PS10.1

The origins of life on Earth have been a longstanding scientific puzzle, prompting scientists from Orgel to Sagan to grapple with the fundamental question of “how did we get here?” While a complete theory of the origin of life on Earth - with experimental support and no unresolved issues - has yet to be elucidated, certain pieces of the puzzle have seen recent progress. We need to have a cohesive model of the origins of life on Earth to better inform which exoplanets should be observational targets for upcoming telescopes and what tools will be necessary in future missions to deduce the presence or absence of life on a potentially habitable world. Fortunately, we have unprecedented access to the one planet where we know circumstances led one way or another to life’s origins: the Earth. While astronomers find exoplanets and planetary scientists explore the possibility for habitability in our Solar System, chemistry can play an invaluable role in facilitating the search for life beyond Earth. If we better understand the chemical reactions and pathways possibly leading to the origins of life on Earth, we can better inform and constrain the search for life in other planetary environments. By working towards a continuous and plausible pathway towards delineating the origins of life on Earth, we can place constraints on the astronomical, planetary, and chemical environments necessary for habitability. 

How to cite: Todd, Z.: From Astronomy to Chemistry: Towards a Continuous Path for the Origins of Life, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6227, https://doi.org/10.5194/egusphere-egu22-6227, 2022.

Ocean chemistry plays a key role in the removal of CO2 from the atmosphere-ocean system in the form of carbonates that are eventually subducted to the mantle. Silicate weathering and CO2 dissolution dictate the steady-state ocean chemistry and thereby the carbonate-silicate cycle (inorganic carbon cycle). Data on stellar elemental abundances suggest a strong diversity in the bulk mineralogy of exoplanets. We study the role of weathering-derived divalent cations (Ca++, Mg++) on ocean pH and carbonate compensation depth (CCD) in exoplanet oceans. If CCD is too shallow, carbonates on the seafloor cannot be subducted to the mantle. We find that the presence of carbonates sets the upper bound on ocean pH and CO2 dissolution sets the lower bound on ocean pH. We show that CCD increases with increasing divalent cations supplied by weathering and decreases with CO2 dissolution. 

How to cite: Hakim, K., Tian, M., Bower, D. J., and Heng, K.: The Role of Carbonates in Regulating Atmospheric CO2 on Earth-like Exoplanets, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11373, https://doi.org/10.5194/egusphere-egu22-11373, 2022.

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 long stretches of apparently clement conditions in the Archean, a stable Proterozoic climate punctuated by major intervals of glaciation at the beginning and end, and fluctuation between warm and cool climates in the Phanerozoic without any further global glaciation events. Deterministic models of the carbonate-silicate cycle on Earth-like planets do not predict such a sequence of transitions, instead yielding either permanently clement conditions, or limit-cycle behavior only for planets receiving low stellar fluxes.

In this work, we take a stochastic approach to modeling atmospheric CO2 evolution. We present a simple model that assumes an imperfect CO2 thermostat, such that pCO2 follows a bounded random walk around a mean value that alone would maintain clement climate conditions. Because less CO2 is required to keep the planet warm as solar luminosity increases, the model predicts an increase in climate variability with time. This implies that unless some mechanism is present to decrease CO2 variance as stellar luminosity increases, the climates of Earth-like planets should become increasingly unstable as they approach the inner edge of their systems’ habitable zones. Implications for exoplanets are discussed, and the model is then applied to the specific problem of Earth’s climate history. In particular, the potential role of the biosphere in forcing and/or inhibiting Snowball transitions both in the Phanerozoic and earlier in Earth history is discussed.

How to cite: Wordsworth, R. and Knoll, A.: Stochastic modeling of CO2 fluctuations and Snowball transitions on Earth and other planets, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3075, https://doi.org/10.5194/egusphere-egu22-3075, 2022.

The origin of life on Earth involves the early appearance of an information-containing molecule such as RNA. Warm little ponds are ideal sites for the emergence of RNA, as their periodic wet-dry cycles provide conditions favorable for polymerization (e.g. Da Silva et al. 2015, Ross & Deamer 2016).

How did the building blocks of RNA come to be in warm little ponds on early Earth? Is it necessary that they were delivered by meteorites or interplanetary dust? Or was early Earth capable of producing them on its own? In the latter case, the process can begin with the production of HCN in the atmosphere, which reacts in aqueous solution to produce several key RNA precursors such as nucleobases, ribose, and 2-aminooxazole (e.g. Yi et al. 2020, Hill & Orgel 2002, Becker et al. 2018, Powner et al. 2009).

