arXiv subjects

Ben K. D. Pearce

Publications and source records attributed to Ben K. D. Pearce.

At least 19 recordsLinked to original sources

arXiv2026

Prebiotic Chemistry Insights for Dragonfly II: Thermodynamic Favorability of Nucleobases, Ribose, and Fatty Acids in Selk Crater on Titan

Saturn's moon Titan is a prime destination for investigating prebiotic chemistry beyond Earth, particularly at impact crater sites where transient liquid water may have enabled aqueous reactions between organic molecules. Selk crater represents one such environment and is a primary target of NASA's Dragonfly mission. Here, we present a thermodynamic assessment of nucleobases, ribose, and fatty acids formed from simple atmospheric precursors (HCN and C2H2) within a Selk-sized aqueous melt pool across varying ammonia (NH3) abundances. We find that ammonia acts as a chemical gatekeeper for molecular accessibility. In NH3-free systems, accessibility is restricted to adenine and butanoic acid. Once >=1% NH3 is introduced, all investigated molecular classes become thermodynamically accessible. Distinct molecular classes have different NH3 sensitivities: nucleobases, ribose, and C2-C6 fatty acids yield peaks at 1% NH3, and C7-C12 fatty acids yield peaks at 2% NH3. The modeled preference for pyrimidines vs. purines and monotonic decline of fatty acid abundance with chain length qualitatively mirror patterns observed in carbonaceous meteorites and returned asteroid samples. We show how molecular distributions and cross-class correlations may provide indirect constraints on Selk's past aqueous environment, help constrain past ammonia availability, and distinguish abiotic production from potential anomalies. By coupling thermodynamic predictions with an assessment of Dragonfly's mass spectrometer (DraMS) capabilities, we posit concrete, testable predictions for evaluating Selk's prebiotic potential in situ.

astro-ph.EP↗

arXiv2025

Prebiotic Chemistry Insights for Dragonfly: Thermodynamics of Amino Acid Synthesis in Selk Crater on Titan

Saturnian moon Titan presents a compelling testbed for probing prebiotic chemistry beyond early Earth. Impact-generated melt pools provide transient aqueous habitats in an otherwise cryogenic environment. We use Cantera equilibrium models to assess whether mixtures of hydrogen cyanide (HCN), acetylene (C2H2), and ammonia (NH3) can drive amino acid synthesis in Selk-sized craters. Across twenty-one amino acids (twenty proteinogenic plus beta-alanine), NH3-free systems yield only proline, alanine, and beta-alanine, whereas adding as little as 1% NH3 (relative to H2O) renders almost the full suite accessible, with yields peaking at 2% and tapering thereafter. The NH3-free alanine result implies alternative pathways beyond classical Strecker or aminonitrile hydrolysis, suggesting acetylene, abundant on Titan but scarce on early Earth, as a plausible feedstock. We identify acrylonitrile (detected on Titan) as a thermodynamically favorable intermediate that can convert to alanine under aqueous conditions in an NH3-free pathway. For glycine and alanine production from nitrile hydrolysis, comparison with laboratory kinetics shows that our equilibrium models predict near-complete conversion, while observed rates yield only partial products over weeks. Yet estimated chemical equilibration times (years-centuries) are far shorter than melt lifetimes, supporting plausibility of equilibrium in situ. These predictions are directly testable with Dragonfly mass spectrometer (DraMS), for which we recommend pre-flight standards to test proline, alanine, beta-alanine, cysteine, and methionine. The first three offer the best chances for amino acid detection regardless of ammonia availability; the latter two offer diagnostic tools for determining the presence of reactive sulfur in post-impact Titan ponds.

astro-ph.EP↗

arXiv2025

Deep Mantle-Atmosphere Coupling and Carbonaceous Bombardment: Options for Biomolecule Formation on an Oxidized Early Earth

