Bridging the Gap: Aqueous Alteration Simulation for Prebiotic Organic Abundances

Introduction

For decades, meteorites' chemical inventory has been studied for astrobiologically relevant organic components like amines and amino acids that may have led to life on Earth and beyond. Prebiotic organics in some carbonaceous chondrite groups may have enabled life to evolve on Earth and elsewhere. However, the creation and chemical development of complex soluble organic compounds from interstellar precursors under parent body circumstances has not been well studied.

A direct collection of elements from carbon-rich asteroids, which are assumed to be the parent bodies of some carbonaceous chondrites, has been done to understand better their origins (e.g., OSIRIS-REx and Hayabusa2 sample-return missions). The creation and molecular distributions of primordial organics in meteoritic and sample-returned materials must be understood. Were meteoritic amines and amino acids produced in the parent body, protoplanetary disc, or interstellar molecular cloud? Understanding the origins of organic matter in meteoritic and sample-returned materials helps explain the prevalence of prebiotic chemicals in our solar system and beyond.

Laboratory studies with irradiated interstellar ice analogs have shown the production of refractory residues, which include amino acids, amines, and other soluble organic compounds, inferring that meteorite parent bodies may have acquired primordial organic content from interstellar materials. Martins et al. demonstrated that the Paris meteorite's chemical composition was significantly connected to interstellar predecessors. However, ice irradiation tests have not consistently matched carbonaceous chondrites in amine and amino acid distribution. For instance, certain ice irradiation experiments have more α-than β-alanine, whereas the most aqueously affected chondrites, such as CI1, CM1, and CR1, have more β-than α-alanine. The ratio reverses in the least altered CM2 and CR2 meteorites. Aqueous modification of ice irradiation leftovers at various temperatures may explain these variances.

Aqueous modification of commercial organics relevant to interstellar materials has been tested in recent lab studies. Kebukawa et al. used calcium hydroxide to aqueously transform simple interstellar important molecules at different temperatures in a "bottom-up" manner. Vinogradoff et al. started with hexamethylenetetramine (HMT), an abundant complex photoproduct of interstellar ice chemistry, using a "top-down" method. These findings shed light on meteorite parent body chemistry but don't explain the changes in α- to β-alanine ratios reported in aqueously altered meteorites. Interstellar residue analogs can fill these voids.

Meteorite parent body aqueous change of interstellar residue analogs affects the distribution of amines and amino acids. Danger et al. mimicked the irradiation of ice using aqueous alteration near the edge of protoplanetary discs, which have greater temperatures and less volatiles than interstellar ice. This work used protons, while Danger et al. used photons. However, radiation-type chemical compounds differ only slightly. We irradiate an H2O: CO2:CH3OH:15NH3 (20:4:2:1) ice combination at 25 K and evaluate its products when heated in aqueous solution at 50 and 125 °C for 2, 7, and 30 days. The ice mixture ratio reflects abundances found in low-mass young stellar objects (YSOs) when interstellar ices are warmed above 20 K. Ices subjected to cosmic rays should create YSO and residue. Some interstellar ice and organic wastes are carried to the protoplanetary disc and integrated into meteorite parent bodies, which undergo aqueous alteration at varying temperatures.

The experiments were conducted at 50 and 125 °C and a relatively short heating time scale to reflect the processes in parent bodies that contain CI, CM, and CR chondrites, which exhibit only moderate aqueous alteration and may explain the extended periods (104–106 years) when liquid water was present. This study's chemical alterations can be compared to comparable research since the literature has documented an experimental heating time range of 2–30 days. This study's main products include ethylamine, methylamine, and alanine. glycine, , β-alanine, and serine. Due to their simple structures, these meteorites may include interstellar materials' most abundant amines and amino acids. Methylamine, ethylamine, and glycine were found orbiting comet 67P/Churyumov-Gerasimenko, in samples from comet 81P/Wild and the interstellar medium (ISM). Comets and interstellar settings lack alanine, β-alanine, and serine.

