Radical reactions on interstellar icy dust grains: Experimental investigations of elementary processes.

Molecular clouds (MCs) in space are the birthplace of various molecular species. Chemical reactions occurring on the cryogenic surfaces of cosmic icy dust grains have been considered to play important roles in the formation of these species. Radical reactions are crucial because they often have low barriers and thus proceed even at low temperatures such as ∼10 K. Since the 2000s, laboratory experiments conducted under low-temperature, high-vacuum conditions that mimic MC environments have revealed the elementary physicochemical processes on icy dust grains. In this review, experiments conducted by our group in this context are explored, with a focus on radical reactions on the surface of icy dust analogues, leading to the formation of astronomically abundant molecules such as H2, H2O, H2CO, and CH3OH and deuterium fractionation processes. The development of highly sensitive, non-destructive methods for detecting adsorbates and their utilization for clarifying the behavior of free radicals on ice, which contribute to the formation of complex organic molecules, are also described.

Tracing the Primordial Chemical Life of Glycine: A Review from Quantum Chemical Simulations.

Glycine (Gly), NH2CH2COOH, is the simplest amino acid. Although it has not been directly detected in the interstellar gas-phase medium, it has been identified in comets and meteorites, and its synthesis in these environments has been simulated in terrestrial laboratory experiments. Likewise, condensation of Gly to form peptides in scenarios resembling those present in a primordial Earth has been demonstrated experimentally. Thus, Gly is a paradigmatic system for biomolecular building blocks to investigate how they can be synthesized in astrophysical environments, transported and delivered by fragments of asteroids (meteorites, once they land on Earth) and comets (interplanetary dust particles that land on Earth) to the primitive Earth, and there react to form biopolymers as a step towards the emergence of life. Quantum chemical investigations addressing these Gly-related events have been performed, providing fundamental atomic-scale information and quantitative energetic data. However, they are spread in the literature and difficult to harmonize in a consistent way due to different computational chemistry methodologies and model systems. This review aims to collect the work done so far to characterize, at a quantum mechanical level, the chemical life of Gly, i.e., from its synthesis in the interstellar medium up to its polymerization on Earth.

Space Exposure of Amino Acids and Their Precursors during the Tanpopo Mission.

Amino acids have been detected in extraterrestrial bodies such as carbonaceous chondrites (CCs), which suggests that extraterrestrial organics could be the source of the first life on Earth, and interplanetary dust particles (IDPs) or micrometeorites (MMs) are promising carriers of extraterrestrial organic carbon. Some amino acids found in CCs are amino acid precursors, but these have not been well characterized. The Tanpopo mission was conducted in Earth orbit from 2015 to 2019, and the stability of glycine (Gly), hydantoin (Hyd), isovaline (Ival), 5-ethyl-5-methylhydantoin (EMHyd), and complex organics formed by proton irradiation from CO, NH3, and H2O (CAW) in space were analyzed by high-performance liquid chromatography and/or gas chromatography/mass spectrometry. The target substances showed a logarithmic decomposition over 1-3 years upon space exposure. Recoveries of Gly and CAW were higher than those of Hyd, Ival, and EMHyd. Ground simulation experiments showed different results: Hyd was more stable than Gly. Solar ultraviolet light was fatal to all organics, and they required protection when carried by IDPs/MMs. Thus, complex amino acid precursors (such as CAW) were possibly more robust than simple precursors during transportation to primitive Earth. The Tanpopo 2 mission is currently being conducted to expose organics to more probable space conditions.

Scientific Targets of Tanpopo: Astrobiology Exposure and Micrometeoroid Capture Experiments at the Japanese Experiment Module Exposed Facility of the International Space Station.