Understanding what environmental conditions prevailed on early Earth during the Hadean eon, and how this set the stage for the origins of life, remains a challenge. Geologic processes such as serpentinization and bombardment by chondritic material during the late veneer might have been very active, shaping an atmospheric composition reducing enough to allow efficient photochemical synthesis of HCN, one of the key precursors of prebiotic molecules. HCN can rain out and accumulate in warm little ponds (WLPs), forming prebiotic molecules such as nucleobases and the sugar ribose. These molecules could condense to nucleotides, the building blocks of RNA molecules, one of the ingredients of life. Here, we perform a systematic study of potential sources of reducing gases on Hadean Earth and calculate the concentrations of prebiotic molecules in WLPs based on a comprehensive geophysical and atmospheric model. We find that in a reduced H$_2$-dominated atmosphere, carbonaceous bombardment can produce enough HCN to reach maximum WLP concentrations of $\sim 1-10\,\mathrm{mM}$ for nucleobases and, in the absence of seepage, $\sim 10-100\,\mathrm{μM}$ for ribose. If the Hadean atmosphere was initially oxidized and CO$_2$-rich ($90\,\%$), we find serpentinization alone can reduce the atmosphere, resulting in WLP concentrations of an order of magnitude lower than the reducing carbonaceous bombardment case. In both cases, concentrations are sufficient for nucleotide synthesis, as shown in experimental studies. RNA could have appeared on Earth immediately after it became habitable (about $100\,\mathrm{Myr}$ after the Moon-forming impact), or it could have (re)appeared later at any time up to the beginning of the Archean.

astro-ph.EP↗

arXiv2024

Towards Prebiotic Chemistry on Titan: Impact experiments on organic haze particles

Impacts are critical to producing the aqueous environments necessary to stimulate prebiotic chemistry on Titan's surface. Furthermore, organic hazes resting on the surface are a likely feedstock of biomolecules. In this work, we conduct impact experiments on laboratory-produced organic haze particles and haze/sand mixtures and analyze these samples for life's building blocks. Samples of unshocked haze and sand particles are also analyzed to determine the change in biomolecule concentrations and distributions from shocking. Across all samples, we detect seven nucleobases, nine proteinogenic amino acids, and five other biomolecules (e.g., urea) using a blank subtraction procedure to eliminate signals due to contamination. We find that shock pressures of 13 GPa variably degrade nucleobases, amino acids, and a few other organics in haze particles and haze/sand mixtures; however, certain individual biomolecules become enriched or are even produced from these events. Xanthine, threonine, and aspartic acid are enriched or produced in impact experiments containing sand, suggesting these minerals may catalyze the production of these biomolecules. On the other hand, thymine and isoleucine/norleucine are enriched or produced in haze samples containing no sand, suggesting catalytic grains are not necessary for all impact shock syntheses. Uracil, glycine, proline, cysteine, and tyrosine are the most unstable to impact-related processing. These experiments suggest that impacts alter biomolecule distributions on Titan's surface, and that organic hazes co-occurring with fine-grained material on the surface may provide an initial source for further prebiotic chemistry on Titan.

astro-ph.IM↗

arXiv2024

Organic hazes as a source of life's building blocks to warm little ponds on the Hadean Earth

Over 4 billion years ago, Earth is thought to have been a hazy world akin to Saturn's moon Titan. The organic hazes in the atmosphere at this time could contain a vast inventory of life's building blocks, and thus may have seeded warm little ponds for life. In this work, we produce organic hazes in the lab in atmospheres with high (5%) and low (0.5%) CH4 abundances and analyze the solid particles for nucleobases, amino acids, and a few other organics using GC/MS/MS to obtain their concentrations. We also analyze heated (200 $^{\circ}$C) samples from the high methane organic haze experiment to simulate these particles sitting on an uninhabitable surface. Finally, we use our experimental results and estimates of atmospheric haze production as inputs for a comprehensive numerical pond model to calculate the concentrations of nucleobases from organic hazes in these environments. We find that organic hazes typically provide up to 0.2-6.5 $μ$M concentrations of nucleobases to warm little ponds for potentially habitable Hadean conditions. However, without seepage, uracil and thymine can reach ~100 $μ$M concentrations, which is the present lower experimental limit to react these species to form nucleotides. Heating samples leads to partial or complete decay of biomolecules, suggesting that biomolecule stockpiling on the hot surface is unlikely. The ideal conditions for the delivery of life's building blocks from organic hazes would be when the Hadean atmosphere is rich in methane, but not so rich as to create an uninhabitable surface.

