the planet. Furthermore, I will emphasize the need for accurate stellar UV spectra to study the photochemistry of exoplanet atmospheres.

The significant concentration of volcanic H2 in early CO2-dominated atmospheres has been proposed recently inspired by the massive work constraining the atmospheric composition of early Mars. Indeed, CO2-H2 collision-induced greenhouse warming can raise surface temperatures above the freezing point of water. It thus becomes a new process capable of extending the outer edge of the habitable zone further away from the host star. The potential detection of these objects is helped by the CO2-H2 spectral features and the higher pressure scale height caused by hydrogen.

The photochemical evolution of these atmospheres is however poorly known. Using a photochemical reactor, I studied the evolution of a CO2-H2 gas mixture under monochromatic irradiation. I will present a new experimental method to retrieve the mixing ratio of every species using mass spectrometry analysis of the gas phase. I will focus on the O2 to H2O ratio to better understand the chemical network. A 0D model accounting for the geometry of the photochemical reactor is used for comparison. The main reactions affecting the composition of the neutral molecules are identified. The impact of photochemistry on the vertical distribution of oxygen and water in the atmosphere is studied using a 1D chemical kinetics code. The potential detection of photochemical gas products will be discussed. 

The vast majority of the known exoplanet population was detected via indirect techniques using transit observations or radial velocity monitoring. For transiting exoplanets, transit spectroscopy, secondary eclipse and phase curve data provide access to information about the objects' atmospheres, and the James Webb Space Telescope and ESA's Ariel mission will leverage these observational approaches and revolutionize our understanding of exoplanet atmospheres. In the long run, however, and in particular in the context of investigating a significant number of terrestrial exoplanets, direct detection methods are needed. They will give access to non-transiting exoplanets and significantly increase the discovery space.

In this lecture I will discuss what we can expect from future ground- and space-based instruments and missions that aim at the direct detection of terrestrial exoplanets. The main challenges will be explained  and the synergies between different approaches will be highlighted. I will argue that over the coming ~25 years, slowly but surely, we will be able to empirically address questions related to the habitability of other worlds and search for indications of biological activity in their atmospheres.

The imminent arrival of the Extremely Large Telescopes (ELTs) will finally deliver the observational power capable of assessing the habitability of nearby rocky exoplanets. The ELT presents us with the exciting opportunity of being able to spatially resolve the terrestrial exoplanet Proxima b, which lies in the habitable zone of Proxima Centuri. This would allow molecule mapping, a technique that uses the spatial separation plus cross correlation high resolution spectroscopy to disentangle the planet's spectrum from the host star and characterise its atmosphere. Here we present simulations in reflected light for HARMONI/ELT using model planet spectra from the Carl Sagan Institute. HARMONI's resolution is well suited to molecule mapping, with access to wavelengths covering multiple biosignatures. Our simulator shows that this first light ELT instrument can characterise the atmosphere of Proxima b, within a reasonable time frame, but requires intervention on the focal plane masks in HARMONI's current instrument design. If changed, HARMONI has the potential to identify CO2 and H2O in Proxima b. The simulator is highly versatile and can be extended to other systems including METIS, and GMagAO-X+IFS.

At the dawn of the search for life in the universe, we are focusing our efforts on designing new instruments that can characterize the atmospheres of terrestrial exoplanets. Using nulling interferometry in the MIR wavelength range, the Large Interferometer for Exoplanets (LIFE, Quanz et al. 2018, 2021) will allow us to further constrain the bulk parameters and the surface conditions of a few dozens of terrestrial planets, as well as to gather information about their atmospheric structure and composition. At this stage of the mission development, atmospheric retrieval studies are essential to determine the technical requirements for LIFE. Because of the lack of observational data, we rely on theoretical spectra of terrestrial exoplanets to develop analysis pipelines that could be most effective for the characterisation of such targets. We feed these spectra to Bayesian retrieval routines to produce a statistically robust analysis of an atmospheric spectrum given a set of parameters (pressure-temperature structure, chemical abundance, planetary dimensions).