Glavin et al. found l-alanine, β-alanine, d- and l-serine, and other amino acids in Stardust aerogels exposed to 81P/Wild; however, these species present in other controls, therefore they should be considered contamination. Glavin et al. found near or entire homochirality of alanine and serine, indicating that they are terrestrial pollutants; however, Elsila et al. suggested that part of the identified β-alanine may be cometary. Numerous interstellar residue analog tests with and without hydrolysis of the produced ice residue have found amino acids. Ehrenfreund et al. suggested that the relatively simple distribution of amino acids in the CI1 meteorites Orgueil and Ivuna, dominated by glycine and β-alanine, was evidence that these meteorites originated from a cometary parent body, making them promising components of interstellar-inherited amino acids in meteorites.

Methods

Radiation Processing and Synthesis of Ice

A cryogenic high-vacuum chamber was used to create residues and ice, and proton irradiation of ice was studied. H2O: CO2:CH3OH:15NH3 ice mixture was used to synthesize residues. Laser interferometry and leak valve calibrations were employed to achieve the correct ice mixture ratio. Laser interferometry tracked molecular species deposition on foil. Ices were grown for 3 h to reach a thickness of ∼15 μm. After the ice developed, an interfaced beamline coupled to a Van de Graaff accelerator allowed protons at 0.9 MeV at 1.5 × 10−7 A into the vacuum chamber.

In situ Nicolet Nexus 670 spectrometer measured ice and residue IR spectra. Ice and residue spectra were averaged from 100 scans at 2 cm−1 from 5000 to 650 cm−1. IR spectroscopy was utilized to examine residue molecular structures and assure uniformity of IR characteristics across studies. The ice got 10 eV/molecule of radiation, similar to interstellar frozen grains in a thick cloud for 107 years. After irradiation, the samples were heated to 300 K at 1.5 K/min to gently release volatiles without damaging the foil. The cryostat was removed from the vacuum chamber, and baked tweezers gently removed residue-covered foils and placed them in baked tubes. The tubes were instantly frozen at −80 °C before the aqueous alteration experiments.

Aqueous Alteration Controls and Residue Preparation

All glassware was rinsed with Milli-Q ultrapure water and baked overnight in a muffle furnace at 500 °C. 850 μL of methanol was pipetted into each foil-containing tube and sonicated for 10 minutes to extract the samples. After sonication, liquids were pipetted into baking vials with 10 μL of 6 M HCl to prevent volatile amine loss. The acidified methanol solutions were split into four 200 μL fractions placed in clean 13 × 100 mm borosilicate tubes. All the tubes were kept in different conditions and for different processing times. Labconco CentriVap was used to remove methanol, after which, in each tube, 200 μL of ultrapure water was added.

Since the samples were unbuffered, a drop of solution was spotted on universal pH paper to measure the pH of each water-filled tube. An oxy-propane torch created ampoule necks at the upper 1/3 of the tubes. To avoid sample contamination, the torch was bracketed, and the tube was manually turned using nitrile-gloved hands. The necks were created by monitoring the glass tube by the sample while manually sealing without warming the solutions. The ampoules were liquid nitrogen freeze−pump−thaw degassed three times to eliminate volatile air impurities and flame-sealed in vacuo.

Aqueous Alteration and Preparation for Analysis

Aqueous alteration samples were treated at 50 or 125 °C for a set time (2, 7, or 30 days). To heat samples, tubes were put in dry bath blocks in a furnace or oven. Freezing the unprocessed samples preserved them.

The sample treated at 125 °C for 30 days has a pH of 8, comparable with CM asteroid fluids after aqueous alteration (pH 7−10). 400 μL of ultrapure water was pipetted into tubes to acquire the most aqueously changed samples. Tube liquids were transferred into baked Total Recovery Autosampler Vials with 20 μL of 6 M HCl and dried in a centrifugal evaporator. Samples were derivatized with AccQ•Tag reagents. Amino acids were separated using chromatography and a ToF-MS analyzer.