astro-ph.EP↗

arXiv2023

What it takes to solve the Origin(s) of Life: An integrated review of techniques

Understanding the origin(s) of life (OoL) is a fundamental challenge for science in the 21st century. Research on OoL spans many disciplines, including chemistry, physics, biology, planetary sciences, computer science, mathematics and philosophy. The sheer number of different scientific perspectives relevant to the problem has resulted in the coexistence of diverse tools, techniques, data, and software in OoL studies. This has made communication between the disciplines relevant to the OoL extremely difficult because the interpretation of data, analyses, or standards of evidence can vary dramatically. Here, we hope to bridge this wide field of study by providing common ground via the consolidation of tools and techniques rather than positing a unifying view on how life emerges. We review the common tools and techniques that have been used significantly in OoL studies in recent years. In particular, we aim to identify which information is most relevant for comparing and integrating the results of experimental analyses into mathematical and computational models. This review aims to provide a baseline expectation and understanding of technical aspects of origins research, rather than being a primer on any particular topic. As such, it spans broadly -- from analytical chemistry to mathematical models -- and highlights areas of future work that will benefit from a multidisciplinary approach to tackling the mystery of life's origin. Ultimately, we hope to empower a new generation of OoL scientists by reviewing how they can investigate life's origin, rather than dictating how to think about the problem.

q-bio.PE↗

arXiv2022

Meteorites and the RNA World: Synthesis of Nucleobases in Carbonaceous Planetesimals and the Role of Initial Volatile Content

Prebiotic molecules, fundamental building blocks for the origin of life, have been found in carbonaceous chondrites. The exogenous delivery of these organic molecules onto the Hadean Earth could have sparked the polymerization of the first RNA molecules in Darwinian ponds during wet-dry cycles. Here, we investigate the formation of the RNA and DNA nucleobases adenine, uracil, cytosine, guanine, and thymine inside parent body planetesimals of carbonaceous chondrites. An up-to-date thermochemical equilibrium model coupled with a 1D thermodynamic planetesimal model is used to calculate the nucleobase concentrations. Different from the previous study (Pearce & Pudritz 2016), we assume initial volatile concentrations more appropriate for the formation zone of carbonaceous chondrite parent bodies. This represents more accurately cosmochemical findings that these bodies have formed inside the inner, $\sim 2\mathrm{-}5\,\mathrm{au}$, warm region of the solar system. Due to these improvements, our model represents the concentrations of adenine and guanine measured in carbonaceous chondrites. Our model did not reproduce per se the measurements of uracil, cytosine, and thymine in these meteorites. This can be explained by transformation reactions between nucleobases and potential decomposition of thymine. The synthesis of prebiotic organic matter in carbonaceous asteroids could be well explained by a combination of i) radiogenic heating, ii) aqueous chemistry involving a few key processes at a specific range of radii inside planetesimals where water can exist in the liquid phase, and iii) a reduced initial volatile content (H$_2$, CO, HCN, CH$_2$O) of the protoplanetary disk material in the parent body region compared to the outer region of comets.

astro-ph.EP↗

arXiv2022

An experimental and theoretical investigation of HCN production in the Hadean Earth atmosphere

