Stephen J. Mojzsis

Estimates for when life emerged on Earth range from ca. 3.8 Ga (Eoarchean) to the Hadean eon (≥4.0-4.5 Ga). Here, we conclude that a proposed early stage of life – the RNA World – appeared at ca. 4.33 Ga by tracking the near-surface thermal stability of key biomolecules comprising postulated RNA organisms. We apply a global three-dimensional numerical model to analyse the thermal evolution profile of Hadean crust in response to late accretion bombardment. Results show that frequent global sterilization events persisted up until about 4.4 Ga owing to hot ejecta deposition, rock vapour condensation, and enhanced geothermal gradients. Later, conditions became favorable to persistent biochemistry by supporting interconnected zones of biomolecule stability associated with extensive energy-rich hydrothermal oases. We identify an optimal time for the RNA World that dovetails with the emergence of the Last Universal Common Ancestor (LUCA) population.

Previous studies report apparent chemical abundance discrepancies between M dwarfs and FGK stars in the solar neighborhood, raising questions about the origin of these differences. Using a statistical pipeline, we revisit whether M dwarf abundance ratios are consistent with non–local thermodynamic equilibrium (non-LTE) measurements of FGK stars, and independently validate these results with predictions from state-of-the-art Galactic chemical evolution (GCE) models of the Milky Way’s thin disk. Abundance trends for [C, O, Na, Mg, Al, Si, Ca, Ti, Ni/H] versus [Fe/H] are derived for a benchmark sample of 47 M dwarfs from the APOGEE, CARMENES, and Subaru/InfraRed Doppler (IRD) surveys. These are compared to non-LTE measurements of FGK stars and to GCE predictions calculated with the open-source code OMEGA+ . Our predictions take into account the intrinsic metallicity scatter of the solar neighbourhood and different stellar yield sets. We find that the M dwarf trends align with FGK stars within uncertainties for most elements (C, O, Na, Al, Si, Ni), resolving prior discrepancies. The observational trends also agree well with GCE predictions. However, the Ti trends are not consistent between spectroscopic samples, likely due to systematic differences in abundance determination techniques. We also show that M dwarf abundances differ significantly between different surveys in the literature. The abundance ratios of M dwarfs and FGK stars are mostly chemically consistent in the solar neighborhood and can be effectively modeled. GCE slopes provide an independent benchmark of these compositional trends.

Experiencing an international childhood, and an innate curiosity of learning the origin of himself and his surroundings, Steve Mojzsis set out to try his best to make sense of the universe he was born into. This quest will never be fulfilled, but it doesn’t prevent him from trying. The ultimate prize is to figure out how the biosphere came into being. The path towards learning began with adventures mostly in North America and Central and Western Europe. To find himself still further meant going, to Africa, Asia and Australia, with expert guidance from the best mentors along the way. Ever the bridge builder, he was not satisfied with staying within one narrow discipline of science for very long before trying a hand at something else, albeit related. In this way he stumbled into biogeodynamics and geoastronomy; we will probably not see spectacular developments in either of these nascent fields until the end of 21st century or long after he is returned to some mineral salts, CO2 and H2O. Back on his home world, it is evident that in its first 500 Myr (q.v. Hadean eon) Earth’s crust co-existed with liquid water; it was affected by late accretion bombardments and harboured chemically diverse hydrothermal systems. Globally active volcanism powered by a hotter mantle, impact generated topography and inherent differential buoyancy of the crust means that scattered emergent land masses existed irrespective of different estimates for plausible Hadean ocean volumes. Consequently, land surfaces in the first few hundred million years – with diverse subaerial and (sub-)aqueous environments – could bring prebiotic chemical ingredients to reactive concentrations. This active chemistry led to the first life. Yet, the inferred complexity for even the minimum biological entity probably means that both operative and persistent biology in a planetary environment is the most difficult developmental stage to reach. Soon after Earth’s formation, its surface had the capacity to host fully fledged (i.e. living) biochemistry. While life emerged at the dynamic interface of the planet’s primordial geosphere, hydrosphere and atmosphere, nobody knows whether any Hadean environment was ideally suited, or merely good enough, to facilitate its emergence. Given that we do not have time machines to go back and see what happened on Earth, or anywhere else for that matter, we are compelled to tell stories that fit the data. That said, comfortable fictions about what “might” have happened, just because somebody said something (without evidence), ought to be taken with a grain of salt. If we ever are to come up with a plausible pathway to a life giving world, it will be more metaphorical than literal. All of this will likely change if we find an independent origin of life, either on purpose or, more likely in Mojzsis’ opinion, by accidental discovery of something strange in the data.

