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The timing of formation for the first planetesimals determines the mode of planetary accretion and their geophysical and compositional evolution. Astronomical observations of circumstellar discs and Solar System geochronology provide evidence for planetesimal formation during molecular cloud collapse, much earlier than previously estimated. Here, we present distinct observational evidence from white dwarf planetary systems for planetesimal formation occurring during the first few hundred thousand years after cloud collapse in exoplanetary systems. A significant fraction of white dwarfs have accreted planetary material rich in iron core or mantle material. In order for the exo-asteroids accreted by white dwarfs to form iron cores, substantial heating is required. By simulating planetesimal evolution and collisional evolution we show that the most likely heat source is short-lived radioactive nuclides such as Al-2 (half life of approximately 0.7 Myr). Core-rich materials in the atmospheres of white dwarfs, therefore, provide independent evidence for rapid planetesimal formation, concurrent with star formation., Comment: Accepted to Nature Astronomy

The scarcity of geochemical constraints on the Hadean climate limits our understanding of the planetary environment that gave rise to life on the earliest Earth. I will describe how rocky exoplanets open a novel observational window into the geodynamic evolution of terrestrial worlds that is unavailable in the Solar System. Some of these planets orbit so closely to their star that they lack an atmosphere, which gives direct access to their surfaces. Geodynamic simulations of the super-Earths LHS 3844b and GJ 486b suggest that solid but tidally-locked rocky exoplanets undergo a hemispherically-forced tectonic regime, unknown in the Solar System, with volcanic activity focused either on the day- or nightside, potentially driving asymmetric outgassing. The dense sub-Earth GJ 367b, on the other hand, likely hosts a day-side magma ocean, enabling inferences on accretion regime and the evolution of melting geometry and surface composition over geologic time. Scheduled high-resolution observations with JWST of these and similar exoplanets will enable novel tests of fundamental models of planetary geodynamics, atmospheric formation, and planetary accretion.

The dawn of high-resolution observations with the James Webb Space Telescope will enable spatially resolved observations of ultrashort-period rocky exoplanets. Some of these planets orbit so closely to their star that they lack an atmosphere [1], which gives direct access to their surfaces and opens a window to infer their geodynamics [2]. The physical parameters of the ultrashort-period sub-Earth GJ 367b have been observationally constrained to a planetary radius of about 0.72 to 0.75 Earth-radii and a mass between 0.48 and 0.55 Earth-masses, implying a density of 6200 to 8500 kg/m3 [3, 4], which puts this planet in a Mercury-like interior regime with a thin mantle overlying a fractionally large core.The dayside temperature ranges between 1500 to 1800 K, thus suggesting the presence of a permanent magma ocean or dayside magma pond on the surface, induced by stellar irradiation. The large uncertainty on the age of the stellar system, between 30 Myr [4] and about 8 Gyr [3], however, introduce severe uncertainties related to the compositional and thermal evolution of the planetary mantle. In this study we perform global 2D spherical annulus StagYY simulations [5, 6] of solid state mantle convection and surface melting with the goal to constrain the geometric and compositional properties ofthe planet. Constraining the spatial dimensions of thermodynamic properties of partially molten, atmosphere-less planets like GJ 367b offers unique opportunities to constrain the compositional fractionation during magma ocean epochs and provides avenues to constrain the delivery and loss cycle of atmophile elements on strongly irradiated exoplanets.References:[1] L. Kreidberg and 18 co-authors. Absence of a thick atmosphere on the terrestrial exoplanet LHS 3844b. Nature, 573:87–90, August 2019.[2] T. G. Meier, D. J. Bower, T. Lichtenberg, P. J. Tackley, and B.-O. Demory. Hemispheric Tectonics on LHS 3844b. Astrophys. J. Lett., 908:L48, February 2021.[3] K.W.F. Lam and 78 co-authors. GJ 367b: A dense, ultrashort-period sub-earth planet transiting a nearby red dwarf star. Science, 374:1271–1275, 2021.[4] W. Brandner, P. Calissendorff, N. Frankel, and F. Cantalloube. High-contrast, high-angular resolution view of the GJ367 exoplanet system. Mon. Notices Royal Astron. Soc., 513:661–669, June 2022.[5] J. W. Hernlund and P. J. Tackley. Modeling mantle convection in the spherical annulus. Phys. Earth Planet. Int., 171:48–54, 2008.[6] P. J. Tackley. Modelling compressible mantle convection with large viscosity contrasts in a three-dimensional spherical shell using the yin-yang grid. Phys. Earth Planet. Int., 171:7–18, 2008.

LIFE (www.life-space-mission.com) is an initiative to develop the science, technology and a roadmap for an ambitious space mission that will allow humankind to detect dozens of warm, terrestrial exoplanets and hundreds of exoplanets overall at mid-infrared (MIR) wavelengths (Quanz et al., 2018, 2022). For most of the detected exoplanets direct estimates of their effective temperature and radius will be available, and a for a significant subset the atmospheric composition will be investigated including the search for potential biosignatures (Des Marais et al., 2002; Léger et al., 2019). Characterizing exoplanet atmospheres using their thermal emission at MIR wavelengths — compared to studies at optical/near-infrared wavelength looking at planets in reflected light — offers the possibility to study a broader set of molecular features (Schwieterman et al., 2018) and get a better understanding of the atmospheric structure (Line et al., 2019). Hence, in particular for questions related to the habitability of exoplanets, a mission like LIFE offers unprecedented scientific potential.The current baseline design of LIFE features a 4-aperture interferometer array with a 6:1 baseline ratio to reduce the impact of instability noise (Lay, 2006). A beam combiner spacecraft is located at the center of the array. The size of the individual apertures is currently under study, but based on detection yield simulations including all relevant astrophysical noise sources, diameters of 2–3.5 m are under consideration. The aperture size is primarily driven by the number of detectable planets and the time-on-target required for in-depth atmospheric characterization. The current wavelength range requirement is 4–18.5 μm, but additional studies are underway for further verification. A spectral resolution of at least R=30, but better R=50, seems required in order to reliably quantify the abundance ratios of main molecular species in the atmosphere of an Earth-twin planet at several pc distance. The minimum mission lifetime is 5-6 years in order to have sufficient time for both a dedicated search phase, to identify the most interesting and promising targets, and a characterization phase for in-depth investigations of a subset of those. LIFE shall be launched to the Earth-Sun L2 point.While the general feasibility of the required null-depth and stability was demonstrated in the context of Darwin and TPF-I (Martin et al., 2012), a corresponding experiment under cryogenic conditions is underway in the form of the Nulling Interferometric Cryogenic Experiment (NICE) at ETH Zurich (Gheorghe et al., in prep.). A more general overview of the readiness of key technologies for a space mission like LIFE was presented in Defrère et al. (2018). Defrère, D., et al. 2018, Exp. Astron., 46, 475Des Marais, D. J., et al. 2002, Astrobiology, 2, 153Lay, O. P. 2006, in SPIE Astronomical Telescopes + Instrumentation (SPIE), 62681A-14Léger, A., et al. 2019, Astrobiology, 19, 797Line, M., et al. 2019, BAAS, 51, 271Martin, S., et al. 2012, Appl. Opt., 51, 3907Quanz, S. P., et al. 2018, SPIE Conf. Ser., 10701, 107011IQuanz, S. P., et al. 2022, A&A, 664, A21Schwieterman, E. W., et al. 2018, Astrobiology, 18, 663