We have built our own retrieval framework and we have validated our routines with an Earth twin orbiting a Sun-like star at a 10 pc distance from the observer (see Konrad et al., this conference). In this contribution, we analyse simulated spectra of the Earth at various stages of its evolution calculated by Rugheimer & Kaltenegger (2018): a prebiotic Earth at 3.9 billion years ago (Ga), the Earth after the Great Oxygenation Event at 2.0 Ga, and after the Neoproterozoic Oxygenation Event at 0.8 Ga, and the modern Earth. We considered an Earth-sized planet on a 1 AU orbit around a Sun-like star at a 10 pc distance, at the minimum LIFE requirements found in the Earth twin study (R=50, S/N=10, ∆λ = 4 − 18.5 μm). We create mock observations with LIFE by running the simulated spectra through the LIFEsim simulator (Dannert et al., 2022) considering all major astrophysical noise sources.

We find that these requirements allow the identification of the main spectral features of all spectra. In particular, LIFE could be able to detect O3 in the atmosphere if the O2 mass fraction is of the order of a few percent in mass fraction. CH4 could be constrained in terrestrial atmospheres if its abundance is of the order of 0.1% in mass fraction. To detect more precise and accurate results of the potential biosignature pair O3-CH4 in the biotic epochs, we suggest increasing the S/N to 20 for the most promising candidates.

The question of what causes global glaciations to occur on Earth-like planets is of great importance to habitability and climate evolution. Earth itself has a complex climate history consisting of a long stretch of apparently clement conditions in the Archean (4 - 2.5 Gy ago), a stable, even boring Proterozoic (2.5 - 0.54 Gy ago) climate punctuated by major intervals of glaciation at the beginning and end, and fluctuation between warm and cool climates in the most recent Phanerozoic eon without any further global glaciation events. Various explanations have been put forward for the mechanisms that drove Earth’s glaciations, but so far there is little consensus.

Here it is argued that much insight into the climate history of Earth, and the potential climates of rocky exoplanets, can be gained via a stochastic modeling approach. Via a series of stochastic carbon cycle models of increasing complexity, it is shown that counterintuitively, the climates of Earth-like planets may become increasingly unstable to Snowball episodes as solar luminosity increases. This provides new insight into Earth’s susceptibility to glaciation in the Neoproterozoic 716 My ago, but it also raises questions about how Earth has avoided additional Snowball events in the last 500 million years. To put it plainly, have we just been lucky, or have new feedbacks come into play since the rise of complex life? In the final part of this presentation we attempt to answer this question.

A long-term goal of exoplanet research is to characterize the atmospheres of a sizable sample of temperate terrestrial exoplanets. Such studies will augment our knowledge about the diversity of terrestrial worlds and might even enable the discovery of habitable or even inhabited worlds. To achieve this goal, missions capable of measuring the spectra of temperate terrestrial exoplanets have been proposed (LUVOIR/HabEx - optical & near-infrared; Large Interferometer For Exoplanets (LIFE) - mid-infrared (MIR) [see talk by Sascha Quanz]). The MIR thermal emission measured by LIFE provides exclusive probes to important molecules (e.g. the potential bioindicators CH4 and O3). Further, the MIR observations can provide direct constraints on a planet’s pressure-temperature (PT) profile, radius, and surface conditions. We present results from our recent atmospheric retrieval studies based on detailed mock-observations with LIFE. Therein, We investigated a cloud-free Earth-twin [1] and, to our knowledge for the first time, a cloudy Venus-twin [Konrad et al., in prep.] exoplanet around a sun-like star at 10 pc. We simulate MIR planet emission spectra with petitRADTRANS (a 1D radiative transfer model) [2] and use LIFESim [3] to estimate the wavelength-dependent astrophysical noise expected for exoplanet observations with LIFE. Our retrieval suite uses the atmospheric model petitRADTRANS, including a parametrized cloud model, and the MultiNest algorithm [4] for parameter estimation. We retrieve the planetary radius, the PT profile, the surface pressure, the molecular abundances and the cloud parameters. Further, we constrain the planetary mass using the statistical mass-radius relation forecaster [5]. By considering input spectra of different wavelength ranges, resolutions (R), and noise levels (S/N), we aim to determine the requirements to:  1. Discriminate Earth- from Venus-like MIR spectra. 2. Characterize the structure and composition of atmospheres. 3. Detect potential biomarkers in Earth-twin. 4. Infer the presence of clouds in atmospheres. 5. Constrain cloud structure and composition in a Venus-twin.  We also discuss challenges in the analysis of MIR exoplanet spectra from LIFE via atmospheric retrievals and how differences in the quality of the spectra affect them. With these studies and an additional retrieval study for Earth at different times [Alei et al., in prep.], we find first constraints for the instrument requirements for the LIFE interferometer and identify important limitations and challenges of MIR atmospheric retrieval studies for exoplanets.