The Tanpopo experiment was the first Japanese astrobiology mission on board the Japanese Experiment Module Exposed Facility on the International Space Station (ISS). The experiments were designed to address two important astrobiological topics, panspermia and the chemical evolution process toward the generation of life. These experiments also tested low-density aerogel and monitored the microdebris environment around low Earth orbit. The following six subthemes were identified to address these goals: (1) Capture of microbes in space: Estimation of the upper limit of microbe density in low Earth orbit; (2) Exposure of microbes in space: Estimation of the survival time course of microbes in the space environment; (3) Capture of cosmic dust on the ISS and analysis of organics: Detection of the possible presence of organic compounds in cosmic dust; (4) Alteration of organic compounds in space environments: Evaluation of decomposition time courses of organic compounds in space; (5) Space verification of the Tanpopo hyper-low-density aerogel: Durability and particle-capturing capability of aerogel; (6) Monitoring of the number of space debris: Time-dependent change in space debris environment. Subthemes 1 and 2 address the panspermia hypothesis, whereas 3 and 4 address the chemical evolution. The last two subthemes contribute to space technology development. Some of the results have been published previously or are included in this issue. This article summarizes the current status of the Tanpopo experiments.

Insoluble organic matter in chondrites: Archetypal melanin-like PAH-based multifunctionality at the origin of life?

An interdisciplinary review of the chemical literature that points to a unifying scenario for the origin of life, referred to as the Primordial Multifunctional organic Entity (PriME) scenario, is provided herein. In the PriME scenario it is suggested that the Insoluble Organic Matter (IOM) in carbonaceous chondrites, as well as interplanetary dust particles from meteorites and comets may have played an important role in the three most critical processes involved in the origin of life, namely 1) metabolism, via a) the provision and accumulation of molecules that are the building blocks of life, b) catalysis (e.g., by templation), and c) protection of developing life molecules against radiation by excited state deactivation; 2) compartmentalization, via adsorption of compounds on the exposed organic surfaces in fractured meteorites, and 3) replication, via deaggregation, desorption and related physical phenomena. This scenario is based on the hitherto overlooked structural and physicochemical similarities between the IOM and the dark, insoluble, multifunctional melanin polymers found in bacteria and fungi and associated with the ability of these microorganisms to survive extreme conditions, including ionizing radiation. The underlying conceptual link between these two materials is strengthened by the fact that primary precursors of bacterial and fungal melanins (collectively referred to herein as allomelanins) are hydroxylated aromatic compounds like homogentisic acid and 1,8-dihydroxynaphthalene, and that similar hydroxylated aromatic compounds, including hydroxynaphthalenes, figure prominently among possible components of the organic materials on dust grains and ices in the interstellar matter, and may be involved in the formation of IOM in meteorites. Inspired by this rationale, a vis-à-vis review of the properties of IOM from various chondrites and non-nitrogenous allomelanin pigments from bacteria and fungi is provided herein. The unrecognized similarities between these materials may pave the way for a novel scenario at the origin of life, in which IOM-related complex organic polymers delivered to the early Earth are proposed to serve as PriME and were preserved and transformed in those primitive forms of life that shared the ability to synthesize melanin polymers playing an important role in the critical processes underlying the establishment of terrestrial eukaryotes.

Analog Experiments for the Identification of Trace Biosignatures in Ice Grains from Extraterrestrial Ocean Worlds.

Reliable identification of biosignatures, such as amino acids, fatty acids, and peptides, on extraterrestrial ocean worlds is a key prerequisite for space missions that search for life or its emergence on these worlds. One promising approach is the use of high-performance in situ impact ionization mass spectrometers to sample water ice grains emerging from ocean-bearing moons such as Europa or Enceladus. A predecessor of such detectors, the Cosmic Dust Analyzer on board the Cassini spacecraft, has proven to be very successful in analyzing inorganic and organic ocean constituents and with that characterizing the habitability of Enceladus ocean. However, biosignatures have not been definitively identified in extraterrestrial ocean environments so far. Here, we investigate with an analog experiment the spectral appearance of amino acids, fatty acids, and peptides in water ice grains, together with their detection limits, as applicable to spaceborne mass spectrometers. We employ a laboratory-based laser induced liquid beam ion desorption technique, proven to simulate accurately the impact ionization mass spectra of water ice grains over a wide range of impact speeds. The investigated organics produce characteristic mass spectra, with molecular peaks as well as clearly identifiable, distinctive fragments. We find the detection limits of these key biosignatures to be at the µM or nM level, depending on the molecular species and instrument polarity, and infer that impact ionization mass spectrometers are most sensitive to the molecular peaks of these biosignatures at encounter velocities of 4-6 km/s.