Many super-Earths are very close-in to their host star and are therefore likely to be tidally locked. Tidally locked super-Earths experience intense solar heating on their permanent dayside, whereas the nightside surface can reach extremely cold temperatures. For the case of super-Earth LHS 3844b, a bare-rock super-Earth with a radius around 1.3 Earth radii, we have shown that this strong contrast between the dayside and nightside surface temperature can lead to a so-called hemispheric tectonic regime. Such a regime is characterised by a strong downwelling on one hemisphere and upwellings that rise on the other side. We further define hemispheric tectonics as a special case of degree-1 convection (one upwelling and one downwelling), where the downwelling gets pinned to either the dayside or nightside and upwellings are preferentially on the other hemisphere. Here, we focus on super-Earth GJ 486b, which has also a radius around 1.3 Earth radii, but for which it is unknown whether it was able to retain an atmosphere. We investigate how different surface temperature contrasts affects the likelihood of hemispheric tectonics. For this, 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 models are basally heated. We use general circulation models (GCMs) of potential atmospheres to constrain the surface temperature assuming different efficiencies of atmospheric heat circulation. We find that degree-1 convection is a consequence of the strong lithosphere, while hemispheric tectonics is favoured for strong surface temperature contrasts between the dayside and nightside, and higher surface temperatures. Our results show that hemispheric tectonics or degree-1 convection could operate on super-Earth GJ 486b (or other 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 abundance of the short-lived radioisotopes 26-Al and 60-Fe in the early Solar system is usually explained by the Sun either forming from pre-enriched material, or the Sun's protosolar disc being polluted by a nearby supernova explosion from a massive star. Both hypotheses suffer from significant drawbacks: the former does not account for the dynamical evolution of star-forming regions, while in the latter the time for massive stars to explode as supernovae can be similar to, or even longer than, the lifetime of protoplanetary discs. In this paper, we extend the disc enrichment scenario to include the contribution of 26-Al from the winds of massive stars before they explode as supernovae. We use N-body simulations and a post-processing analysis to calculate the amount of enrichment in each disc, and we vary the stellar density of the star-forming regions. We find that stellar winds contribute to disc enrichment to such an extent that the Solar system's 26-Al/60-Fe ratio is reproduced in up to 50 per cent of discs in dense (rho = 1000Msun pc^-3) star-forming regions. When winds are a significant contributor to the SLR enrichment, we find that Solar system levels of enrichment can occur much earlier (before 2.5 Myr) than when enrichment occurs from supernovae, which start to explode at later ages (>4 Myr). We find that Solar system levels of enrichment all but disappear in low-density star-forming regions (rho < 10Msun pc^-3), implying that the Solar system must have formed in a dense, populous star-forming region if 26-Al and 60-Fe were delivered directly to the protosolar disc from massive-star winds and supernovae., 16 pages, 10 figures, 1 appendix with 3 further figures, accepted for publication in MNRAS

Active comets have been detected in several exoplanetary systems, although so far only indirectly, when the dust or gas in the extended coma has transited in front of the stellar disk. The large optical surface and relatively high temperature of an active cometary coma also makes it suitable to study with direct imaging, but the angular separation is generally too small to be reachable with present-day facilities. However, future imaging facilities with the ability to detect terrestrial planets in the habitable zones of nearby systems will also be sensitive to exocomets in such systems. Here we examine several aspects of exocomet imaging, particularly in the context of the Large Interferometer for Exoplanets (LIFE), which is a proposed space mission for infrared imaging and spectroscopy through nulling interferometry. We study what capabilities LIFE would have for acquiring imaging and spectroscopy of exocomets, based on simulations of the LIFE performance as well as statistical properties of exocomets that have recently been deduced from transit surveys. We find that for systems with extreme cometary activities such as ß Pictoris, sufficiently bright comets may be so abundant that they overcrowd the LIFE inner field of view. More nearby and moderately active systems such as ÑÂ "Eridani or Fomalhaut may turn out to be optimal targets. If the exocomets have strong silicate emission features, such as in comet Hale-Bopp, it may become possible to study the mineralogy of individual exocometary bodies. We also discuss the possibility of exocomets as false positives for planets, with recent deep imaging of a Centauri as one hypothetical example. Such contaminants could be common, primarily among young debris disk stars, but should be rare among the main sequence population. We discuss strategies to mitigate the risk of any such false positives., Astronomy & Astrophysics, 671, ISSN:0004-6361, ISSN:1432-0746

Ocean-vaporizing impacts of chemically reduced planetesimals onto the early Earth have been suggested to catalyse atmospheric production of reduced nitrogen compounds and trigger prebiotic synthesis despite an oxidized lithosphere. While geochemical evidence supports a dry, highly reduced late veneer on Earth, the composition of late-impacting debris around lower-mass stars is subject to variable volatile loss as a result of their hosts' extended pre-main sequence phase. We perform simulations of late-stage planet formation across the M-dwarf mass spectrum to derive upper limits on reducing bombardment epochs in Hadean analog environments. We contrast the Solar System scenario with varying initial volatile distributions due to extended primordial runaway greenhouse phases on protoplanets and desiccation of smaller planetesimals by internal radiogenic heating. We find a decreasing rate of late-accreting reducing impacts with decreasing stellar mass. Young planets around stars $\leq$ 0.4 $M_\odot$ experience no impacts of sufficient mass to generate prebiotically relevant concentrations of reduced atmospheric compounds once their stars have reached the main sequence. For M-dwarf planets to not exceed Earth-like concentrations of volatiles, both planetesimals and larger protoplanets must undergo extensive devolatilization processes and can typically emerge from long-lived magma ocean phases with sufficient atmophile content to outgas secondary atmospheres. Our results suggest that transiently reducing surface conditions on young rocky exoplanets are favoured around FGK- stellar types relative to M-dwarfs., 17 pages, 3 figures, 1 table; accepted for publication in ApJL