References: [1] Konrad, B.S., et al., 2021, arXiv:2112.02054v2 (A&A preprint); [2] Mollière, P., et al., 2019, A&A, 627:A67; [3] Dannert, F., et al., 2022, arXiv:2203.00471v2 (A&A preprint); [3] Feroz, F., et al., 2009, MNRAS, 398(4):1601–1614; [4] Chen, J., Kipping, D.M., 2016, arXiv:1603.08614

Upcoming missions such as JWST will allow for spectroscopically characterizing the atmospheres of terrestrial exoplanets with unprecedented detail. These planets pose unfamiliar challenges to the current Bayesian atmospheric retrievals. One such challenge is the presence of a planetary surface, which may impact the observed emission spectrum if the planet has an optically thin atmosphere. In Whittaker et al. (submitted), we found that the albedo spectrum of the surface can bias the inferred atmospheric abundances of existent gases and mimic the signatures of absent ones. In the work presented here, we incorporate realistic spectral surfaces into our atmospheric retrieval model and test the feasibility of identifying the surface composition and surface pressure via emission spectroscopy. We explore planets of various atmospheric and surface properties. We find that a grey surface albedo model does not correctly retrieve these properties and thus a wavelength-dependent albedo is necessary. We find that the surface pressure is a robustly estimated parameter. We find that, broadly speaking, simultaneously retrieving the atmospheric chemistry, thermal structure, and surface composition is challenging, but which piece of information is correctly estimated depends on the specific planet. I will talk about how to thus best implement and interpret retrievals for the thermal emission of rocky planets. We also test various parameterizations of the surface albedo spectrum and discuss degeneracy with aerosols. We also identify potential targets for surface characterization with JWST to test the connection between interior evolution and surface types, and a temperature-dependent dichotomy between solid surfaces and magma oceans. 

Super-Earth and Neptune-like planets are found to be abundant in our galaxy. The observed radii of these worlds indicate that some of them are bare rocky objects while others contain lighter materials - gases or volatiles. While the terms “super-Earth” and “Neptune-like” may hint at similarity with our solar system planets, characterisation of these rocky worlds has to be done independently of the solar system related assumptions.

One way to shed light on the nature of these planets is by studying the effects of various compositions on planet formation, thermal evolution, and long term internal structure. I will show some findings based on recent planet formation and evolution models, and on data from laboratory experiments. In particular I will show that the planet formation process involves significant envelope pollution by rock vapor, that wet-worlds can have fundamentally different interiors than dry worlds, and that interiors of close-in and further-out Neptune twins are quite similar.

Planets smaller than Neptune are ubiquitous in the Galaxy and those around M stars constitute the bulk of warm and temperate worlds amenable for detailed atmospheric characterization. In this talk, we present a re-analysis of all the available data on small transiting planets around M dwarfs, refining their masses and radii (Luque & Pallé 2022, in press). Our precisely characterized sample reveals that this population is well described by only three discrete planet density populations, with bulk densities centered at 1.0, 0.5 and 0.24 relatively to Earth's. The first are rocky planets, the second are water worlds, and the third are puffy planets with Neptune-like densities. This density classification offers a much better insight to disentangle planet formation and evolution mechanisms, which are degenerate when using a radius-based classification. Our results are at odds with atmospheric mass-loss models aiming to explain the bimodal radius distribution and suggest that the gap separates dry from water worlds rather than rocky planets with or without H/He envelopes. Formation models including type I migration explain naturally the observations independently of the accretion mechanism: rocky planets form within the snow line, water worlds form beyond and later migrate inwards.