A critical early stage for the origin of life on Earth may have involved the production of hydrogen cyanide (HCN) in a reducing, predominantly H$_2$ atmosphere. HCN is crucial for the origin of life as it is a possible precursor to several biomolecules that make up RNA and proteins including nucleobases, nucleotides, amino acids, and ribose. In this work, we perform an in depth experimental and theoretical investigation of HCN production in reducing atmospheric conditions (89-95% H$_2$) possibly representing the earliest stages of the Hadean eon, ~4.5-4.3 billion years ago. We make use of cold plasma discharges - a laboratory analog to shortwave UV radiation - to simulate HCN production in the upper layers of the atmosphere for CH$_4$ abundances ranging from 0.1-6.5%. We then combine experimental mass spectrum measurements with our theoretical plasma models to estimate the HCN concentrations produced in our experiments. We find that upper atmospheric HCN production scales linearly with CH$_4$ abundance with the relation [HCN] = 0.13 $\pm$ 0.01[CH$_4$]. Concentrations of HCN near the surface of the Hadean Earth are expected to be about 2-3 orders of magnitude lower. The addition of 1% water to our experiments results in a ~50% reduction in HCN production. We find that four reactions are primarily responsible for HCN production in our experiments: (i) $^4$N + CH$_3$ -> H$_2$CN + H -> HCN + H$_2$, (ii) $^4$N + CH -> CN + H followed by CN + CH$_4$ -> HCN + CH$_3$, (iii) C$_2$H$_4$ + $^4$N -> HCN + CH$_3$, and (iv) $^4$N + $^3$CH$_2$ -> HCN + H. The most prebiotically favorable Hadean atmosphere would have been very rich in CH$_4$ (> 5%), and as a result of greenhouse effects the surface would be likely very hot. In such a prebiotic scenario, it may have been important to incorporate HCN into organic hazes that could later release biomolecules and precursors into the first ponds.

astro-ph.EP↗

arXiv2022

Possible Ribose Synthesis in Carbonaceous Planetesimals

The origin of life might be sparked by the polymerization of the first RNA molecules in Darwinian ponds during wet-dry cycles. The key life-building block ribose was found in carbonaceous chondrites. Its exogenous delivery onto the Hadean Earth could be a crucial step toward the emergence of the RNA world. Here, we investigate the formation of ribose through a simplified version of the formose reaction inside carbonaceous chondrite parent bodies. Following up on our previous studies regarding nucleobases with the same coupled physico-chemical model, we calculate the abundance of ribose within planetesimals of different sizes and heating histories. We perform laboratory experiments using catalysts present in carbonaceous chondrites to infer the yield of ribose among all pentoses (5Cs) forming during the formose reaction. These laboratory yields are used to tune our theoretical model that can only predict the total abundance of 5Cs. We found that the calculated abundances of ribose were similar to the ones measured in carbonaceous chondrites. We discuss the possibilities of chemical decomposition and preservation of ribose and derived constraints on time and location in planetesimals. In conclusion, the aqueous formose reaction might produce most of the ribose in carbonaceous chondrites. Together with our previous studies on nucleobases, we found that life-building blocks of the RNA world could be synthesized inside parent bodies and later delivered onto the early Earth.

physics.chem-ph↗

arXiv2022

Towards RNA life on Early Earth: From atmospheric HCN to biomolecule production in warm little ponds

The origin of life on Earth involves the early appearance of an information-containing molecule such as RNA. The basic building blocks of RNA could have been delivered by carbon-rich meteorites, or produced in situ by processes beginning with the synthesis of hydrogen cyanide (HCN) in the early Earth's atmosphere. Here, we construct a robust physical and non-equilibrium chemical model of the early Earth atmosphere. 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, and in which lightning and external UV-driven chemistry produce HCN. This 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) the limits of adenine concentrations in ponds for habitable surfaces is 0.05$μ$M in the absence of seepage. These concentrations can be maintained for over 100 Myr. Meteorite delivery of adenine to WLPs can provide boosts in concentration by 2-3 orders of magnitude, but these boosts deplete 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.

astro-ph.EP↗

arXiv2020

CRAHCN-O: A Consistent Reduced Atmospheric Hybrid Chemical Network Oxygen Extension for Hydrogen Cyanide and Formaldehyde Chemistry in CO2-, N2-, H2O-, CH4-, and H2-Dominated Atmospheres