The recently adopted Ariel ESA mission will measure the atmospheric composition of a large number of exoplanets. This information will then be used to better constrain planetary bulk compositions. While the connection between the composition of a planetary atmosphere and the bulk interior is still being investigated, the combination of the atmospheric composition with the measured mass and radius of exoplanets will push the field of exoplanet characterisation to the next level, and provide new insights of the nature of planets in our galaxy. In this white paper, we outline the ongoing activities of the interior working group of the Ariel mission, and list the desirable theoretical developments as well as the challenges in linking planetary atmospheres, bulk composition and interior structure., Experimental Astronomy, 53, ISSN:0922-6435, ISSN:1572-9508

Super-Earth LHS 3844b is a rocky exoplanet with a radius around 1.3 Earth radii. Its thermal phase curve suggests that the dayside temperature is around 1040 K and the nightside temperature is around 0 - 700 K, indicating inefficient atmospheric heat circulation. Therefore, this planet most likely lacks an atmosphere. In a previous study, we have shown that such a strong surface temperature dichotomy can lead to a so-called hemispheric tectonic regime. In such a regime, a cold downwelling forms preferentially on one side and hot upwellings are getting pushed towards the other hemisphere. GJ 486b is a super-Earth that 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. We also investigate the stability of hemispheric tectonics. 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 criteria 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. By reducing the temperature contrast, the location of the downwelling becomes less stable and it can start to move from one side towards the other over very long timescales (Gyrs). 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 nucleosynthetic isotope dichotomy between carbonaceous (CC) and non-carbonaceous (NC) meteorites has been interpreted as evidence for spatial separation and the coexistence of two distinct planet-forming reservoirs for several million years in the solar protoplanetary disk. The rapid formation of Jupiter's core within one million years after the formation of calcium-aluminium-rich inclusions (CAIs) has been suggested as a potential mechanism for spatial and temporal separation. In this scenario, Jupiter's core would open a gap in the disk and trap inward-drifting dust grains in the pressure bump at the outer edge of the gap, separating the inner and outer disk materials from each other. We performed simulations of dust particles in a protoplanetary disk with a gap opened by an early-formed Jupiter core, including dust growth and fragmentation as well as dust transport, using the dust evolution software DustPy. Our numerical experiments indicate that particles trapped in the outer edge of the gap rapidly fragment and are transported through the gap, contaminating the inner disk with outer disk material on a timescale that is inconsistent with the meteoritic record. This suggests that other processes must have initiated or at least contributed to the isotopic separation between the inner and outer Solar System., Comment: Accepted for publication in Astronomy & Astrophysics Letters

Ariel, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey, was adopted as the fourth medium-class mission in ESA's Cosmic Vision programme to be launched in 2029. During its 4-year mission, Ariel will study what exoplanets are made of, how they formed and how they evolve, by surveying a diverse sample of about 1000 extrasolar planets, simultaneously in visible and infrared wavelengths. It is the first mission dedicated to measuring the chemical composition and thermal structures of hundreds of transiting exoplanets, enabling planetary science far beyond the boundaries of the Solar System. The payload consists of an off-axis Cassegrain telescope (primary mirror 1100 mm x 730 mm ellipse) and two separate instruments (FGS and AIRS) covering simultaneously 0.5-7.8 micron spectral range. The satellite is best placed into an L2 orbit to maximise the thermal stability and the field of regard. The payload module is passively cooled via a series of V-Groove radiators; the detectors for the AIRS are the only items that require active cooling via an active Ne JT cooler. The Ariel payload is developed by a consortium of more than 50 institutes from 16 ESA countries, which include the UK, France, Italy, Belgium, Poland, Spain, Austria, Denmark, Ireland, Portugal, Czech Republic, Hungary, the Netherlands, Sweden, Norway, Estonia, and a NASA contribution., Comment: Ariel Definition Study Report, 147 pages. Reviewed by ESA Science Advisory Structure in November 2020. Original document available at: https://www.cosmos.esa.int/documents/1783156/3267291/Ariel_RedBook_Nov2020.pdf/

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 on 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 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 one order of magnitude., 13 pages, 5 figures, ApJL, video summary can be found here: https://bit.ly/DornLichtenberg21video

Central stages in the evolution of rocky, potentially habitable planets may play out under atmospheric conditions with a large inventory of non-dilute condensable components. Variations in condensate retention and accompanying changes in local lapse rate may substantially affect planetary climate and surface conditions, but there is currently no general theory to effectively describe such atmospheres. In this article, expanding on the work by Li et al. (2018), we generalize the single-component moist pseudoadiabat derivation in Pierrehumbert (2010) to allow for multiple condensing components of arbitrary diluteness and retained condensate fraction. The introduction of a freely tunable retained condensate fraction allows for a flexible, self-consistent treatment of atmospheres with non-dilute condensable components. To test the pseudoadiabat's capabilities for simulating a diverse range of climates, we apply the formula to planetary atmospheres with compositions, surface pressures, and temperatures representing important stages with condensable-rich atmospheres in the evolution of terrestrial planets: a magma ocean planet in a runaway greenhouse state; a post-impact, late veneer-analogue planet with a complex atmospheric composition; and an Archean Earth-like planet near the outer edge of the classical circumstellar habitable zone. We find that variations in the retention of multiple non-dilute condensable species can significantly affect the lapse rate and in turn outgoing radiation and the spectral signatures of planetary atmospheres. The presented formulation allows for a more comprehensive treatment of the climate evolution of rocky exoplanets and early Earth analogues., Comment: Accepted for publication in Planetary Science Journal on 8/24/2021. For a video summary see https://bit.ly/3ztVhfF

Internal redox reactions may irreversibly alter the mantle composition and volatile inventory of terrestrial and super-Earth exoplanets and affect the prospects for atmospheric observations. The global efficacy of these mechanisms, however, hinges on the transfer of reduced iron from the molten silicate mantle to the metal core. Scaling analysis indicates that turbulent diffusion in the internal magma oceans of sub- Neptunes can kinetically entrain liquid iron droplets and quench core formation. This suggests that the chemical equilibration between core, mantle, and atmosphere may be energetically limited by convective overturn in the magma flow. Hence, molten super-Earths possibly retain a compositional memory of their accretion path. Redox control by magma ocean circulation is positively correlated with planetary heat flow, internal gravity, and planet size. The presence and speciation of remanent atmospheres, surface mineralogy, and core mass fraction of atmosphere-stripped exoplanets may thus constrain magma ocean dynamics.