We analyze the TESS Mission data of a volume-complete sample of 363 mid-to-late M dwarfs with masses between 10% and 30% the solar value and falling within 15 parsecs. We search the TESS 2-minute cadence light curves for transiting planets with orbital periods below 7 days (corresponding to an insolation similar to Venus) and recover all 6 currently known planets within the sample as well as a likely planet candidate orbiting LHS 475 (previously reported as TESS Object of Interest 910.01). We perform a transit injection and recovery analysis for each of the stars to quantify the transit detection sensitivity as a function of planet radius, insolation, and orbital period. We deduce a cumulative occurrence rate of 0.67 +/- 0.28 planets per M dwarf, for planets with radii greater than half the Earth's value. We estimate that rocky terrestrials vastly outnumber enveloped terrestrials around mid-to-late M dwarfs, in contrast with Sun-like stars where their relative abundance is close to 1, despite receiving similar amounts of total radiation. We detect a narrowly peaked distribution of planet size, with planet radii between 0.9-1.4 that of the Earth, and a significant dearth of planets with radii above 1.4 the Earth's value. We also obtain suggestive evidence of a downturn in rocky terrestrial occurrence rate with decreasing planet size: planets with radii between 0.6-0.9 that of the Earth may be intrinsically less common than those with radii close to or above the Earth's value.

The SPECULOOS (Search for habitable Planets EClipsing ULtra-cOOl Stars) project, aimed at detecting transiting terrestrial planets around ultracool dwarfs, began its scientific operations three years ago. SPECULOOS aims to provide first-class planets for atmospheric characterisation in the age of JWST. This survey also provides the unique possibility to study these mysterious red dwarfs and their planetary populations in detail. In this talk, I will present an update on the current status of the survey and an overview of recent results. In particular, I will describe how an efficient synergy with the TESS mission and other ground-based facilities led to the exciting new discovery of two temperate super-Earths transiting a nearby M6 dwarf, with the outer one orbiting in the habitable zone. In terms of potential for atmospheric characterization, we estimate that this planet is the second-most favorable habitable-zone terrestrial planet found so far after the TRAPPIST-1 planets. The discovery of this remarkable system offers another rare opportunity to study temperate terrestrial planets around our smallest and coolest neighbours.

Exoplanets are ubiquitous across the Milky Way. In the next decades we will find hundreds of rocky planets, but if we are to truly understand what these planets are like, we must characterise their interiors. White dwarfs that have swallowed exoplanets provide a unique opportunity to probe their composition. Spectral features, including Mg, Fe, Ca, O, C, Ni, Cr, etc tell us their bulk elemental composition, from which we probe the geological process of core formation, as well as the volatile content. We present results from a new large survey studying some of the heaviest polluted white dwarfs (Rogers et al, in prep). What can the composition tell us about exoplanetary material and geological processes in exoplanetary systems? During planet formation it is hypothesised that planets form from the same cloud of gas as the star. Wide binaries with a polluted white dwarf, which gives the composition of the planetary body, and a main sequence companion, which acts as a proxy for the composition of the white dwarf progenitor, tests whether the refractory composition of the planets is the same as that of the host star. This is assumed true for interior modelling of planets, but we test whether this assumption is indeed the case in a pilot study (Bonsor et al. 2021). Does the refractory composition of the accreted planetary body match that of the wide binary companion? 

TOI-561 is a galactic thick disk star hosting an ultra-short period (USP, 0.45 day orbit) planet of 1.425 R⊕. As one of the few thick disk stars known to host an exoplanet, TOI-561 is one of the most iron depleted (-0.41 dex), alpha abundance enriched (0.23 dex) and oldest (∼10 Gyr) sites where an Earth-sized planet has formed. The equilibrium temperature of the planet (∼2300 K) is comparable to other USPs that have rocky surfaces. Unlike the majority of USPs, however, TOI-561 b has an anomalously low density suggestive of a bulk composition that is iron-poor and contains a thin gas-phase envelope. To improve the interpretation of TOI-561 b’s bulk composition, we conducted a radial velocity (RV) campaign gathering simultaneous measurements from Gemini-N/Maroon-X and Keck/HIRES, which we combined with literature RVs from HIRES and TNG/HARPS-N to find a mass of 2.25 ± 0.2 M⊕. We used two new sectors of TESS photometry to improve the radius determination, finding 1.425 ± 0.037 𝑅⊕. We confirm TOI-561 b is the lowest-density super-Earth measured to date, with 𝜌𝑏 = 4.3± 0.3 g/cm3. We then explore the range of possible atmospheric compositions, and rule out a Hydrogen/Helium envelope–the most abundant form of exoplanet atmospheres. Instead, TOI-561b likely hosts a thin envelope of high mean molecular weight species, such as CO2 or silicate vapor. TOI-561 b appears to be unlike any other planet yet discovered, and challenges our assumptions on the compositions of super-Earth size rocky worlds.