Here, we construct a robust physical and non-equilibrium chemical model of the early Earth atmosphere in which lightning and external UV-driven chemistry produce HCN. The atmosphere is supplied with hydrogen from impact degassing of meteorites, sourced with water evaporated from the oceans, carbon dioxide from volcanoes, and methane from undersea hydrothermal vents. This model allows us to calculate the rain-out of HCN into warm little ponds (WLPs). We then use a comprehensive sources and sinks numerical model to compute the resulting abundances of nucleobases, ribose, and nucleotide precursors such as 2-aminooxazole resulting from aqueous and UV-driven chemistry within them. We find that at 4.4 bya (billion years ago) peak adenine concentrations in ponds can be maintained at ∼2.8μM for more than 100 Myr. Meteorite delivery of adenine to WLPs produce similar peaks in concentration, but are destroyed within months by UV photodissociation, seepage, and hydrolysis. The early evolution of the atmosphere is dominated by the decrease of hydrogen due to falling impact rates and atmospheric escape, and the rise of oxygenated species such as OH from H₂O photolysis. Our work points to an early origin of RNA on Earth within ~200 Myr of the Moon-forming impact.

How to cite: Pearce, B. K. D., Molaverdikhani, K., Pudritz, R., Henning, T., and Cerrillo, K.: Towards RNA life on Early Earth: From atmospheric HCN to biomolecule production in warm little ponds, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1778, https://doi.org/10.5194/egusphere-egu22-1778, 2022.

The formation of life on Earth is generally understood to have required the presence of liquid water, as well as an atmosphere within which the feedstock molecules — such as HCN — for more complex biomolecules are able to form. From the precipitation of these simple molecules, RNA can be built. The thermal profile and surface pressure of early Earth that was necessary for a liquid water cycle may have been created by a large impact, or series of larger impacts, following the formation of our Moon. Models which feature the consequences of very large impacts (e.g. Zahnle 2020) have dense, hydrogen-rich atmospheres that can be conductive to both the formation of HCN and a temperate surface temperature under the faint young Sun. In this work, we developed detailed self-consistent thermochemical equilibrium PT structures for post-large-impact atmospheres. We use a 1D radiative-convective equilibrium modelling code to obtain these thermal profiles and equilibrium chemistry. We found that the 5 optically thick cases for a dry atmosphere have a self-consistent surface temperature that is 742K on average; however, without the collisional opacity from H₂ molecules contributing to the radiative transfer, this self-consistent surface temperature is an average of 394K. For a wet atmosphere, these values are 842K and 568K, respectively. Our current results suggest that, in the work of Zahnle et al. (2020), early post-impact HCN yields were computed for atmospheres that are initially too hot for the necessary liquid surface water and too hot for these molecules to be stable.

How to cite: Cerrillo, K. E., Pearce, B. K. D., Mollière, P., and Pudritz, R. E.: On The Habitability Of An Impacted Young Earth, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10187, https://doi.org/10.5194/egusphere-egu22-10187, 2022.

Conditions at the surface of terrestrial type worlds inhabitable by Earth-like life are hugely variable. Earth itself has explored much of this extensive parameter space over time, as evidenced via the rock record, which contains evidence of both multi-million year global glaciations as well as hot house conditions. The area of emergent land,  the partial pressure of atmospheric carbon dioxide, and the geochemistry of crustal rocks have all evolved, and imply that terrestrial type exoplanets may be extremely diverse. Unfortunately, all of these parameters remain uncertain for Earth during the Era of Prebiotic chemistry. Understanding how the availability of critical molecules for prebiotic chemistry vary as a function of planetary conditions is therefore crucial for constructing self-consistent scenarios for the origin of life. We focus on phosphate, modelling 1) mineral hosts in crustal rocks, 2) the weathering of those minerals as a function of atmospheric composition, 3) the lithological composition of continental crust, 4) the ratio of continental crust to oceanic crust, 5) the ratio of emergent to submerged crust, and 6) the efficiency of sedimentary crustal reworking. Recent work has suggested that phosphate may be most available on worlds with high atmospheric pCO₂, where abundant dissolved inorganic carbon in surface waters can help solubilise the P-bearing phase apatite.  Provocatively, our modelling suggests that, on primitive worlds where apatite is rare, the weathering of rock-forming silicate and carbonate minerals may supply higher P fluxes - up to an order of magnitude higher than weathering of apatite rich crust at low pCO₂, and roughly competitive with or, for mafic crust rich in basaltic glass, 1-2 orders of magnitue higher than the weathering of apatite rich crust at high pCO₂. Finally, our results also strongly suggest that high rates of sedimentary reworking are needed to access the highest P weathering fluxes on Earth-like worlds. Our results point towards settings of active sedimentary cycling as crucial for fuelling prebiotic chemistry with endogenous P sources, and reveal a broad mineralogical and climatic parameter space for Earth-like worlds under which that chemistry may have plausibly taken place.