The earliest atmospheres of rocky planets originate from extensive volatile release during magma ocean epochs that occur during assembly of the planet. These establish the initial distribution of the major volatile elements between different chemical reservoirs that subsequently evolve via geological cycles. Current theoretical techniques are limited in exploring the anticipated range of compositional and thermal scenarios of early planetary evolution. However, these are of prime importance to aid astronomical inferences on the environmental context and geological history of extrasolar planets. In order to advance the potential synergies between exoplanet observations and inferrences on the earliest history and climate state of the solar system terrestial planets, I will present a novel numerical framework that links an evolutionary, vertically-resolved model of the planetary silicate mantle with a radiative-convective model of the atmosphere. Numerical simulations using this framework illustrate the sensitive dependence of mantle crystallization and atmosphere build-up on volatile speciation and predict variations in atmospheric spectra with planet composition that may be detectable with future observations of exoplanets. Magma ocean thermal sequences fall into three general classes of primary atmospheric volatile with increasing cooling timescale: CO, N2, and O2 with minimal effect on heat flux, H2O, CO2, and CH4 with intermediate influence, and H2 with several orders of magnitude increase in solidification time and atmosphere vertical stratification. In addition to these time-resolved results, I will present a novel formulation and application of a multi-species moist-adiabat for condensable-rich magma ocean and archean earth analog atmospheres, and outline how the cooling of such atmospheres can lead to exotic climate states that provide testable predictions for terrestrial exoplanets.

Finding and characterizing extrasolar Earth analogs will rely on interpretation of the planetary system’s environmental context. The total budget and fractionation between C–H–O species sensitively affect the climatic and geodynamic state of terrestrial worlds, but their main delivery channels are poorly constrained. We connect numerical models of volatile chemistry and pebble coagulation in the circumstellar disk with the internal compositional evolution of planetesimals during the primary accretion phase. Our simulations demonstrate that disk chemistry and degassing from planetesimals operate on comparable timescales and can fractionate the relative abundances of major water and carbon carriers by orders of magnitude. As a result, individual planetary systems with significant planetesimal processing display increased correlation in the volatile budget of planetary building blocks relative to no internal heating. Planetesimal processing in a subset of systems increases the variance of volatile contents across planetary systems. Our simulations thus suggest that exoplanetary atmospheric compositions may provide constraints on when a specific planet formed.

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.The thermal phase curve for super-Earth LHS 3844b suggests a solid surface and lack of a substantial atmosphere. The dayside temperature is around 1040 K and the nightside temperature is around 0 K, which is unlike any temperature contrast observed at present day for planets in the Solar System. On the other hand, the thermal phase curve of super-Earth 55 Cnc e suggests much hotter temperatures with a nightside temperature around 1380 K and a substellar point temperature around 2700 K. The substellar point is also substantially shifted eastwards, which requires efficient energy circulation in the atmosphere.Here, we use constraints from thermal phase curve observations to model the interior mantle flow. To constrain the surface temperature of 55 Cnc e, we use the results from general circulation models varying the atmospheric composition and optical depth.Depending on how strong the surface temperature contrast is, this can lead to hemispheric tectonic regimes. Such a regime could influence a planet's atmosphere through interior-exterior coupling mechanisms (e.g. volcanic outgassing). We run 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 modelled with an upper mantle, a perovskite-layer and a post-perovskite layer. The lithospheric strength is modelled through a plastic yielding criteria and the heating mode is either basal heating only or mixed heating (basal and internal heating). For LHS 3844b we find that the surface temperature dichotomy can lead to a hemispheric tectonic regime depending on the strength of the lithosphere and the heating mode in the mantle. In a hemispheric tectonic regime, downwellings occur preferentially on one side and upwellings rise on the other side (Fig. A). We compare these results to the case of 55 Cnc e, where large parts of the surface could be molten. At first order we expect that a magma ocean could homogenise the temperatures at the interface between the magma ocean and the underlying solid mantle and therefore reduce the likelihood of hemispheric tectonics operating on 55 Cnc e (Fig. B). For LHS 3844b, the contribution of the interior flux to the thermal phase curve is on the order of 15-30 K, and therefore below the detecting capabilities of current and near-future observations. However, for hemispheric tectonics, upwellings might lead to preferential melt generation and outgassing on one hemisphere that could manifest as a secondary signal in phase curve observations. Such signals could also be produced on hotter planets such as 55 Cnc e where parts of the surface are hot enough to melt.

The tectonic regime of rocky planets fundamentally influences their long-term evolution and cycling of volatiles between interior and atmosphere. Earth is the only known planet with active plate tectonics, but observations of exoplanets may deliver insights into the diversity of tectonic regimes beyond the solar system. Observations of the thermal phase curve of super-Earth LHS 3844b reveal a solid surface and lack of a substantial atmosphere, with a temperature contrast between the substellar and antistellar point of around 1000 K. Here, we use these constraints on the planet's surface to constrain the interior dynamics and tectonic regimes of LHS 3844b using numerical models of interior flow. We investigate the style of interior convection by assessing how upwellings and downwellings are organized and how tectonic regimes manifest. We discover three viable convective regimes with a mobile surface: (1) spatially uniform distribution of upwellings and downwellings, (2) prominent downwelling on the dayside and upwellings on the nightside, and (3) prominent downwelling on the nightside and upwellings on the dayside. Hemispheric tectonics is observed for regimes (2) and (3) as a direct consequence of the day-to-night temperature contrast. Such a tectonic mode is absent in the present-day solar system and has never been inferred from astrophysical observations of exoplanets. Our models offer distinct predictions for volcanism and outgassing linked to the tectonic regime, which may explain secondary features in phase curves and allow future observations to constrain the diversity of super-Earth interiors., Comment: Accepted for publication in The Astrophysical Journal Letters; 9 pages, 5 figures; summary available at http://exoplanet-talks.org/talk/263

Geochemical and astronomical evidence demonstrate that planet formation occurred in two spatially and temporally separated reservoirs. The origin of this dichotomy is unknown. We use numerical models to investigate how the evolution of the solar protoplanetary disk influenced the timing of protoplanet formation and their internal evolution. Migration of the water snow line can generate two distinct bursts of planetesimal formation that sample different source regions. These reservoirs evolve in divergent geophysical modes and develop distinct volatile contents, consistent with constraints from accretion chronology, thermo-chemistry, and the mass divergence of inner and outer Solar System. Our simulations suggest that the compositional fractionation and isotopic dichotomy of the Solar System was initiated by the interplay between disk dynamics, heterogeneous accretion, and internal evolution of forming protoplanets., Comment: Published 21 January 2021; authors' version; 30 pages, 18 figures; summary available at http://bit.ly/BifurcationBlog (blog) and https://bit.ly/BifurcationVideo (video)