Disintegrating rocky exoplanets are planets that due to proximity to their host stars are evaporating and present comet-like dust tails (e.g. KIC 1255b and K2-22b). Such planets are potentially the key to a better understanding of planet formation, if one is able to determine the composition of the dust tail. This can be achieved by modelling the dynamics of the tail of dust and comparing it to observations. Despite this importance, these planets are yet to be modelled self-consistently. In this talk, I will present simulations with a new self-consistent model of the dust tail which includes the dust dynamics, sublimation and optical depth evolution. 

With the new simulations, we are able to constrain properties of the planetary outflow and dust, as well as constraining for the first time the geometry of the outflow, which is not launched spherically. The simulations can be used to constrain potential dust compositions of any disintegrating planets, allowing us insights into the composition of extra-solar rocky bodies, and hence their origins.

K2-141 b is a transiting, small (1.5 Re) ultra-short-period (USP) planet discovered by the Kepler space telescope orbiting a K-dwarf host star every 6.7 hours. The planet’s high surface temperature of more than 2000 K makes it an excellent target for thermal emission observations. Here we present 65 hours of continuous photometric observations of K2-141 b collected with Spitzer’s IRAC Channel 2 at 4.5 micron spanning 10 full orbits of the planet. We measure an infrared eclipse depth of 143 +/- 39 ppm and a peak to trough amplitude variation of 121 +/- 43 ppm. The best fit model to the Spitzer data shows no significant thermal hotspot offset, in contrast to the previously observed offset for the well-studied USP planet 55 Cnc e. We also jointly analyze the new Spitzer observations with the photometry collected by Kepler during two separate K2 campaigns. We model the planetary emission with a range of toy models that include a reflective and a thermal contribution. With a two-temperature model, we measure a dayside temperature of 2049 +/- 361 K and a night-side temperature that is consistent with zero (< 1712 K at 2 sigma). Models with a steep dayside temperature gradient provide a better fit to the data than a uniform dayside temperature (∆BIC = 22.2). We also find evidence for a non-zero geometric albedo Ag = 0.28 +/- 0.07. We also compare the data to a physically motivated, pseudo-2D rock vapor model and a 1D turbulent boundary layer model. Both models fit the data well. Notably, we find that the optical eclipse depth can be explained by thermal emission from a hot inversion layer, rather than reflected light. A thermal inversion may also be responsible for the deep optical eclipse observed for another USP, Kepler-10 b. Finally, we significantly improve the ephemerides for K2-141 b and c, which will facilitate further follow-up observations of this interesting system with state-of-the art observatories like JWST.

Lava worlds belong to a class of short orbital period planets reaching surface temperatures high enough to melt their silicate crust. Theory predicts that the resulting lava oceans outgas their volatile components, attaining equilibrium with the overlying vapour. This creates a tenuous, silicate-rich atmosphere that may be confined to the permanent dayside of the planet. With the recently successful deployment of JWST it is now possible to characterise these worlds. We assess JWST observability of key spectral features by self-consistently modelling silicate atmospheres for all the currently confirmed targets having sufficient substellar temperatures. We use outgassed equilibrium chemistry and radiative transfer methods to compute temperature-pressure profiles, atmospheric chemical compositions and emission spectra. Our results indicate that SiO and SiO2 infrared features are the best, unique identifiers of silicate atmospheres, detectable using the MIRI instrument of JWST. Detection of these two species in emission would allow for strong constraints on atmospheric thermal structure and possibly the composition of the melt. We also propose that certain species, e.g., TiO or MgO, may be directly tied to different classes of melts, possibly revealing surface and interior dynamics. Currently, there are nearly a dozen confirmed lava planets ideal for characterisation using JWST, but only two of these have been accepted for the initial General Observers program.