Hydrogen cyanide (HCN) and formaldehyde (H2CO) are key precursors to biomolecules such as nucleobases and amino acids in planetary atmospheres; However, many reactions which produce and destroy these species in atmospheres containing CO2 and H2O are still missing from the literature. We use a quantum chemistry approach to find these missing reactions and calculate their rate coefficients using canonical variational transition state theory and Rice-Ramsperger-Kassel-Marcus/master equation theory at the BHandHLYP/aug-cc-pVDZ level of theory. We calculate the rate coefficients for 126 total reactions, and validate our calculations by comparing with experimental data in the 39% of available cases. Our calculated rate coefficients are most frequently within an factor of 2 of experimental values, and generally always within an order of magnitude of these values. We discover 45 previously unknown reactions, and identify 6 from this list that are most likely to dominate H2CO and HCN production and destruction in planetary atmospheres. We highlight $^1$O + CH3 $\rightarrow$ H2CO + H as a new key source, and H2CO + $^1$O $\rightarrow$ HCO + OH as a new key sink, for H2CO in upper planetary atmospheres. In this effort, we develop an oxygen extension to our consistent reduced atmospheric hybrid chemical network (CRAHCN-O), building off our previously developed network for HCN production in N2-, CH4-, and H2-dominated atmospheres (CRAHCN). This extension can be used to simulate both HCN and H2CO production in atmospheres dominated by any of CO2, N2, H2O, CH4, and H2.

physics.chem-ph↗

arXiv2020

HCN production in Titan's Atmosphere: Coupling quantum chemistry and disequilibrium atmospheric modeling

Hydrogen cyanide (HCN) is a critical reactive source of nitrogen for building key biomolecules relevant for the origin of life. Still, many HCN reactions remain uncharacterized by experiments and theory, and the complete picture of HCN production in planetary atmospheres is not fully understood. To improve this situation, we develop a novel technique making use of computational quantum chemistry, experimental data, and atmospheric numerical simulations. First, we use quantum chemistry simulations to explore the entire field of possible reactions for a list of primary species in N2-, CH4-, and H2-dominated atmospheres. In this process, we discover 33 new reactions with no previously known rate coefficients. From here, we develop a consistent reduced atmospheric hybrid chemical network (CRAHCN) containing experimental values when available, and our calculated rate coefficients otherwise. Next, we couple CRAHCN to a 1D chemical kinetic model (ChemKM) to compute the HCN abundance as a function of atmospheric depth on Titan. Our simulated atmospheric HCN profile agrees very well with the Cassini observations. CRAHCN contains 104 reactions however nearly all of the simulated atmospheric HCN profile can be obtained using a scaled down network of only 19 dominant reactions. From here, we form a complete picture of HCN chemistry in Titan's atmosphere, from the dissociation of the main atmospheric species, down to the direct production of HCN along 4 major channels. One of these channels was first discovered and characterized in Pearce et al. (2019) and this work.

astro-ph.EP↗

arXiv2019

A Consistent Reduced Network for HCN Chemistry in Early Earth and Titan Atmospheres: Quantum Calculations of Reaction Rate Coefficients

HCN is a key ingredient for synthesizing biomolecules such as nucleobases and amino acids. We calculate 42 reaction rate coefficients directly involved with or in competition with the production of HCN in the early Earth or Titan atmospheres. These reactions are driven by methane and nitrogen radicals produced via UV photodissociation or lightning. For every reaction in this network, we calculate rate coefficients at 298 K using canonical variational transition state theory (CVT) paired with computational quantum chemistry simulations at the BHandHLYP/augcc-pVDZ level of theory. We also calculate the temperature dependence of the rate coefficients for the reactions that have barriers from 50 to 400 K. We present 15 new reaction rate coefficients with no previously known value; 93% of our calculated coefficients are within an order of magnitude of the nearest experimental or recommended values. Above 320 K, the rate coefficient for the new reaction H2CN -> HCN + H dominates. Contrary to experiments, we find the HCN reaction pathway, N + CH3 -> HCN + H2, to be inefficient and suggest that the experimental rate coefficient actually corresponds to an indirect pathway, through the H2CN intermediate. We present CVT using energies computed with density functional theory as a feasible and accurate method for calculating a large network of rate coefficients of small-molecule reactions.