Quartz is a ubiquitous, durable mineral abundant in most lithologies and in sedimentary systems. Its stability during weathering and diagenesis makes it a robust provenance tracer. Quartz optically stimulated luminescence (OSL) sensitivity and electron spin resonance (ESR) signals have been empirically proposed as indicators for sediment provenance based on differences in quartz sensitivity. Sensitivity is defined as luminescence produced per unit dose (Gray: Gy) per unit mass (mg). The mechanisms driving OSL sensitisation during weathering from source rocks remain an open question in the luminescence community. While it is largely believed to be acquired by earth surface processes, recent studies bring evidence that sensitisation processes depend on source geology. Moreover, most studies focus on quartz grains deposited in the sink and infer possible sources rock by clustering.The present study investigated a diverse suite of source rocks a wide range of geological settings, including granites, sandstones, volcanic and metamorphic rocks, and, where applicable, their derived sediments, independently dated from millions (Ma) to billions (Ga) of years. To understand the mechanism of OSL sensitivity we combined thermoluminescence (TL), OSL, ESR, cathodoluminescence (CL) and Raman spectroscopy measurements, with geochemical data obtained through laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Only in those quartzes that exhibited the [TiO4-Li+]0 paramagnetic centre in ESR, had notably high concentration of titanium and lithium impurities and had high blue emission in SEM-CL showed sensitisation following repeated cycles of laboratory dosing and bleaching. Samples lacking these centres did not show any laboratory OSL sensitisation by dosing and bleaching. We find a strong correlation of the degree of laboratory OSL sensitisation with the [TiO4-Li+]0 electron centre from ESR signal, and the titanium and lithium concentrations measured by LA-ICPMS. Our data on rocks of various ages and regions reinforce the idea that the potential for OSL sensitisation originates from specific lattice defect structures acquired by quartz during crystallisation. The results also highlight the role of Li⁺-ion impurities in forming paramagnetic centres such as [AlO₄/h]0 and [TiO4-Li+]0, advancing our understanding on interaction of radiations with specific defects in quartz.This study is funded by the European Research Council Consolidator Grant - PROGRESS, (ERC-CoG-101043356) awarded to Prof. Alida Timar-Gabor.

Do the various continental crustal growth curves formulated from disparate geochemical models robustly inform us as to why the Hadean (pre-4 Ga) rock record is basically non-existent? Is its absence due to extrinsic effects (bombardment)? Or, could it be that little or no continental crust existed at first? On the other hand, was this record essentially lost over time by recycling processes? For instance, the biggest problem with searching for any information about the history of plate tectonics is that the process erases evidence of its own existence. The age of oceanic crust averages about 70 Ma and is not older than 200 Ma because plate tectonics keeps recycling it (except for some old ophiolites). Most of the crust by surface area is oceanic, whereas most crust by volume is continental. The mean age of continental crust (ca. 2 Ga) is 36× greater than that of oceanic crust because its buoyancy prevents it from subducting except for loss to subduction via erosion. The overall decline in preserved continental crust based simply on the detrital zircon record shows a roughly 1.4 Gyr e-folding time. The residence time of the lithosphere is the average length of time that it will remain as a geochemical entity; this is estimated to be about 500-750 Myr. The value is about half of the observed e-folding time for the pre-Phanerozoic (>542 Ma) continental crust, but is close to the average mixing timescale since the Archean of about 420-440 Myr for primitive mantle, recycled continental crust and mantle residue. Assuming the residence time of 750 Myr is a good estimate for the half-life of continental crust, then the e-folding time is in broad agreement with both the zircon record and model calculations of crustal reworking. The zircon record is strongly biased to continental crust, because zircon is most commonly found in granites and granitoids, which constitute the major rock fraction of the continents. The trends in the detrital zircon data can be interpreted to represent decreasing preservation rather than increasing production, of continental crust.

The stellar sources that contributed to the chemical and isotopic composition of the solar system, their mixing in the Galactic environment, and the evolution and longevity of the molecular cloud from which our solar system formed can be probed with isotopic measurements of presolar grains, meteorites, and comets. The isotopic compositions of cometary xenon (Xe) and krypton (Kr) measured in the coma of 67P/Churyumov–Gerasimenko (67P/C-G) are distinct from those of the Sun. As noble gases in cometary ices are thought to be derived from the same molecular cloud from which the Sun formed, such nonsolar isotopic compositions suggest that a compositionally distinct reservoir of noble gases was present in the presolar molecular cloud that is currently absent from 67P/C-G. This likely requires that a noble gas component was cotrapped in volatile ice which was later destroyed by processing within the protoplanetary disk. Here, we show that this hypothetical component was enriched in r -process and potentially p -process isotopes compared to the solar composition, and that these isotopes formed over a hundred million years before coalescence and cosmochemical isolation of the solar system. We discuss a scenario where this component was accreted by the presolar molecular cloud sometime after the formation of water ice containing cotrapped Xe and Kr with the composition of 67P/C-G. Such a component could have been derived from a diffuse, heterogeneous molecular portion of the interstellar medium accreted by the presolar molecular cloud, or incorporated from another molecular cloud via a cloud–cloud collision. These observations support two distinct r -processes contributing to the overall solar r -process abundance pattern.