Ultra-short period small planets (R<2 R⊕, P < 1 d) are believed to be naked cores of former Neptunian planets that have lost their atmospheres during the migration process from beyond the ice line to their current close-in orbits. How this migration occurred is currently unclear and needs to be explored on a case-by-case basis once the inner architecture of the hosting system is known. GJ 367 is an M1 V star that has been recently found to host a transiting ultra-short period sub-Earth on a 8 h orbit (Lam et al. 2021). With the aim of improving the planetary mass and radius and unveiling the architecture of the system, we performed an intensive RV follow-up campaign with the HARPS spectrograph - collecting nearly 300 high-precision radial velocities - and combined our Doppler measurements with new TESS observations from Sectors 35 and 36. We found that GJ 367 b has a mass of Mb=0.611 ± 0.068 M⊕ (11 %), a radius of Rb=0.704 ± 0.025 R⊕ (3.6 %), implying a bulk density of ρb = 9.6 ± 1.5 g cm-3. We revealed the presence of two additional low mass companions with orbital periods of ~11.5 and 34 days and minimum masses of ~4.5 and 6.6 M⊕, respectively, and explored the different secular migration scenarios that could account for the current architecture of the planetary system.

Ultra-short period small planets (R<2 R⊕, P < 1 d) are believed to be naked cores of former Neptunian planets that have lost their atmospheres during the migration process from beyond the ice line to their current close-in orbits. How this migration occurred is currently unclear and needs to be explored on a case-by-case basis once the inner architecture of the hosting system is known. GJ 367 is an M1 V star that has been recently found to host a transiting ultra-short period sub-Earth on a 8 h orbit (Lam et al. 2021). With the aim of improving the planetary mass and radius and unveiling the architecture of the system, we performed an intensive RV follow-up campaign with the HARPS spectrograph - collecting nearly 300 high-precision radial velocities - and combined our Doppler measurements with new TESS observations from Sectors 35 and 36. We found that GJ 367 b has a mass of Mb=0.611 ± 0.068 M⊕ (11 %), a radius of Rb=0.704 ± 0.025 R⊕ (3.6 %), implying a bulk density of ρb = 9.6 ± 1.5 g cm-3. We revealed the presence of two additional low mass companions with orbital periods of ~11.5 and 34 days and minimum masses of ~4.5 and 6.6 M⊕, respectively, and explored the different secular migration scenarios that could account for the current architecture of the planetary system.

The study of exoplanets and especially their atmospheres can reveal key insights on their evolution by identifying specific atmospheric species. Towards the atmospheric characterization of terrestrial planets, the super-Earth exoplanet 55 Cnc e is one of the most promising exoplanets studied to date. Here, we present a high-resolution spectroscopic transit observation of this planet, acquired with the PEPSI instrument at the Large Binocular Telescope. Assuming the presence of Earth-like crustmspecies on the surface of 55 Cnc e, from which a possible silicate-vapor atmosphere could have originated, we search in its transmission spectrum for absorption of various atomic and ionized species such as Fe, Fe+, Ca, Ca+, Mg and K , among others. 

The vigour and style of mantle convection in tidally locked super-Earths may be substantially different from Earth's regime where the surface temperature is spatially uniform and sufficiently cold to drive downwellings into the mantle. Super-Earth LHS 3844b is a rocky exoplanet with a radius around 1.3 Earth radii and its thermal phase curve suggests a solid surface and lack of a substantial atmosphere with a dayside temperature around 1040 K and a nightside temperature around 0 - 700 K. GJ 486b is a super-Earth, which is very similar to LHS 3844b in terms of size and it is currently unknown whether this planet has an atmosphere. In this study, we are investigating under which circumstances hemispheric tectonics can operate on GJ 486b and how stable such a hemispheric tectonic regime is.

We run 2D geodynamic simulations of the interior mantle flow using the mantle convection code StagYY. The models are fully compressible with an Arrhenius-type viscosity law where the mantle is mostly composed of perovskite and post-perovskite. The lithospheric strength is modelled through a plastic yielding criterion and the heating mode is either basal heating only or mixed heating (basal and internal heating). We use general circulation models (GCMs) of potential atmospheres to constrain the surface temperature assuming different efficiencies of atmospheric heat circulation.

We find that a hemispheric tectonic regime is also possible for surface temperature contrasts with moderate heat redistribution. The location of the strong downwelling depends on several factors such as the surface temperature contrast and strength of the lithosphere. Our results show that hemispheric tectonics could operate on tidally locked super-Earths, even if the surface temperature contrast between the dayside and nightside is not as strong as for LHS 3844b. Upwellings that rise preferentially on one hemisphere could lead to generation of melt and subsequent outgassing of volatiles on that side. Imprints of such outgassing on the atmospheric composition could possibly be probed by current and future observations such as JWST, ARIEL or the ELT. 