Presentation at Cambridge Earth Sciences journal club, 5 February 2021

The earliest atmospheres of rocky planets originate from extensive volatile release during magma ocean epochs that occur during assembly of the planet. These establish the initial distribution of the major volatile elements between different chemical reservoirs that subsequently evolve via geological cycles. Current theoretical techniques are limited in exploring the anticipated range of compositional and thermal scenarios of early planetary evolution, even though these are of prime importance to aid astronomical inferences on the environmental context and geological history of extrasolar planets. Here, we present a coupled numerical framework that links an evolutionary, vertically-resolved model of the planetary silicate mantle with a radiative-convective model of the atmosphere. Using this method we investigate the early evolution of idealized Earth-sized rocky planets with end-member, clear-sky atmospheres dominated by either H$_2$, H$_2$O, CO$_2$, CH$_4$, CO, O$_2$, or N$_2$. We find central metrics of early planetary evolution, such as energy gradient, sequence of mantle solidification, surface pressure, or vertical stratification of the atmosphere, to be intimately controlled by the dominant volatile and outgassing history of the planet. Thermal sequences fall into three general classes with increasing cooling timescale: CO, N$_2$, and O$_2$ with minimal effect, H$_2$O, CO$_2$, and CH$_4$ with intermediate influence, and H$_2$ with several orders of magnitude increase in solidification time and atmosphere vertical stratification. Our numerical experiments exemplify the capabilities of the presented modeling framework and link the interior and atmospheric evolution of rocky exoplanets with multi-wavelength astronomical observations.

Presentation at Origins 2021 conference, Netherlands, January 2021

We investigated the hydrogen isotopic compositions and water contents of pyroxenes in two recent ordinary chondrite falls, namely, Chelyabinsk (2013 fall) and Benenitra (2018 fall), and compared them to three ordinary chondrite Antarctic finds, namely Graves Nunataks GRA 06179, Larkman Nunatak LAR 12241, and Dominion Range DOM 10035. The pyroxene minerals in Benenitra and Chelyabinsk are hydrated ($\sim$0.018-0.087 wt.$\%$ H$_2$O) and show D-poor isotopic signatures ($\delta$D$_{SMOW}$ from -444$\unicode{x2030}$ to -49$\unicode{x2030}$). On the contrary, the ordinary chondrite finds exhibit evidence of terrestrial contamination with elevated water contents ($\sim$0.039-0.174 wt.$\%$) and values (from -199$\unicode{x2030}$ to -14$\unicode{x2030}$). We evaluated several small parent body processes that are likely to alter the measured compositions in Benenitra and Chelyabinsk, and inferred that water-loss in S-type planetesimals is minimal during thermal metamorphism. Benenitra and Chelyabinsk hydrogen compositions reflect a mixed component of D-poor nebular hydrogen and water from the D-rich mesostases. 45-95$\%$ of water in the minerals characterized by low $\delta$D$_{SMOW}$ values was contributed by nebular hydrogen. S-type asteroids dominantly composed of nominally anhydrous minerals can hold 254-518 ppm of water. Addition of a nebular water component to nominally dry inner Solar System bodies during accretion suggests a reduced need of volatile delivery to the terrestrial planets during late accretion., Comment: 18 pages, 7 figures, accepted for publication in The Planetary Science Journal

Seminar/colloquium talk in February/March 2022 at: University of Rochester, Earth and Environmental Sciences University of Washington in St. Louis, Department of Physics European Astrobiology Institute seminar series

Recent astronomical (1,2) and geochemical (3,4) evidence point to early spatial and temporal fragmentation of the planet formation process, whose physical origins remain disputed. Here, using a coupled numerical model, we investigate the influence of the build-up of the solar protoplanetary disk (5) on the timing and internal evolution (6) of forming protoplanets. We find that the orbital drift of the water iceline can generate two temporally and spatially distinct bursts of planetesimal formation, which sample different source regions of interstellar materials and experience limited intermixture. Driven by internal radiogenic heating, these planetary reservoirs compositionally evolve in two modes and recover accretion chronology, thermo-chemical pattern, and mass divergence of inner and outer Solar System. Our numerical experiments suggest that the earliest interplay between disk physics and geophysical evolution of accreting planetesimals initiated the present-day observed chemical and isotopic dichotomy of the Solar System planets. Figure caption: Schematic illustration of early Solar System accretion chronology suggested by our numerical experiments. Isotope variability (left) across the disk due to varying composition of infall material is retained by the traffic-jam of inward-drifting dust grains at the water iceline. The formation of two distinct planetesimal populations initiates divergent evolutionary pathways of inner and outer Solar System (right) due to the combined effects of internal radiogenic heating and secular variation of the dominant mode of planetary growth. References Najita, J. R., & Kenyon, S. J. (2014). Mon. Not. R. Astron. Soc. 445, 3315-3329. Tychoniec, Ł., Manara, C. F., Rosotti, G. P., et al. (2020). Astron. Astrophys., in press., arXiv:2006.02812. Trinquier, A., Birck, J. L., Allegre, C. J. (2007). Astrophys. J. 655, 1179. Leya, I., Schönbächler, M., Wiechert, U., et al. (2008). Earth Planet. Sci. Lett. 266, 233-244. Dra̧żkowska, J., & Dullemond, C. P. (2018). Astron. Astrophysics 614, A62. Lichtenberg, T., Golabek, G. J., Burn, R., et al. (2019). Nat. Astron. 3, 307-313.

The earliest atmospheres of rocky planets originate from extensive volatile release during one or more magma ocean epochs that occur during primary and late-stage assembly of the planet (1). These epochs represent the most extreme cycling of volatiles between the interior and atmosphere in the history of a planet, and establish the initial distribution of the major volatile elements (C, H, N, O, S) between different chemical reservoirs that subsequently evolve via geological cycles. Crucially, the erosion or recycling of primary atmospheres bear upon the nature of the long-lived secondary atmospheres that will be probed with current and future observing facilities (2). Furthermore, the chemical speciation of the atmosphere arising from magma ocean processes can potentially be probed with present-day observations of tidally-locked rocky super-Earths (3). The speciation in turn strongly influences the climatic history of rocky planets, for instance the occurrence rate of planets that are locked in long-term runaway greenhouse states (4). We will present an integrated framework to model the build-up of the earliest atmospheres from magma ocean outgassing using a coupled model of mantle dynamics and atmospheric evolution. We consider the diversity of atmospheres that can arise for a range of initial planetary bulk compositions, and show how even small variations in volatile abundances can result in dramatically different atmospheric compositions and affect earliest mantle geochemistry and atmospheric speciation relevant for surficial prebiotic chemical environments (5). Only through the lense of coupled evolutionary models of terrestrial interiors and atmospheres can we begin to deconvolve the imprint of formation from that of evolution, with consequences for how we interpret the diversity revealed by astrophysical observables, and their relation to the earliest planetary conditions of our home world. References Bower, D. J., Kitzmann, D., Wolf, A. S., et al. (2019). Astron. Astrophys. 631, A103. Bonati, I., Lichtenberg, T., Bower, D. J., et al. (2019). Astron. Astrophys. 621, A125. Kreidberg, L., Koll, D. D., Morley, C., et al. (2019). Nature 573, 87-90. Hamano, K., Abe, Y., Genda, H. (2013). Nature 497, 607-610. Sasselov, D. D., Grotzinger, J. P., Sutherland, J. D. (2020). Sci. Adv. 6, eaax3419.

