Large Uncertainties in the Thermodynamics of Phosphorus (III) Oxide (P4_4O6_6) Have Significant Implications for Phosphorus Species in Planetary Atmospheres

Phosphorus (III) oxide (P$_4$O$_6$) has been suggested to be a major component of the gas phase phosphorus chemistry in the atmospheres of gas giant planets and of Venus. However, P$_4$O$_6$'s proposed role is based on thermodynamic modeling, itself based on values for the free energy of formation of P$_4$O$_6$ estimated from limited experimental data. Values of the standard Gibbs free energy of formation ($\Delta$Go(g)) of P$_4$O$_6$ in the literature differ by up to ~656 kJ/mol, a huge range. Depending on which value is assumed, P$_4$O$_6$ may either be the majority phosphorus species present or be completely absent from modeled atmospheres. Here, we critically review the literature thermodynamic values and compare their predictions to observed constraints on P$_4$O$_6$ geochemistry. We conclude that the widely used values from the NIST/JANAF database are almost certainly too low (predicting that P$_4$O$_6$ is more stable than is plausible). We show that, regardless of the value of $\Delta$Go(g) for P$_4$O$_6$ assumed, the formation of phosphine from P$_4$O$_6$ in the Venusian atmosphere is thermodynamically unfavorable. We conclude that there is a need for more robust data on both the thermodynamics of phosphorus chemistry for astronomical and geological modeling in general and for understanding the atmosphere of Venus and the gas giant planets in particular.Comment: Article published in ACS Earth Space Chem. https://pubs.acs.org/doi/full/10.1021/acsearthspacechem.3c0001

Phosphine Generation Pathways on Rocky Planets

The possibility of life in the venusian clouds was proposed in the 1960s, and recently this hypothesis has been revived with the potential detection of phosphine (PH3) in Venus\u27 atmosphere. These observations may have detected ∼5–20 ppb phosphine on Venus (Greaves et al., 2020), which raises questions about venusian atmospheric/geochemical processes and suggests that this phosphine could possibly be generated by biological processes. In such a claim, it is essential to understand the abiotic phosphorus chemistry that may occur under Venus-relevant conditions, particularly those processes that may result in phosphine generation. Here, we discuss two related abiotic routes for phosphine generation within the atmosphere of Venus. Based on our assessment, corrosion of large impactors as they ablate near Venus\u27 cloud layer, and the presence of reduced phosphorus compounds in the subcloud layer could result in production of phosphine and may explain the phosphine detected in Venus\u27 atmosphere or on other rocky planets. We end on a cautionary note: although there may be life in the clouds of Venus, the detection of a simple, single gas, phosphine, is likely not a decisive indicator

Prebiotic synthesis of phosphoenol pyruvate by α-phosphorylation-controlled triose glycolysis

Phosphoenol pyruvate is the highest-energy phosphate found in living organisms and is one of the most versatile molecules in metabolism. Consequently, it is an essential intermediate in a wide variety of biochemical pathways, including carbon fixation, the shikimate pathway, substrate-level phosphorylation, gluconeogenesis and glycolysis. Triose glycolysis (generation of ATP from glyceraldehyde 3-phosphate via phosphoenol pyruvate) is among the most central and highly conserved pathways in metabolism. Here, we demonstrate the efficient and robust synthesis of phosphoenol pyruvate from prebiotic nucleotide precursors, glycolaldehyde and glyceraldehyde. Furthermore, phosphoenol pyruvate is derived within an α-phosphorylation controlled reaction network that gives access to glyceric acid 2-phosphate, glyceric acid 3-phosphate, phosphoserine and pyruvate. Our results demonstrate that the key components of a core metabolic pathway central to energy transduction and amino acid, sugar, nucleotide and lipid biosyntheses can be reconstituted in high yield under mild, prebiotically plausible conditions

The Forensics of Fulgurite Formation

Natural disasters such as forest fires can result in extensive and costly property damage. These events may be the result of a human error or system failure triggered by electrical discharge, and in such circumstances may form a fulgurite. Understanding fulgurites and their formation may be critical in determining the cause of the fire or other, shock-related event. Here we identify several distinguishing features of fulgurites formed in association with downed power lines, including the presence of melted conductors, transformation of quartz to cristobalite, and morphological differences including increased glass percentage and smaller internal voids. These features are consequences of how heat is transferred to and through a target rock material as it melts and forms a fulgurite, and are predicted from both first principles of diffusive heat transfer, and empirically-derived reaction kinetics for mineral transformations

Schreibersite on the Early Earth: Scenarios for Prebiotic Phosphorylation

The mineral schreibersite, (Fe,Ni)3P, provides a reactive source of phosphorus capable of forming phosphorylated molecules. These molecules may have been an important component of prebiotic chemistry, allowing their build-up and eventual commencement of autopoiesis. Discussed here are potential geochemical routes to providing schreibersite, as a potentially important prebiotic mineral, to the Hadean Earth. Two routes are identified: delivery of phosphides by meteoritic material and the reduction of phosphates to phosphides by high-temperature, low-redox conditions. About 1–10% of all crustal phosphorus is estimated to have been in schreibersite during the Hadean, making the long-term reaction of this mineral with organic-laden water plausible for many years. Ultimately, such conditions would have been conducive to the formation of life as we know it today