The actual mantle water content in Earth today is poorly constrained, but its water storage capacity may amount to a few times the modern surface ocean mass (OM). Knowledge of mantle water storage capacity informs our understanding of how water may have cycled between the surface and mantle and changed the volumes of the oceans. We parameterized high pressure- temperature experimental data on water storage capacities in major rock-forming minerals to track the bulk water storage capacity in Earth’s mantle, finding that storage capacity increases as temperature decreases with secular cooling. For today’s mantle potential temperature (TP = 1600 K), the Earth’s median mantle water storage capacity is 2.29 OM, compared to a median of 0.72 OM for TP = 1900 K, suggesting that there may have been more voluminous surface oceans during the early Archean. We performed the same analysis for Mars, taking into account the effect of Fe on minerals’ water storage capacities. For today’s mantle potential temperature (TP = 1600 K), the Martian mantle has an average bulk water storage capacity of 9.0 km Global Equivalent Layer (GEL); however, for the early Martian mantle with TP = 1900 K, the average bulk water storage capacity is only 4.9 km GEL. The mantle water storage capacities of both Earth and Mars increase significantly with secular cooling through time, but due to the lack of an efficient water recycling mechanism on Mars, its actual mantle water content may be significantly lower than its storage capacity today, while subduction on Earth may have acted to keep the Earth’s mantle water content close to its storage capacity over time.

The Sun, Earth, and Moon trio dances to rhythms of mutual gravitational tidal interactions. Consequently, ever-since the Moon formed close to the Earth, it has been forced to drift away. Available geological data provide snapshots of the lunar orbital history, but a complete theoretical reconstruction is yet to be established. Namely, previous models of this reconstruction are either empirical, or numerically costly, and are always incompatible with the estimated lunar age. In this talk, we undertake a systematic exploration of the time-varying tidal dissipation in the Earth's paleo-oceans to provide a history of the lunar orbit that fits the present measurement of its recession, the lunar age, and the available geological proxies. The resulting evolution involves multiple crossings of resonances in the oceanic dissipation that are associated with significant and rapid variations in the lunar orbital distance, the Earth’s length of the day, obliquity, and precession frequency. Our effective tidal model couples oceanic and solid planetary interactions and requires a reduced number of free parameters, thus allowing its utilization outside our home. A classical dynamical tale revived for the exoplanetary 21st century!

The inferred density of Enceladus’ core, together with evidence of hydrothermal activity within the moon, suggests that the core is porous. Tidal dissipation in an unconsolidated core has been proposed as the power source for Enceladus' geological activity. However, the tidal response of the core has generally been modeled assuming it behaves viscoelastically rather than poro-viscoelastically. We analyze the poroviscoelastic response to better constrain the distribution of tidal dissipation within Enceladus. A poroviscoelastic body has a different tidal response than a viscoelastic one; pressure within the pores alters the stress field and induces a Darcian porous flow. This flow represents an additional pathway for energy dissipation. Using Biot’s theory of poroviscoelasticity, we develop a new framework to obtain the tidal response of a spherically symmetric, self-gravitating moon with porous layers. We apply this theory to Enceladus and show that the boundary conditions at the interface of the core and overlying ocean play a key role in the tidal response. The ocean hinders the development of a large-amplitude Darcian flow, making negligible the Darcian contribution to the dissipation budget. We therefore infer that Enceladus’ core can be the source of its geological activity only if it has a low rigidity and a very low viscosity. A future mission to Enceladus could test this hypothesis by measuring the phase lags of tidally induced changes of gravitational potential and surface displacements.