The DNA of Bacteria of the World Ocean and the Earth in Cosmic Dust at the International Space Station.

Cosmic dust samples from the surface of the illuminator of the International Space Station (ISS) were collected by a crew member during his spacewalk. The sampler with tampon in a vacuum container was delivered to the Earth. Washouts from the tampon's material and the tampon itself were analyzed for the presence of bacterial DNA by the method of nested PCR with primers specific to DNA of the genus Mycobacteria, DNA of the strains of capsular bacteria Bacillus, and DNA encoding 16S ribosomal RNA. The results of amplification followed by sequencing and phylogenetic analysis indicated the presence of the bacteria of the genus Mycobacteria and the extreme bacterium of the genus Delftia in the samples of cosmic dust. It was shown that the DNA sequence of one of the bacteria of the genus Mycobacteria was genetically similar to that previously observed in superficial micro layer at the Barents and Kara seas' coastal zones. The presence of the wild land and marine bacteria DNA on the ISS suggests their possible transfer from the stratosphere into the ionosphere with the ascending branch of the global electric circuit. Alternatively, the wild land and marine bacteria as well as the ISS bacteria may all have an ultimate space origin.

Following the Interstellar History of Carbon: From the Interiors of Stars to the Surfaces of Planets.

The chemical history of carbon is traced from its origin in stellar nucleosynthesis to its delivery to planet surfaces. The molecular carriers of this element are examined at each stage in the cycling of interstellar organic material and their eventual incorporation into solar system bodies. The connection between the various interstellar carbon reservoirs is also examined. Carbon has two stellar sources: supernova explosions and mass loss from evolved stars. In the latter case, the carbon is dredged up from the interior and then ejected into a circumstellar envelope, where a rich and unusual C-based chemistry occurs. This molecular material is eventually released into the general interstellar medium through planetary nebulae. It is first incorporated into diffuse clouds, where carbon is found in polyatomic molecules such as H2CO, HCN, HNC, c-C3H2, and even C60+. These objects then collapse into dense clouds, the sites of star and planet formation. Such clouds foster an active organic chemistry, producing compounds with a wide range of functional groups with both gas-phase and surface mechanisms. As stars and planets form, the chemical composition is altered by increasing stellar radiation, as well as possibly by reactions in the presolar nebula. Some molecular, carbon-rich material remains pristine, however, encapsulated in comets, meteorites, and interplanetary dust particles, and is delivered to planet surfaces. Key Words: Carbon isotopes-Prebiotic evolution-Interstellar molecules-Comets-Meteorites. Astrobiology 16, 997-1012.

Silica aerogel for capturing intact interplanetary dust particles for the Tanpopo experiment.

In this paper, we report the progress in developing a silica-aerogel-based cosmic dust capture panel for use in the Tanpopo experiment on the International Space Station (ISS). Previous studies revealed that ultralow-density silica aerogel tiles, comprising two layers with densities of 0.01 and 0.03 g/cm(3) developed using our production technique, were suitable for achieving the scientific objectives of the astrobiological mission. A special density configuration (i.e., box framing) aerogel with a holder was designed to construct the capture panels. Qualification tests for an engineering model of the capture panel as an instrument aboard the ISS were successful. Sixty box-framing aerogel tiles were manufactured in a contamination-controlled environment.

Organic compounds in star forming regions.