astro-ph.EP↗

arXiv2018

Meteoritic Abundances of Fatty Acids and Potential Reaction Pathways in Planetesimals

The origin of fatty acids on the prebiotic Earth is important as they likely formed the encapsulating membranes of the first protocells. Carbon-rich meteorites (i.e., carbonaceous chondrites) such as Murchison and Tagish Lake are well known to contain these molecules, and their delivery to the early planet by intense early meteorite bombardments constitutes a key prebiotic source. We collect the fatty acid abundances measured in various carbonaceous chondrites from the literature and analyze them for patterns and correlations. Fatty acids in meteorites include straight-chain and branched-chain monocarboxylic and dicarboxylic acids up to 12 carbons in length---fatty acids with at least 8 carbons are required to form vesicles, and modern cell membranes employ lipids with ~12--20 carbons. To understand the origin of meteoritic fatty acids, we search the literature for abiotic fatty acid reaction pathways and create a candidate list of 11 reactions that could potentially produce these fatty acids in meteorite parent bodies. Straight-chain monocarboxylic acids (SCMA) are the dominant fatty acids in meteorites, followed by branched-chain monocarboxylic acids (BCMA). SCMA are most abundant in CM2 and Tagish Lake (ungrouped) meteorites, ranging on average from 10$^2$ ppb to 4x10$^5$ ppb, and 10$^4$ ppb to 5x10$^6$ ppb, respectively. In CM, CV, and Tagish Lake meteorites, SCMA abundances generally decrease with increasing carbon chain length. Conversely, SCMA abundances in CR meteorites peak at 5 and 6 carbons in length, and decrease on either side of this peak. This unique CR fatty acid distribution may hint at terrestrial contamination, or that certain fatty acid reactions mechanisms are active in different meteorite parent bodies (planetesimals). We identify Fischer-Tropsch-type synthesis as the most promising pathway for further analysis in the production of fatty acids in planetesimals.

astro-ph.EP↗

arXiv2018

Constraining the Time Interval for the Origin of Life on Earth

Estimates of the time at which life arose on Earth make use of two types of evidence. First, astrophysical and geophysical studies provide a timescale for the formation of Earth and the Moon, for large impact events on early Earth, and for the cooling of the early magma ocean. From this evidence, we can deduce a habitability boundary, which is the earliest point at which Earth became habitable. Second, biosignatures in geological samples, including microfossils, stromatolites, and chemical isotope ratios, provide evidence for when life was actually present. From these observations we can deduce a biosignature boundary, which is the earliest point at which there is clear evidence that life existed. Studies with molecular phylogenetics and records of the changing level of oxygen in the atmosphere give additional information that helps to determine the biosignature boundary. Here, we review the data from a wide range of disciplines to summarize current information on the timings of these two boundaries. The habitability boundary could be as early as 4.5 Ga, the earliest possible estimate of the time at which Earth had a stable crust and hydrosphere, or as late as 3.9 Ga, the end of the period of heavy meteorite bombardment. The lack of consensus on whether there was a late heavy meteorite bombardment that was significant enough to prevent life is the largest uncertainty in estimating the time of the habitability boundary. The biosignature boundary is more closely constrained. Evidence from carbon isotope ratios and stromatolite fossils both point to a time close to 3.7 Ga. Life must have emerged in the interval between these two boundaries. The time taken for life to appear could, therefore, be within 200 Myr or as long as 800 Myr.