The earliest atmospheres of rocky planets originate from extensive volatile release during magma ocean epochs that occur during assembly of the planet. These establish the initial distribution of the major volatile elements between different chemical reservoirs that subsequently evolve via geological cycles. Current theoretical techniques are limited in exploring the anticipated range of compositional and thermal scenarios of early planetary evolution, even though these are of prime importance to aid astronomical inferences on the environmental context and geological history of extrasolar planets. Here, we present a coupled numerical framework that links an evolutionary, vertically resolved model of the planetary silicate mantle with a radiative‐convective model of the atmosphere. Using this method, we investigate the early evolution of idealized Earth‐sized rocky planets with end‐member, clear‐sky atmospheres dominated by either H2, H2O, CO2, CH4, CO, O2, or N2. We find central metrics of early planetary evolution, such as energy gradient, sequence of mantle solidification, surface pressure, or vertical stratification of the atmosphere, to be intimately controlled by the dominant volatile and outgassing history of the planet. Thermal sequences fall into three general classes with increasing cooling timescale: CO, N2, and O2 with minimal effect, H2O, CO2, and CH4 with intermediate influence, and H2 with several orders of magnitude increase in solidification time and atmosphere vertical stratification. Our numerical experiments exemplify the capabilities of the presented modeling framework and link the interior and atmospheric evolution of rocky exoplanets with multiwavelength astronomical observations., Key Points Magma oceans with different volatiles display varying timescales of solidification, outgassing, and atmospheric stratificationAtmospheric blanketing and cooling time increase in three principle classes from N2, CO, O2 to H2O, CO2, CH4 to H2 Coupled mantle–atmosphere systems display distinctive features that link the interior and surface state to atmospheric spectrum and climate

The vigour and style of mantle convection in tidally-locked super-Earths may be substantially different from Earth's regime. Earth's surface temperature is spatially uniform at 300 K, which is sufficiently cold to drive strong downwellings into the interior (i.e. subduction). In contrast, a tidally-locked super-Earth can have a large temperature contrast between the dayside and nightside, which we infer could lead to a dichotomy of the interior dynamics. We therefore use constraints from astrophysical observations to infer the possible pattern of flow in the interior of a tidally-locked super-Earth, using super-Earth LHS 3844b as a case study. We run mantle convection models using the code StagYY with two-dimensional spherical annulus geometry and parameters from the literature that are appropriate for LHS 3844b. The majority of the mantle is either perovskite or post-perovskite with the phase transition occurring around 1700 km depth (the total mantle depth is 3757 km). An upper and lower bound for the viscosity of post-perovskite is provided by previous theoretical calculations. We include plastic yielding to model the brittle nature of the lithosphere; plastic yielding occurs when the local stress state exceeds a prescribed yielding criteria and is commonly applied in studies of Earth to produce surface behaviour similar to plate tectonics.For a low yield stress criteria (promoting a weak lithosphere), we find that plumes are generally evenly distributed between the dayside and nightside, albeit strong downwellings form on the nightside. Plumes on the nightside have less lateral mobility than on the dayside because they are confined by downwellings either side. In contrast, for a high yield stress criteria, the interior dynamics are mostly driven by a prominent downwelling on the dayside which flushes hot material from the lower thermal boundary layer around the CMB towards the nightside where plumes preferentially arise. This, in turn, leads to a return flow of colder material from the near surface of the nightside towards the dayside. This seemingly counterintuitive pattern of flow is a consequence of weak lithosphere (due to temperature) on the dayside that is able to deform and thereby subduct, whereas lithosphere on the nightside is too stiff to subduct.Our models therefore show that the vigour of convection and the distribution of upwellings and downwellings of tidally locked super-Earths are sensitive to the strength of the lithosphere: plumes can either be equally distributed around the planet or preferentially occur on the nightside. In the first case, the cold downwellings are also equally distributed but more prominent on the nightside, whereas in the second case they are preferentially on the dayside. Somewhat unexpected, we do not observe a preference for hot plumes to congregate on the dayside. Our results have implications for space missions such as TESS, CHEOPS, JWST, PLATO and ARIEL that will discover and characterise super-Earths, thereby potentially probing for signals of volatile outgassing and volcanism.

The earliest secondary atmosphere of a rocky planet originates from extensive volatile release during one or more magma ocean epochs that occur during and after the assembly of the planet. Magma oceans set the stage for the long-term evolution of terrestrial planets by establishing the major chemical reservoirs of the iron core and silicate mantle, chemical stratification within the mantle, and outgassed atmosphere. Furthermore, current and future exoplanet observations will favour the detection and characterisation of hot and warm planets, potentially with large outgassed atmospheres. In this study, we highlight the potential to combine models of coupled interior–atmosphere evolution with static structure calculations and modelled atmospheric spectra (transmission and emission). By combining these components in a common modelling framework, we acknowledge planets as dynamic entities and leverage their evolution to bridge planet formation, interior-atmosphere interaction, and observations. An interior–atmosphere model is combined with static structure calculations to track the evolving radius of a hot rocky mantle that is outgassing volatiles. We consider oxidised species CO2 and H2O and generate synthetic emission and transmission spectra for CO2 and H2O dominated atmospheres. Atmospheres dominated by CO2 suppress the outgassing of H2O to a greater extent than previously realised, since previous studies have applied an erroneous relationship between volatile mass and partial pressure. Furthermore, formation of a lid at the surface can tie the outgassing of H2O to the efficiency of heat transport through the lid, rather than the radiative timescale of the atmosphere. We extend this work to explore the speciation of a primary atmosphere that is constrained using meteoritic materials as proxies for the planetary building blocks, and find that a range of reducing and oxidising atmospheres are possible. Our results demonstrate that a hot molten planet can have a radius several percent larger (about 5%, assuming Earth-like core size) than its equivalent solid counterpart, which may explain the larger radii of some close-in exoplanets. Outgassing of a low molar mass species (such as H2O, compared to CO2) can combat the continual contraction of a planetary mantle and even marginally increase the planetary radius. We further use our models to generate synthetic transmission and emission data to aid in the detection and characterisation of rocky planets via transits and secondary eclipses. Atmospheres of terrestrial planets around M-stars that are dominated by CO2 versus H2O could be distinguished by future observing facilities that have extended wavelength coverage (e.g., JWST). Incomplete magma ocean crystallisation, as may be the case for close-in terrestrial planets, or full or part retention of an early outgassed atmosphere, should be considered in the interpretation of observational data from current and future observing facilities.