Tidal evolution is playing a critical role in the development of the architecture of the solar and extra-solar systems. On the one hand, tides add a lot to the n-body interactions in these systems. On the other hand, tidal heating contributes greatly to the thermal processes and internal evolution of planets. Among the numerous examples of the working of tides in the solar system are: (1) the Martian moons Phobos and Deimos, which are migrating towards and away from its host planet, respectively; (2) the Jovian moon Io with its strong tidal volcanism; (3) the Saturnian moon Enceladus renown for its water-rich plumes venting into space. Inarguably, tides and tidal heating are playing a major role in determining planetary habitability. Modeling tidal evolution of orbits and interiors thus becomes a key to our understanding the evolution and current state of planetary systems. Historically, the Constant Phase Lag (CPL) and Constant Time Lag (CTL) models were often employed for this purpose. Simplistic, and not always consistent mathematically, these models often yield incorrect predictions. Relying on the modern theory of tides, which has its origins in the work of Darwin and Kaula, we developed a tidal model applicable to describing highly eccentric orbits and higher-order spin-orbit resonances. The tidal model is combined with self-consistently built interior structure models, and is used to explore evolution of terrestrial planets and moons. The model has been employed to study, e.g., the orbital evolution of the Martian moons, the Pluto-Charon binary, exoplanet TRAPPIST-1e, and the dwarf planet Gonggong. We show that the use of this tidal model is essential to obtain precise results. We illustrate this on several solar-system and exoplanetary objects.

The history of Earth’s atmospheric molecular oxygen (O₂) remains loosely constrained. Geological evidence supports the view that there were two major episodes where O₂ increased by an order of magnitude or more: the Great Oxidation Event (GOE) and the Neoproterozoic Oxidation Event. Since the GOE ~2.4 Gyr ago, O₂ concentrations have likely fluctuated between 10⁻⁴ and 1.5 times the present atmospheric level (PAL), resulting in a time-varying ozone (O₃) layer. I will present simulations from a three-dimensional (3D) chemistry-climate model where oxygen has been varied between 10⁻³ and 1.5 times PAL. I will discuss changes in the abundance and distribution of O₃ in Earth's atmosphere since the GOE and consider the implications for biological surface habitability, as well as CH₄ abundance and its impact on glaciation during the Mesoproterozoic. The calculated O₃ columns are lower (reduced by up to 4.68 times for a given O₂ concentration) when compared to previous work. Consequently, higher fluxes of biologically harmful UV radiation would have reached the surface under this reduced O₃ shield. Additionally, this leads to enhanced tropospheric production of the hydroxyl radical (OH) which then substantially reduces the lifetime of methane (CH₄). I will show that a CH₄ supported greenhouse effect during the Mesoproterozoic may be even more unlikely than previously thought. The simulated O₃ columns have significant implications for planetary habitability and demonstrate the importance of 3D chemistry-climate simulations when assessing paleoclimates and climates on faraway worlds.

Life has played a key role in shaping the atmosphere since its origin on Earth and is likely to have played a similar role on inhabited exoplanets. However, modelling the biosphere’s impact on climate is complicated by the range of time and spatial scales involved. 3D climate models have successfully been used to spatially resolve key processes, but on relatively short time scales. Whereas, biogeochemical modelling allows us to estimate biosphere driven gas fluxes in and out of the atmosphere over longer time scales, but lacks a sophisticated treatment of a spatially resolved atmosphere. Here, we look to bridge these two modelling approaches to better understand the biosphere’s impact on the climate, and the possible implications this may have on terrestrial exoplanets. We use a biogeochemical model to understand the limits on the potential evolution of the atmosphere, as well as a state-of-the-art 3D climate model to explore potential atmospheric compositions produced by early biospheres. The biogeochemical model, coupled to a 1D photochemical model, has been developed to explore the effects of early biospheres driven by anoxic phototrophs. There is a particular focus on the effect of methane on the early climate, which has predominantly biotic sources. We then use the 3D climate model to explore the climate of varying plausible methane concentrations during the Archean. This work begins to explore how models of the early biosphere can be coupled to 3D climate models, to understand the biosphere’s impact on the climate of Early Earth and potentially inhabited terrestrial exoplanets.

I will present new results obtained using 3-D Global Climate Model simulations of very hot, water-rich dominated planetary atmospheres (Turbet et al. 2021, Nature) designed to evaluate the ability of initially hot terrestrial/super-Earth-sized planets (typical of post-magma-ocean conditions) to form primordial surface liquid water oceans. This work shows that a significant fraction of exoplanets located within the so-called "Habitable Zone" (HZ) should be unable to host liquid water oceans, because their primordial water reservoir never had the opportunity to condense at the surface due to day-night asymmetric distribution of water clouds. This defines a surface liquid water “Condensation Zone” (CZ). I will show – using the inner planets of the TRAPPIST-1 system (b,c,d) as a proof of concept – how to use JWST observations to test this day-night cloud feedback and thus the CZ.