The light odd- Z elements P, Cl, K, and Sc are underproduced in galactic chemical evolution models compared to spectroscopic observations of stars in the Milky Way. The most promising solution to this puzzle is that some massive stars experience O-C shell mergers, boosting their yields through dynamic, convective-reactive nucleosynthesis. We report how convective macro physics based on 3D 4 π hydrodynamic simulations impacts production in the O shell by postprocessing the M _ZAMS = 15 M _⊙ Z = 0.02 model from the NuGrid dataset. We explore a mixing downturn, boosted velocities, reduced ingestion rate, and convective quenching. Across 24 mixing cases, the pre-explosive yields for [P/Fe], [Cl/Fe], [K/Fe], and [Sc/Fe] are modified by [−0.33, 0.23], [−0.84, 0.64], [−0.78, 1.48], and [−0.36, 1.29] dex, respectively. Cases with a convective downturn with the fastest ingestion rate have the largest enhancement, and production is nonmonotonic with boosted velocities. Which reactions are most important for the convective-reactive element production pathways depends on the mixing case. We parameterize production of ⁴⁰ K ( t _1/2 = 1.248 Gyr), an important radiogenic heat source for younger (2–3 Gyr) rocky planets, and find a yield variation exceeding 3 orders of magnitude. This range of initial abundances for ⁴⁰ K implies the early geodynamic behavior of silicate mantles in rocky planets can differ greatly from that of Earth. These results underscore the importance of investigating the 3D macro physics of shell merger convection through hydrodynamic simulations to develop a predictive understanding of the origin and variability of the light odd- Z elements and the ⁴⁰ K/K ratio in planet host stars.

Aims . The inferred compositions of the Solar System’s terrestrial (rocky) bodies are fractionated from that of the Sun, where elemental depletions in the bulk rocky bodies correlate with element volatility, expressed in its 50% condensation temperature. However, because element volatility depends on disk gas composition, it is not mandated that elemental fractionation trends derived from the solar-terrestrial scenario apply to other planetary systems. Here, we expand upon previous efforts to quantify elemental volatility during disk condensation, and how this affects rocky planet compositional diversity. Methods . We simulated condensation sequences for a sample of 1000 initial disk compositions based on observed stellar abundances. Based on these simulations, we present parametrisations of how element 50% condensation temperatures depend on disk composition and apply element fractionation trends with appropriate element volatility to stellar abundances to simulate compositions of rocky exoplanets with the same volatile depletion pattern as the Earth, providing a robust and conservative lower limit to the compositional diversity of rocky exoplanets. Results . Here we show that Earth-like planets emerge from low-C/O disks (C/O ≤ 0.75) and graphite-bearing planets from medium-to-high-C/O disks (C/O > 0.75). Furthermore, we identify an intermediate-C/O (0.84–1.04) class of planets characterised by Mg and Si depletion, leading to relatively high abundances of Fe, Ca, and A1. We show that devolatilisation patterns could be adapted potentially with disk composition-dependent condensation temperatures to make predictions of rocky planet bulk compositions within individual systems, although such patterns could be further modified by the dynamics of planetary accretion, which remains under-constrained for most exoplanetary systems. The outcomes of our analysis suggest that accounting for disk composition-dependent condensation temperatures means that we can expect an even broader range of possible rocky planet compositions than has previously been considered.

In the Exoplanetary zoo revealed by astrophysical observations, the Super-Earth to Sub-Neptune planets constitute an intriguing array as they lie at the boundary between rocky and gaseous bodies. In order to build Sub-Neptune planets, gravitational capture of the nebular gas must occur; this forms a dense gaseous atmosphere which encapsulates the growing planet. This dense gas is expected to interact with the planetary interior causing some elemental redistributions, modifying planetary redox state, and potentially changing the internal structure. At some points, this dense nebular atmosphere can be lost, leaving behind a rocky world with an internal geochemistry that has been profoundly affected by its nebular history. Here, we simulate the geochemical consequences of an H2-rich atmosphere effectively accreting around a young and molten rocky planet. We compute a series of reactions involving three phases: the gaseous fluid reacts with a molten silicate at the pressure prevailing at the base of the atmosphere, and the molten silicate reacts with a molten metal alloys at the pressure of the core-magma ocean boundary.The nebular gas, mostly composed of molecular H2, has a great reducing capacity, causing internal redox redistribution. The magnitude of such redox redistribution depends on the availability of oxygen in the planetary interior, which is, itself, governed by reactions between the molten silicate and the molten core. In particular, molecular H2 can produce massive amounts of water if oxygen is available in the molten silicate. This occurs if the magma ocean is rich in iron oxides (FeO & Fe2O3) or if silicon is incorporated in the metallic core, a reaction which produces oxygen. In contrast, if oxygen is itself incorporated in to the metallic core, this diminishes the amount of molecular hydrogen that can be converted into water. Finally, a redox neutral process is also identified with the direct solubilization of H2 into the molten metallic core. The simultaneous resolution of these equilibria indicates that the system tends to self-regulate with diverse initial scenarios converging to a unique final configuration in which abundant water is produced.