Planets smaller than Neptune and larger than Earth make up the majority of the discovered exoplanets. Those with H$_2$-rich atmospheres are prime targets for atmospheric characterization. The transition between the two main classes, super-Earths and sub-Neptunes, is not clearly understood as the rocky surface is likely not accessible to observations. Tracking several trace gases (specifically the loss of ammonia (NH$_3$) and hydrogen cyanide (HCN)) has been proposed as a proxy for the presence of a shallow surface. In this work, we revisit the proposed mechanism of nitrogen conversion in detail and find its timescale on the order of a million years. NH$_3$ exhibits dual paths converting to N$_2$ or HCN, depending on the UV radiation of the star and the stage of the system. In addition, methanol (CH$_3$OH) is identified as a robust and complementary proxy for a shallow surface. We follow the fiducial example of K2-18b with a 2D photochemical model (VULCAN) on an equatorial plane. We find a fairly uniform composition distribution below 0.1 mbar controlled by the dayside, as a result of slow chemical evolution. NH$_3$ and CH$_3$OH are concluded to be the most unambiguous proxies to infer surfaces on sub-Neptunes in the era of the James Webb Space Telescope (JWST)., Comment: Accepted for publication in ApJL

Presentation slides at Imperial College London, 11 November 2019

The presence of an unseen ‘Planet 9’ on the outskirts of the Solar system has been invoked to explain the unexpected clustering of the orbits of several Edgeworth–Kuiper Belt Objects. We use N-body simulations to investigate the probability that Planet 9 was a free-floating planet (FFLOP) that was captured by the Sun in its birth star formation environment. We find that only 1–6 per cent of FFLOPs are ensnared by stars, even with the most optimal initial conditions for capture in star-forming regions (one FFLOP per star, and highly correlated stellar velocities to facilitate capture). Depending on the initial conditions of the star-forming regions, only 5–10 of 10 000 planets are captured on to orbits that lie within the constraints for Planet 9. When we apply an additional environmental constraint for Solar system formation – namely the injection of short-lived radioisotopes into the Sun’s protoplanetary disc from supernovae – we find the probability for the capture of Planet 9 to be almost zero., Monthly Notices of the Royal Astronomical Society. Letters, 472 (1), ISSN:1745-3933, ISSN:1745-3925

Chondrules are the dominant bulk silicate constituent of chondritic meteorites and originate from highly energetic, local processes during the first million years after the birth of the Sun. So far, an astrophysically consistent chondrule formation scenario, explaining major chemical, isotopic and textural features, remains elusive. Here, we examine the prospect of forming chondrules from planetesimal collisions. We show that intensely melted bodies with interior magma oceans became rapidly chemically equilibrated and physically differentiated. Therefore, collisional interactions among such bodies would have resulted in chondrule-like but basaltic spherules, which are not observed in the meteoritic record. This inconsistency with the expected dynamical interactions hints at an incomplete understanding of the planetary growth regime during the protoplanetary disk phase. To resolve this conundrum, we examine how the observed chemical and isotopic features of chondrules constrain the dynamical environment of accreting chondrite parent bodies by interpreting the meteoritic record as an impact-generated proxy of planetesimals that underwent repeated collision and reaccretion cycles. Using a coupled evolution-collision model we demonstrate that the vast majority of collisional debris feeding the asteroid main belt must be derived from planetesimals which were partially molten at maximum. Therefore, the precursors of chondrite parent bodies either formed primarily small, from sub-canonical aluminum-26 reservoirs, or collisional destruction mechanisms were efficient enough to shatter planetesimals before they reached the magma ocean phase. Finally, we outline the window in parameter space for which chondrule formation from planetesimal collisions can be reconciled with the meteoritic record and how our results can be used to further constrain early solar system dynamics., 20 pages, 11 figures, 2 tables; accepted for publication in Icarus; associated blog article at goo.gl/5bDqGC

Talk at EPSC 2019 in Geneva, session TP14.

Doctoral Thesis Award 2019 prize talk at the annual meeting of the German Astronomical Society, Stuttgart.

Poster presentation @ Gordon Research Conference – Origins of Solar Systems 2019

Talk at UKEXOM 2019 @ ICL, 15 April 2019

Heating by short-lived radioisotopes (SLRs) such as aluminum-26 and iron-60 fundamentally shaped the thermal history and interior structure of Solar System planetesimals during the early stages of planetary formation. The subsequent thermo-mechanical evolution, such as internal differentiation or rapid volatile degassing, yields important implications for the final structure, composition and evolution of terrestrial planets. SLR-driven heating in the Solar System is sensitive to the absolute abundance and homogeneity of SLRs within the protoplanetary disk present during the condensation of the first solids. In order to explain the diverse compositions found for extrasolar planets, it is important to understand the distribution of SLRs in active planet formation regions (star clusters) during their first few Myr of evolution. By constraining the range of possible effects, we show how the imprint of SLRs can be extrapolated to exoplanetary systems and derive statistical predictions for the distribution of aluminum-26 and iron-60 based on N-body simulations of typical to large clusters (1000-10000 stars) with a range of initial conditions. We quantify the pollution of protoplanetary disks by supernova ejecta and show that the likelihood of enrichment levels similar to or higher than the Solar System can vary considerably, depending on the cluster morphology. Furthermore, many enriched systems show an excess in radiogenic heating compared to Solar System levels, which implies that the formation and evolution of planetesimals could vary significantly depending on the birth environment of their host stars., Comment: 15 pages, 8 figures, 4 tables; accepted for publication in MNRAS; associated video files can be found at http://timlichtenberg.net/2016_enrichment.html

Presentation slides EANA 24 Sep 2018 invited talk

In contrast to the water-poor inner solar system planets, stochasticity during planetary formation and order of magnitude deviations in exoplanet volatile contents suggest that rocky worlds engulfed in thick volatile ice layers are the dominant family of terrestrial analogues among the extrasolar planet population. However, the distribution of compositionally Earth-like planets remains insufficiently constrained, and it is not clear whether the solar system is a statistical outlier or can be explained by more general planetary formation processes. Here we employ numerical models of planet formation, evolution, and interior structure, to show that a planet's bulk water fraction and radius are anti-correlated with initial $^{26}$Al levels in the planetesimal-based accretion framework. The heat generated by this short-lived radionuclide rapidly dehydrates planetesimals prior to accretion onto larger protoplanets and yields a system-wide correlation of planet bulk abundances, which, for instance, can explain the lack of a clear orbital trend in the water budgets of the TRAPPIST-1 planets. Qualitatively, our models suggest two main scenarios of planetary systems' formation: high-$^{26}$Al systems, like our solar system, form small, water-depleted planets, whereas those devoid of $^{26}$Al predominantly form ocean worlds, where the mean planet radii between both scenarios deviate by up to about 10%., Comment: Preprint version; free-to-read journal version at https://rdcu.be/bmdlw; blog article at https://t.co/p6SValG1ip