astro-ph.EP↗

arXiv2017

Origin of the RNA World: The Fate of Nucleobases in Warm Little Ponds

Prior to the origin of simple cellular life, the building blocks of RNA (nucleotides) had to form and polymerize in favourable environments on the early Earth. At this time, meteorites and interplanetary dust particles delivered organics such as nucleobases (the characteristic molecules of nucleotides) to warm little ponds whose wet-dry cycles promoted rapid polymerization. We build a comprehensive numerical model for the evolution of nucleobases in warm little ponds leading to the emergence of the first nucleotides and RNA. We couple Earth's early evolution with complex prebiotic chemistry in these environments. We find that RNA polymers must have emerged very quickly after the deposition of meteorites (< a few years). Their constituent nucleobases were primarily meteoritic in origin and not from interplanetary dust particles. Ponds appeared as continents rose out of the early global ocean but this increasing availability of "targets" for meteorites was offset by declining meteorite bombardment rates. Moreover, the rapid losses of nucleobases to pond seepage during wet periods, and to UV photodissociation during dry periods means that the synthesis of nucleotides and their polymerization into RNA occurred in just one to a few wet-dry cycles. Under these conditions, RNA polymers likely appeared prior to 4.17 billion years ago. Significance: There are two competing hypotheses for the site at which an RNA world emerged: hydrothermal vents in the deep ocean and warm little ponds. Because the former lacks wet and dry cycles, which are well known to promote polymerization (in this case, of nucleotides into RNA), we construct a comprehensive model for the origin of RNA in the latter sites. Our model advances the story and timeline of the RNA world by constraining the source of biomolecules, the environmental conditions, the timescales of reaction, and the emergence of first RNA polymers.

astro-ph.EP↗

arXiv2016

Meteorites and the RNA World: A Thermodynamic Model of Nucleobase Synthesis within Planetesimals

The possible meteorite parent body origin of Earth's pregenetic nucleobases is substantiated by the guanine (G), adenine (A) and uracil (U) measured in various meteorites. Cytosine (C) and thymine (T) however are absent in meteorites, making the emergence of a RNA and later RNA/DNA/protein world problematic. We investigate the meteorite parent body (planetesimal) origin of all nucleobases by computationally modeling 18 reactions that potentially contribute to nucleobase formation in such environments. Out of this list, we identify the two most important reactions for each nucleobase and find that these involve small molecules such as HCN, CO, NH3, and water that ultimately arise from the protoplanetary disks in which planetesimals are built. The primary result of this study is that cytosine is unlikely to persist within meteorite parent bodies due to aqueous deamination. Thymine has a thermodynamically favourable reaction pathway from uracil, formaldehyde and formic acid, but likely did not persist within planetesimals containing H2O2 due to an oxidation reaction with this molecule. Finally, while FT synthesis is found to be the dominant source of nucleobases within our model planetesimal, NC synthesis may still be significant under certain chemical conditions (e.g. within CR2 parent bodies). We discuss several major consequences of our results for the origin of the RNA world.

astro-ph.EP↗

arXiv2015

Nature's Starships II: Simulating the Synthesis of Amino Acids in Meteorite Parent Bodies

Carbonaceous chondrite meteorites are known for having high water and organic material contents, including amino acids. Here we address the origin of amino acids in the warm interiors of their parent bodies (planetesimals) within a few million years of their formation, and connect this with the astrochemistry of their natal protostellar disks. We compute both the total amino acid abundance pattern as well as the relative frequencies of amino acids within the CM2 (e.g. Murchison) and CR2 chondrite subclasses based on Strecker reactions within these bodies. We match the relative frequencies to well within an order of magnitude among both CM2 and CR2 meteorites for parent body temperatures $<$ 200$^{\circ}$C. These temperatures agree with 3D models of young planetesimal interiors. We find theoretical abundances of approximately 7x10$^5$ parts-per-billion (ppb), which is in agreement with the average observed abundance in CR2 meteorites of 4$\pm$7x10$^5$, but an order of magnitude higher than the average observed abundance in CM2 meteorites of 2$\pm$2x10$^4$. We find that the production of hydroxy acids could be favoured over the production of amino acids within certain meteorite parent bodies (e.g. CI1, CM2) but not others (e.g. CR2). This could be due to the relatively lower NH$_3$ abundances within CI1 and CM2 meteorite parent bodies, which leads to less amino acid synthesis. We also find that the water content in planetesimals is likely to be the main cause of variance between carbonaceous chondrites of the same subclass. We propose that amino acid abundances are primarily dependent on the ammonia and water content of planetesimals that are formed in chemically distinct regions within their natal protostellar disks.

astro-ph.EP↗