How to cite: Walton, C., Shorttle, O., Jenner, F., and Pasek, M.: Which planets best liberate phosphate for prebiotic chemistry?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1141, https://doi.org/10.5194/egusphere-egu22-1141, 2022.

Life has played a key role in shaping the atmosphere since its origin on Earth, but 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 compared to those at which the biosphere interacts with the climate system. Whereas, biogeochemical modelling allows us to estimate biosphere driven gas fluxes in and out of the atmosphere over longer time scales [1], 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.

We use a biogeochemical model [2] to understand the limits on the potential evolution of the atmosphere, as well as a state-of-the-art 3D climate model [3] 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 use the 3D climate model to extend a 1D exploration of methane’s diminished greenhouse potential during the Archean [4] by looking at how methane concentrations affect the cloud distribution, atmospheric dynamics and surface temperature.

We find that global surface temperature peaks for pCH₄ between 30-100 Pa, with the peak shifting to higher pCH₄ as pCO₂ is increased. Equator-to-pole temperature differences also have a peaked response driven by changes in the radiative balance. These changes come about from the balance between the effect of methane and carbon dioxide on atmospheric dynamics due to changes in heating rates vertically and meridionally, which also affects the cloud formation. This work begins to explore how our understanding of early biospheres can be coupled to 3D climate models, to understand the biosphere’s impact on the climate of Earth and terrestrial exoplanets following the origin of life.

References

[1] Kharecha, Kasting & Siefert (2005) Geobiology 3, 53-76.

[2] Lenton & Daines (2017) Ann. Rev. Mar. Sci. 9:1, 31-58.

[3] Mayne et al. (2014) Geosci. Model Dev. 7, 3059–3087.

[4] Byrne & Goldblatt (2015) Clim. Past 11, 559–570.

How to cite: Eager, J., Mayne, N., and Lenton, T.: Towards Coupled Modelling of the Biosphere and Atmosphere for the Archean Climate: the importance of Methane, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8099, https://doi.org/10.5194/egusphere-egu22-8099, 2022.

Life in the clouds of Venus, if present, has been proposed to extract energy from its environment using sulfur-based metabolisms. These metabolisms link life to the chemistry of Venus's atmosphere and thus provide testable predictions of life's presence given current observations. In particular, these hypothetical metabolisms raise the possibility of Venus's enigmatic cloud-layer SO2-depletion being explained by life. We couple each proposed metabolic pathway to a photochemical-kinetics code and self-consistently predict the composition of Venus's atmosphere under the scenario that life produces the observed SO2-depletion. Using this photo-bio-chemical kinetics code, we show that all three metabolisms produce SO2-depletions which violate other observational constraints on Venus's atmospheric chemistry. For each metabolism, we estimate the maximum potential biomass density in the cloud layer before the observational constraints are violated. Our analysis shows that either the observed SO2-depletion is due to a currently unknown metabolism, or there is not a high-mass biosphere in Venus's clouds. The methods employed are equally applicable to aerial biospheres on Venus-like exoplanets, planets that are optimally poised for atmospheric characterisation in the near-future.

How to cite: Jordan, S., Shorttle, O., and Rimmer, P.: Metabolic Signatures of an Aerial Biosphere in the Clouds of Venus: A Self-Consistent Photo-Bio-Chemical Model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2528, https://doi.org/10.5194/egusphere-egu22-2528, 2022.