Results and Discussion

Major products obtained are methylamine, ethylamine, glycine, serine, alanine, and beta-alanine. Other products like gamma-aminobutyric acid, isopropylamine, beta-amino-isobutyric acid, and propylamine were also obtained as minor products. Following these experiments, it was clear that the ice residue forming conditions are responsible for determining the concentration of amino acids and amines. Amines were obtained in higher concentration when compared to amino acids in every residue formed. It was found that varying the conditions for synthesizing the interstellar residue analogs did not affect the molecular distributions of amino acids and amines formed. Aqueous alteration affected amine and amino acid distributions based on temperature, time, and molecule.

Precursors recovered from the residue by sonication extraction may have generated amines and amino acids during aqueous alteration. Acid hydrolysis of meteorite extracts returns more amines and amino acids from the partial breakage of insoluble organic matter (IOM) and other structurally related species. The experiments proved that the amines and the amino acids did not have a parent-daughter relationship. Methylamine and ethylamine were at least an order of magnitude greater than amino acids in non-aqueously altered samples. Both amines desorb below room temperature in an ice experiment and are volatile in ambient air; thus, their large presence in non-aqueously altered samples would arise if they were retained by and released from the ice residue or generated from precursor molecules in the extraction process.

Nitrile-based synthesis may affect aqueous alteration patterns. Radiation may convert amines to nitriles; therefore, the ice might have generated cyanide and aliphatic nitriles from 15NH3. Unprocessed samples included cyanide. CN− generated from ice irradiation should engage in several chemical processes during aqueous alteration and influence the product abundance trend. Our experiments showed that the efficiency of processes involving cyanides or aliphatic nitriles decreased equally across time and temperature. Methylamine, ethylamine, glycine, serine, alanine, and βalanine had the same abundance trend with or without aqueous alteration. Thus, interstellar cloud conditions and chemical inventories may strongly influence the abundance trend of amines and amino acids in meteorite parent bodies, even after the aqueous alteration.

The trend's resilience to aqueous alteration and initial conditions suggests that for parent bodies experiencing aqueous alteration at temperatures of 125 °C, the amine and amino acid abundance trend can be somewhat linked to that from the parent molecular cloud, assuming the molecules were interstellar-inherited. However, IOM-relevant elements, minerals, iron, magnesium, aluminum, and other inorganic species with different oxidation states were not employed in the studies and may be crucial for the synthesis (and destruction) of chondritic amines and amino acids. CI-like chondrites and sample-returned materials differ in amino acid distributions. Thus, both studies must advance to grasp the interstellar−meteorite parent body link.

Conclusion

To understand how interstellar-inherited residues affect meteorite amine and amino acid distributions, meteorite parent body aqueous alteration of interstellar residue analogs was simulated. This study suggests that the circumstances and organics from the parental interstellar cloud affect meteoritic soluble organic matter abundances and that the meteorite parent body may influence specific interstellar organic abundances. The abundance trend of methylamine > ethylamine > glycine > serine > alanine > β-alanine remained constant before and after aqueous alteration, even with cyanides, suggesting that the chemical inventory and conditions of the parental molecular cloud may strongly influence meteoritic amine and amino acid distribution. To study the development of soluble organics from the interstellar cloud to the parent body, IOM-relevant materials, minerals, and inorganic species should be tested.

During aqueous alteration, methylamine, serine, glycine, and β-alanine increased 2-fold, but alanine and ethylamine did not. Interstellar and meteoritic abundances may be unrelated due to molecular sensitivity to aqueous modification. The ratio of α-alanine/β-alanine increased with aqueous alteration duration and temperature, similar to carbonaceous chondrites. Aqueous alteration increased product abundances most significantly after seven days at 125 °C. The greater abundance of α-than β-alanine matches aqueously treated commercial sample tests but differs from aqueously changed chondrites.

Compared to prior experimental models, cyanide/nitriles interacting with relevant minerals may relate interstellar to aqueously changed meteoritic amine and amino acid abundances. The comparatively large quantities of volatile amines in the non-aqueously altered samples imply that volatiles may be transferred from the ISM to asteroids and comets, strengthening the case for interstellar organics beyond the ISM. Laboratory experiments and chondritic investigations are needed to comprehend interstellar and meteoritic amines and amino acids.