The influence of complex dust composition on the general chemical evolution of a prestellar core and the content of complex organic compounds is studied. It is shown that various component groups respond differently to the presence of a small dust population. At early stages the difference is determined primarily by changes in the balance of photo processes due to effective absorption of ultraviolet photons by small dust grains of the second population and collisional reactions with dust particles. At later stages differences are also caused by the growing dominance of additional reaction channels related to surface organic synthesis.

Low-energy electron-induced chemistry of condensed methanol: implications for the interstellar synthesis of prebiotic molecules.

In the interstellar medium, UV photolysis of condensed methanol (CH3OH), contained in ice mantles surrounding dust grains, is thought to be the mechanism that drives the formation of "complex" molecules, such as methyl formate (HCOOCH3), dimethyl ether (CH3OCH3), acetic acid (CH3COOH), and glycolaldehyde (HOCH2CHO). The source of this reaction-initiating UV light is assumed to be local because externally sourced UV radiation cannot penetrate the ice-containing dark, dense molecular clouds. Specifically, exceedingly penetrative high-energy cosmic rays generate secondary electrons within the clouds through molecular ionizations. Hydrogen molecules, present within these dense molecular clouds, are excited in collisions with these secondary electrons. It is the UV light, emitted by these electronically excited hydrogen molecules, that is generally thought to photoprocess interstellar icy grain mantles to generate "complex" molecules. In addition to producing UV light, the large numbers of low-energy (< 20 eV) secondary electrons, produced by cosmic rays, can also directly initiate radiolysis reactions in the condensed phase. The goal of our studies is to understand the low-energy, electron-induced processes that occur when high-energy cosmic rays interact with interstellar ices, in which methanol, a precursor of several prebiotic species, is the most abundant organic species. Using post-irradiation temperature-programmed desorption, we have investigated the radiolysis initiated by low-energy (7 eV and 20 eV) electrons in condensed methanol at - 85 K under ultrahigh vacuum (5 x 10(-10) Torr) conditions. We have identified eleven electron-induced methanol radiolysis products, which include many that have been previously identified as being formed by methanol UV photolysis in the interstellar medium. These experimental results suggest that low-energy, electron-induced condensed phase reactions may contribute to the interstellar synthesis of "complex" molecules previously thought to form exclusively via UV photons.

Fluid-induced organic synthesis in the solar nebula recorded in extraterrestrial dust from meteorites.

Isotopically anomalous carbonaceous grains in extraterrestrial samples represent the most pristine organics that were delivered to the early Earth. Here we report on gentle aberration-corrected scanning transmission electron microscopy investigations of eight (15)N-rich or D-rich organic grains within two carbonaceous Renazzo-type (CR) chondrites and two interplanetary dust particles (IDPs) originating from comets. Organic matter in the IDP samples is less aromatic than that in the CR chondrites, and its functional group chemistry is mainly characterized by C-O bonding and aliphatic C. Organic grains in CR chondrites are associated with carbonates and elemental Ca, which originate either from aqueous fluids or possibly an indigenous organic source. One distinct grain from the CR chondrite NWA 852 exhibits a rim structure only visible in chemical maps. The outer part is nanoglobular in shape, highly aromatic, and enriched in anomalous nitrogen. Functional group chemistry of the inner part is similar to spectra from IDP organic grains and less aromatic with nitrogen below the detection limit. The boundary between these two areas is very sharp. The direct association of both IDP-like organic matter with dominant C-O bonding environments and nanoglobular organics with dominant aromatic and C-N functionality within one unique grain provides for the first time to our knowledge strong evidence for organic synthesis in the early solar system activated by an anomalous nitrogen-containing parent body fluid.

Laboratory studies of electron and ion irradiation of solid acetonitrile (CH3CN).