The discovery of gypsum (CaSO₄●2H₂O) on Mars by the NASA Mars Exploration Rover Opportunity has corroborated past models about the early composition of the Red Planet. In extreme environments, minerals, such as gypsum, which are formed through the evaporation of water, can act as a refuge for extremophilic microorganisms. After providing a refuge from desiccation, rapid temperature fluctuations, and elevated levels of UV-radiation, gypsum can preserve biomarkers by sealing them. To better understand the geobiological interactions of pigments and other biomarkers possibly encapsulated in a gypsum matrix, samples of gypsum collected from a depth of 25cm within microbial mats in the Dohat Faishakh sabkha in Qatar were examined. The Dohat Faishakh sabkha is considered an Earth analogue to past evaporitic environments on Mars due to its extremely high salinity, harsh desiccation, and intense levels of UV-radiation. The aim of this work was to holistically evaluate the buried microbial community and gypsum-hosted biomarkers to gain insight into the best practices for Raman signal detection. 16s rRNA analyses was employed to determine organisms present and their aptitude for producing biomarkers. Raman microscopic analysis was applied to prove whether any biomarkers were trapped within the gypsum matrix. We observed that gypsum formed in a layer heavily dominated by halophilic archaea (>50% total abundance) and organic matter produced by microorganisms was encapsulated resulting in distinct Raman spectra. Several types of organic molecules were identified including carotenoids, chlorophylls, scytonemin and phycobiliproteins suggesting that complex signatures were preserved in gypsum.

How to cite: Diloreto, Z., Bontognali, T., Shaharyar Ahmad, M., and Dittrich, M.: Raman Spectroscopic and Microbial Analysis of Microbial Mat Hosted Gypsum from the Dohat Faishakh Sabkha in Qatar and its Astrobiological Implications, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7007, https://doi.org/10.5194/egusphere-egu22-7007, 2022.

The Anti-Atlas belt of Morocco preserves exceptional record of an Ediacaran microbial biosphere. The Amane Tazgart Formation of the Ouarzazate Group consist of an Ediacaran volcanic alkaline lake depositional system (ca. 571 Ma) were microbial buildups accreted in an extreme environment. These microbial accumulations are exceptional not only for their wide scope of extreme setting but also for their significance for understanding the early biosphere and earth habitability. A description of these buildups provides insights into their spatio-temporal distribution, in a 11 m-thick section. Specifically, the lower part consists mainly of thrombolitic limestone, usually displaying irregular to patchy mesoclots and occasionally arranged in dendritic pattern. The upper part dominated by clastic stromatolites, exhibit a variety of morphotypes ranging vertically from planar wrinkly laminated to large domes. The transitional morphotypes are made of linked and vertically oriented or inclined columns, grading upward to cone-shaped domes. The change from planar to columnar forms has been considered to indicate a shallowing trend, whereas the transition from columnar to domal morphotypes indicate a deepening trend. Spherulitic carbonate particles usually found within thrombolites, comprise radiating, wedge-shaped crystals. The analyses of spherulites-bearing samples using diluted acetic acids reveal the presence of microbial aggregates. They preserve spherical or globular shape and often irregular morphologies showing alignment along specific direction. Microfabric typical of Extra-polymeric substances (EPS) is preserved within these carbonate aggregates, suggesting their biological origin. The mineralogy of Amane Tazgart microbialites was studied using microscopical observation and XRD analyses. XRD show significant change in fabric composition from carbonate-dominated to clastic- and epicalstic-dominated microbialites, revealing the role of calcium carbonate saturation on microbialites genesis. Several features are preserved in the microbialites fabrics including micro-tufts and gas-bubbles and gas escape structure, forming evidence for mat growth and metabolic processes related to oxygenic photosynthesis and oxygen production.

How to cite: Chraiki, I., Bouougri, E. H., Chi Fru, E., Lazreq, N., Youbi, N., Boumehdi, A., Aubineau, J., Fontaine, C., and El Albani, A.: An exceptionally record of microbial mats thriving in a volcanic caldera setting: The Ediacaran microbialites and mat-related structures of the Anti-Atlas, Morocco, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-335, https://doi.org/10.5194/egusphere-egu22-335, 2022.