Rocky planetesimals in the early solar system melted internally and evolved chemically due to radiogenic heating from Al-26. Here we quantify the parametric controls on magma genesis and transport using a coupled petrological and fluid mechanical model of reactive two-phase flow. We find the mean grain size of silicate minerals to be a key control on magma ascent. For grain sizes larger than $\approx$ 1 mm, melt segregation produces distinct radial structure and chemical stratification. This stratification is most pronounced for bodies formed at around 1 Myr after formation of Ca,Al-rich inclusions. These findings suggest a link between the time and orbital location of planetesimal formation and their subsequent structural and chemical evolution. According to our models, the evolution of partially molten planetesimal interiors falls into two categories. In the magma ocean scenario, the whole interior of a planetesimal experiences nearly complete melting, which would result in turbulent convection and core-mantle differentiation by the rainfall mechanism. In the magma sill scenario, segregating melts gradually deplete the deep interior of the radiogenic heat source. In this case, magma may form melt-rich layers beneath a cool and stable lid, while core formation would proceed by percolation. Our findings suggest that grain sizes prevalent during the internal heating stage governed magma ascent in planetesimals. Regardless of whether evolution progresses toward a magma ocean or magma sill structure, our models predict that temperature inversions due to rapid Al-26 redistribution are limited to bodies formed earlier than $\approx$ 1 Myr after CAIs. We find that if grain size was smaller than $\approx$ 1 mm during peak internal melting, only elevated solid-melt density contrasts (such as found for enstatite chondrite compositions) would allow substantial melt segregation to occur., 15 pages, 6 figures, 1 table; associated video file at http://goo.gl/V1UK1i; associated blog post at http://bit.ly/2GZZ0e9

During their formation and early evolution, rocky planets undergo multiple global melting events due to accretionary collisions with other protoplanets. The detection and characterization of their post-collision afterglows (magma oceans) can yield important clues about the origin and evolution of the solar and extrasolar planet population. Here, we quantitatively assess the observational prospects to detect the radiative signature of forming planets covered by such collision-induced magma oceans in nearby young stellar associations with future direct imaging facilities. We have compared performance estimates for near- and mid-infrared instruments to be installed at ESO's Extremely Large Telescope (ELT), and a potential space-based mission called Large Interferometer for Exoplanets (LIFE). We modelled the frequency and timing of energetic collisions using \textit{N}-body models of planet formation for different stellar types, and determine the cooling of the resulting magma oceans with an insulating atmosphere. We find that the probability of detecting at least one magma ocean planet depends on the observing duration and the distribution of atmospheric properties among rocky protoplanets. However, the prospects for detection significantly increase for young and close stellar targets, which show the highest frequencies of giant impacts. For intensive reconnaissance with a K band (2.2 $\mu m$) ELT filter or a 5.6 $\mu m$ LIFE filter, the $\beta$ Pictoris, Columba, TW Hydrae, and Tucana-Horologium associations represent promising candidates for detecting a molten protoplanet. Our results motivate the exploration of magma ocean planets using the ELT and underline the importance of space-based direct imaging facilities to investigate and characterize planet formation and evolution in the solar vicinity., Comment: 12 pages, 7 figures, 6 tables; accepted for publication in Astronomy & Astrophysics; associated blog article at https://medium.com/@irene.bonati/spotting-protoplanet-collision-afterglows-in-the-stellar-neighbourhood-f3b65572ec2

The short-lived $^{182}$Hf-$^{182}$W decay system is a powerful chronometer for constraining the timing of metal-silicate separation and core formation in planetesimals and planets. Neutron capture effects on W isotopes, however, significantly hamper the application of this tool. In order to correct for neutron capture effects, Pt isotopes have emerged as a reliable in-situ neutron dosimeter. This study applies this method to IAB iron meteorites, in order to constrain the timing of metal segregation on the IAB parent body. The $\epsilon^{182}$W values obtained for the IAB iron meteorites range from -3.61 $\pm$ 0.10 to -2.73 $\pm$ 0.09. Correlating $\epsilon^{\mathrm{i}}$Pt with $^{182}$W data yields a pre-neutron capture $^{182}$W of -2.90 $\pm$ 0.06. This corresponds to a metal-silicate separation age of 6.0 $\pm$ 0.8 Ma after CAI for the IAB parent body, and is interpreted to represent a body-wide melting event. Later, between 10 and 14 Ma after CAI, an impact led to a catastrophic break-up and subsequent reassembly of the parent body. Thermal models of the interior evolution that are consistent with these estimates suggest that the IAB parent body underwent metal-silicate separation as a result of internal heating by short-lived radionuclides and accreted at around 1.4 $\pm$ 0.1 Ma after CAIs with a radius of greater than 60 km., Comment: 11 pages, 8 figures, 2 tables; open access article under the CC BY-NC-ND license (see http://creativecommons.org/licenses/by-nc-nd/4.0/)

The thermal history and internal structure of chondritic planetesimals, assembled before the giant impact phase of chaotic growth, potentially yield important implications for the final composition and evolution of terrestrial planets. These parameters critically depend on the internal balance of heating versus cooling, which is mostly determined by the presence of short-lived radionuclides (SLRs), such as aluminum-26 and iron-60, as well as the heat conductivity of the material. The heating by SLRs depends on their initial abundances, the formation time of the planetesimal and its size. It has been argued that the cooling history is determined by the porosity of the granular material, which undergoes dramatic changes via compaction processes and tends to decrease with time. In this study we assess the influence of these parameters on the thermo-mechanical evolution of young planetesimals with both 2D and 3D simulations. Using the code family I2ELVIS/I3ELVIS we have run numerous 2D and 3D numerical finite-difference fluid dynamic models with varying planetesimal radius, formation time and initial porosity. Our results indicate that powdery materials lowered the threshold for melting and convection in planetesimals, depending on the amount of SLRs present. A subset of planetesimals retained a powdery surface layer which lowered the thermal conductivity and hindered cooling. The effect of initial porosity was small, however, compared to those of planetesimal size and formation time, which dominated the thermo-mechanical evolution and were the primary factors for the onset of melting and differentiation. We comment on the implications of this work concerning the structure and evolution of these planetesimals, as well as their behavior as possible building blocks of terrestrial planets., 19 pages, 11 figures, 5 tables; accepted for publication in Icarus; for associated video files, see http://timlichtenberg.net/2015_porosity.html or http://dx.doi.org/10.1016/j.icarus.2016.03.004