The structure and bonding of solid acetonitrile (CH3CN) films on amorphous silica are studied, and chemical and physical processes under irradiation with 200 keV protons and 250-400 eV electrons are quantified using transmission infrared spectroscopy, reflection-absorption infrared spectroscopy and temperature-programmed desorption, with the assistance of basic computational chemistry and nuclear materials calculations. The thermal desorption profiles are found to depend strongly on the balance between CH3CN-surface and CH3CN-CH3CN interactions, passing from a sub-monolayer regime (binding energy: 35-50 kJ mol⁻¹) to a multilayer regime (binding energy: 38.2±1.0 kJ mol⁻¹) via a fractional order desorption regime characteristic of islanding as the coverage increases. Calculations using the SRIM code reveal that the effects of the ion irradiation are dominated by electronic stopping of incident protons, and the subsequent generation of secondary electrons. Therefore, ion irradiation and electron irradiation experiments can be quantitatively compared. During ion irradiation of thicker CH3CN films, a cross section for secondary electron-promoted chemical destruction of CH3CN of 4 (±1) × 10⁻¹8 cm² was measured, while electron-promoted desorption was not detected. A significantly higher cross section for electron-promoted desorption of 0.82-3.2 × 10⁻¹5 cm² was measured during electron irradiation of thinner CH3CN films, while no chemical products were detected. The differences between the experimental results can be rationalized by recognizing that chemical reaction is a bulk effect in the CH3CN film, whereas desorption is a surface sensitive process. In thicker films, electron-promoted desorption is expected to occur a rate that is independent of the film thickness; i.e. show zeroth-order kinetics with respect to the surface concentration.

Insights into H2 formation in space from ab initio molecular dynamics.

Hydrogen formation is a key process for the physics and the chemistry of interstellar clouds. Molecular hydrogen is believed to form on the carbonaceous surface of dust grains, and several mechanisms have been invoked to explain its abundance in different regions of space, from cold interstellar clouds to warm photon-dominated regions. Here, we investigate direct (Eley-Rideal) recombination including lattice dynamics, surface corrugation, and competing H-dimers formation by means of ab initio molecular dynamics. We find that Eley-Rideal reaction dominates at energies relevant for the interstellar medium and alone may explain observations if the possibility of facile sticking at special sites (edges, point defects, etc.) on the surface of the dust grains is taken into account.

Ocular toxicity of authentic lunar dust.

BACKGROUND: Dust exposure is a well-known occupational hazard for terrestrial workers and astronauts alike and will continue to be a concern as humankind pursues exploration and habitation of objects beyond Earth. Humankind's limited exploration experience with the Apollo Program indicates that exposure to dust will be unavoidable. Therefore, NASA must assess potential toxicity and recommend appropriate mitigation measures to ensure that explorers are adequately protected. Visual acuity is critical during exploration activities and operations aboard spacecraft. Therefore, the present research was performed to ascertain the ocular toxicity of authentic lunar dust. METHODS: Small (mean particle diameter = 2.9 ± 1.0 µm), reactive lunar dust particles were produced by grinding bulk dust under ultrapure nitrogen conditions. Chemical reactivity and cytotoxicity testing were performed using the commercially available EpiOcularTM assay. Subsequent in vivo Draize testing utilized a larger size fraction of unground lunar dust that is more relevant to ocular exposures (particles <120 µm; median particle diameter = 50.9 ± 19.8 µm). RESULTS: In vitro testing indicated minimal irritancy potential based on the time required to reduce cell viability by 50% (ET50). Follow-up testing using the Draize standard protocol confirmed that the lunar dust was minimally irritating. Minor irritation of the upper eyelids was noted at the 1-hour observation point, but these effects resolved within 24 hours. In addition, no corneal scratching was observed using fluorescein stain. CONCLUSIONS: Low-titanium mare lunar dust is minimally irritating to the eyes and is considered a nuisance dust for ocular exposure. No special precautions are recommended to protect against ocular exposures, but fully shielded goggles may be used if dust becomes a nuisance.