The Late Heavy Bombardment (LHB) and other late accretion impactor scenarios are often invoked as habitability constraints on the Hadean/Eoarchean Earth. These hypotheses either describe an “impact frustration” where life would not arise until high impact fluxes abated, or “impact bottlenecks” with Bacteria and Archaea representing surviving lineages that subsequently diversified. Phylogenomics studies using relaxed molecular clocks have frequently used these early impact fluxes, especially the LHB, as older-bound constraints on extant life’s early diversification. However, the intensity, timing, and sterilization potential of these scenarios is poorly constrained, and lacks consensus. We propose inverting this hypothesis testing, evaluating late accretion impact hypotheses using molecular clocks that do not presuppose impact frustration or bottlenecks as constraints. However, in the absence of these constraints, previous studies lack the precision to discriminate between these hypotheses. Our recently developed molecular clock approach, using horizontal gene transfers as “cross cutting events” between lineages, overcomes this limitation, and provides sufficient precision to test the proposed biological impact of specific planetary hypotheses such as the LHB.   Using this methodology, we show that major bacterial groups likely diversified between 3.75 and 3.55 Ga, with the last common ancestor of extant Bacteria likely existing shortly after 3.8 Ga.  These ages are consistent with the LHB impact bottleneck hypothesis, wherein bacteria and archaea represent survivors of early Archean cataclysms that extinguished most primordial biodiversity ~3.9 Ga. Future work extending this methodology to Archaea can potentially provide an independent test of the impact bottleneck hypothesis.

How to cite: Fournier, G., Rangel, L. T., Moore, K., Payette, J., Momper, L., and Bosak, T.: Molecular Clock Dates for Bacterial Origins are consistent with Impact Bottleneck Scenarios, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8987, https://doi.org/10.5194/egusphere-egu22-8987, 2022.

The intersection of environmental conditions with the conditions permissive for life defines habitability. Consequently, our understanding of habitability is fundamentally limited by our understanding of the multidimensional niche space for life, which up to now, is based on our one known data point: life on Earth. Terrestrial life has evolved to tolerate environmental conditions found on Earth, and as most physiological studies are limited to extant organisms, it is likely that life has potential for a far broader niche space than observed today.

Potentially habitable extraterrestrial environments present challenges not only in single environmental dimensions (temperature, pH, radiation, etc.), but also in combination. For example, Martian brines feature both low temperature and high concentrations of perchlorate, while Venusian clouds feature both desiccating conditions and extreme acidity. We do not know whether the inability of known life to reproduce under analogous conditions reflects a fundamental boundary condition or simply a lack of terrestrial selection pressure. A mixture of environmental challenges may be similarly common among exoplanets and other potentially habitable environments within our solar system.

We are addressing this key gap in our understanding of habitability by using adaptive laboratory evolution, functional metagenomics, and synthetic biology to expand the known environmental limits of life. First, we are determining and pushing the limits of pH (acidic and basic), salt (both chloride and perchlorate), and UV tolerance individually and in combination with temperature for B. subtilis, E. coli, and D. radiodurans through adaptive laboratory evolution. This will define a multidimensional niche-space for these organisms and assess how firm these boundaries are. Second, we are taking advantage of the rich genetic diversity present on Earth to identify genetic elements providing transferable survival benefits under extreme environmental conditions. One of the most powerful resources available to us for this endeavor to expand the boundary conditions of life is the extensive biodiversity present on Earth, particularly those capable of surviving in extreme environments. Prior work demonstrates that extremophile genes can expand an organism’s niche space, including increased resistance to desiccation, salinity, radiation, and low temperatures. However, despite all we have learned from them, at present it remains difficult and laborious to characterize their genetic mechanisms of adaptation and test their ability to facilitate an enlarged environmental niche. Through a combination of cDNA- and DNA-based libraries, we aim to establish a high throughput method of assaying novel organisms for additional mechanisms of expanding the niche-space of life. Third, we will use codon-optimized cassettes containing genes either identified in our screen or from published research to verify the synthetic acquisition of functional capabilities and to test if the same genetic constructs can expand the niche-space of multiple species.

Through these approaches, we will provide both selection pressure and genetic resources to challenge life to evolve beyond environmental conditions found naturally on Earth. Such work will improve our understanding of what environmental conditions are compatible with life as we know it and allow firm reclassification of some extraterrestrial environments from “probably habitable” to “definitely habitable.”

How to cite: Roberts Kingman, G. and Rothschild, L.: Expanding the known limits of life through adaptive laboratory evolution, functional metagenomics, and synthetic biology, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6794, https://doi.org/10.5194/egusphere-egu22-6794, 2022.