Phosphorus Volatility in the Early Solar Nebula

Phosphorus is a minor element that controls the formation of several key planetary minerals. It is also an element critical to the development of life. A common assumption of phosphorus chemistry is that at low temperatures, phosphorus would have been a volatile component of ices or gases in the outer Solar System. Here I propose that phosphorus was depleted as a volatile throughout the developing Solar System, and as a result, volatile forms of phosphorus would have been minimal, even in the colder regions of the Solar nebula. Based on thermodynamic equilibrium models and metal phosphidation kinetics coupled to a simple 1D gas diffusion model, phosphorus migrated rapidly to the inner Solar System, forming solids such as phosphides and phosphates, and removing volatile phosphorus across large portions of the Solar System

Schreibersite on the Early Earth: Scenarios for Prebiotic Phosphorylation

The mineral schreibersite, (Fe,Ni)3P, provides a reactive source of phosphorus capable of forming phosphorylated molecules. These molecules may have been an important component of prebiotic chemistry, allowing their build-up and eventual commencement of autopoiesis. Discussed here are potential geochemical routes to providing schreibersite, as a potentially important prebiotic mineral, to the Hadean Earth. Two routes are identified: delivery of phosphides by meteoritic material and the reduction of phosphates to phosphides by high-temperature, low-redox conditions. About 1–10% of all crustal phosphorus is estimated to have been in schreibersite during the Hadean, making the long-term reaction of this mineral with organic-laden water plausible for many years. Ultimately, such conditions would have been conducive to the formation of life as we know it today

Fulgurite Morphology: A Classification Scheme and Clues to Formation

Fulgurites are natural glasses formed by cloud-to-ground lightning. Several different morphologies of fulgurites have been reported in previous studies, including sand fulgurites, rock fulgurites, and clay fulgurites. Herein, we examine sand, clay, and caliche fulgurites and demonstrate that these differ systematically in their morphology. We further use morphological features to constrain properties of fulgurite-forming lightning strikes. We classify fulgurites into four types of morphologies with an additional minor type. Type I fulgurites are sand fulgurites consisting of thin, glass walls; type II fulgurites are clay fulgurites, consisting of thick, melt rich walls; type III fulgurites are caliche fulgurites, consisting of thick, glass poor walls; and type IV fulgurites are rock fulgurites, consisting of glasses with walls consisting of surrounding, unmelted rock. Fulgurite morphology shows that the energy of fulgurite-forming strikes is between 1 and 30 MJ/m of fulgurite formed, suggests heating rates in the order of 1,000 K/s, and lightning channel thicknesses of about 1 mm diameter. Lightning generates mixtures of at least two components in most fulgurites: an SiO2 glass identified as lechatelierite and a groundmass of more varied composition. In addition to these four primary types, a fifth type—droplet fulgurites—is morphologically dissimilar from the other types, but is compositionally related to the type II or IV fulgurites. Additionally, two fulgurites, both from York County, Pennsylvania, USA, showed the reduction of iron to iron metal with an assortment of Fe–Ti and Si–P compounds with stoichiometry that ranges from nearly pure Fe metal to FeSi. These metal silicides include stoichiometric Fe3Si, Fe2Si, and Fe5Si3, and possibly Fe8Si3 and Fe7Si3, and provide a terrestrial source for these phases, which are typically associated with extraterrestrial material

Implications of Extraterrestrial Material on the Origin of Life

Meteoritic organic material may provide the best perspective on prebiotic chemistry. Meteorites have also been invoked as a source of prebiotic material. This study suggests a caveat to extraterrestrial organic delivery: that prebiotic meteoritic organics were too dilute to promote prebiotic reactions. However, meteoritic material provides building material for endogenous synthesis of prebiotic molecules, such as by hydrolysis of extraterrestrial organic tars, and corrosion of phosphide minerals

Is Struvite a Prebiotic Mineral?

The prebiotic relevance of mineral struvite, MgNH4PO4·6H2O, was studied experimentally as a phosphorylating reagent and, theoretically, to understand the geochemical requirements for its formation. The effectiveness of phosphorylation by the phosphate mineral, monetite, CaHPO4, was also studied to compare to the efficiency of struvite. The experiments focused on the phosphorylation reactions of the minerals with organic compounds, such as nucleosides, glycerol and choline chloride, and heat at 75 °C for about 7–8 days and showed up to 28% phosphorylation of glycerol. In contrast, the compositional requirements for the precipitation of struvite are high ammonium and phosphate concentrations, as well as a little Ca2+ dissolved in the water. Combined, these requirements suggest that it is not likely that struvite was present in excess on the early Earth to carry out phosphorylation reactions. The present study focuses on the thermodynamic aspects of struvite formation, complementing the results given by Orgel and Handschuh (1973), which were based on the kinetic effects