Spectroscopy and astronomy: H3+ from the laboratory to the Galactic center.

Since the serendipitous discovery of the Fraunhofer spectrum in the Sun in 1814 which initiated spectroscopy and astrophysics, spectroscopy developed hand in hand with astronomy. I discuss my own work on the infrared spectrum of H3+ from its discovery in the laboratory in 1980, in interstellar space in 1996, to recent studies in the Galactic center as an example of astronomical spectroscopy. Its spin-off, the spectroscopy of simple molecular ions, is also briefly discussed.

[Effect of lunar dust on humans: -lunar dust: regolith-].

We reviewed the effect of lunar dust (regolith) on humans by the combination of the hazard/exposure of regolith and microgravity of the moon. With regard to the physicochemical properties of lunar dust, the hazard-related factors are its components, fibrous materials and nanoparticles. Animal exposure studies have been performed using a simulant of lunar dust, and it was speculated that the harmful effects of the simulant lies between those of crystalline silica and titanium dioxide. Fibrous materials may not have a low solubility judging from their components. The nanoparticles in lunar dust may have harmful potentials from the view of the components. As for exposure to regolith, there is a possibility that particles larger than ones in earth (1 gravity) are respirable. In microgravity, 1) the deposition of particles of less than 1 µm in diameter in the human lung did not decrease, 2) the functions of macrophages including phagocytosis were suppressed, 3) pulmonary inflammation was changed. These data on hazard/exposure and microgravity suggest that fine and ultrafine particles in regolith may have potential hazards and risks for humans.

Cosmic carbon chemistry: from the interstellar medium to the early Earth.

Astronomical observations have shown that carbonaceous compounds in the gas and solid state, refractory and icy are ubiquitous in our and distant galaxies. Interstellar molecular clouds and circumstellar envelopes are factories of complex molecular synthesis. A surprisingly large number of molecules that are used in contemporary biochemistry on Earth are found in the interstellar medium, planetary atmospheres and surfaces, comets, asteroids and meteorites, and interplanetary dust particles. In this article we review the current knowledge of abundant organic material in different space environments and investigate the connection between presolar and solar system material, based on observations of interstellar dust and gas, cometary volatiles, simulation experiments, and the analysis of extraterrestrial matter. Current challenges in astrochemistry are discussed and future research directions are proposed.

Mass spectrometry of hyper-velocity impacts of organic micrograins.

The study of hyper-velocity impacts of micrometeoroids is important for the calibration of dust sensors in space applications. For this purpose, submicron-sized synthetic dust grains comprising either polystyrene or poly[bis(4-vinylthiophenyl)sulfide] were coated with an ultrathin overlayer of an electrically conductive organic polymer (either polypyrrole or polyaniline) and were accelerated to speeds between 3 and 35 km s(-1) using the Heidelberg Dust Accelerator facility. Time-of-flight mass spectrometry was applied to analyse the resulting ionic impact plasma using a newly developed Large Area Mass Analyser (LAMA). Depending on the projectile type and the impact speed, both aliphatic and aromatic molecular ions and cluster species were identified in the mass spectra with masses up to 400 u. Clusters resulting from the target material (silver) and mixed clusters of target and projectile species were also observed. Impact velocities of between 10 and 35 km s(-1) are suitable for a principal identification of organic materials in micrometeoroids, whereas impact speeds below approximately 10 km s(-1) allow for an even more detailed analysis. Molecular ions and fragments reflect components of the parent molecule, providing determination of even complex organic molecules embedded in a dust grain. In contrast to previous measurements with the Cosmic Dust Analyser instrument, the employed LAMA instrument has a seven times higher mass resolution--approximately 200--which allowed for a detailed analysis of the complex mass spectra. These fundamental studies are expected to enhance our understanding of cometary, interplanetary and interstellar dust grains, which travel at similar hyper-velocities and are known to contain both aliphatic and aromatic organic compounds.